Critical IQ — Haemodynamic Coupling in Shock

Why Should I Care About Haemodynamic Coupling?

Start here — the clinical motivation before the physiology

🎯 The Problem We're Solving at the Bedside

You are looking after a patient in cardiogenic shock. Their mean arterial pressure (MAP — the average blood pressure in the arteries) is 68 mmHg on noradrenaline. You think they're "stable." But their lactate (a marker of how starved the tissues are of oxygen) keeps rising — 4, then 5, then 6 mmol/L. The organs are failing despite what looks like an acceptable blood pressure.

Or: you have a post-cardiac arrest patient. You've given fluids and vasopressors. MAP is 70. ScvO₂ (central venous oxygen saturation — how much oxygen the tissues are extracting) looks OK. But the kidneys are shutting down. Why?

The answer in both cases is haemodynamic uncoupling. The different layers of the cardiovascular system — the heart, the large vessels, the small vessels, the capillaries, the veins — have stopped working together. You cannot fix what you cannot see. And you cannot see it unless you understand coupling.

When Do We Use This Framework?

Any patient in shock not responding to standard resuscitation
Rising lactate despite "adequate" MAP and cardiac output (CO)
Deciding whether to add, increase, or reduce vasopressors
Deciding whether to add an inotrope (a drug that strengthens heart contractions)
Evaluating the right ventricle (RV) in ARDS, pulmonary embolism, or post-cardiac surgery
Guiding mechanical circulatory support (MCS — devices that help the failing heart)
Understanding why an apparently "stable" patient is deteriorating

What "Coupling" Actually Means

The word "coupling" simply means the heart and the blood vessels are working together efficiently — like an engine perfectly matched to its gearbox. Energy generated by the heart is transferred into useful blood flow rather than wasted as heat or pressure.

"Uncoupling" means this match is broken. Either the heart is too weak, or the vessels are too stiff or too floppy — and the result is the same: organs don't get enough oxygen despite what the numbers on the monitor might suggest.

🧒 In Plain Language

It's like driving a car in the wrong gear. The engine is revving (heart working hard), fuel is burning (energy being used), but the wheels are barely moving (blood isn't reaching the organs). The problem isn't always the engine — sometimes it's the gearbox.

🏥 Clinical Case — Follow This Patient Through the Module

Meet David, 58 years old. He presented with an anterior ST-elevation myocardial infarction (STEMI — a heart attack caused by complete blockage of a coronary artery). He underwent emergency percutaneous coronary intervention (PCI — a procedure to open the blocked artery with a stent) but arrived late. His left ventricle (LV — the main pumping chamber of the heart) is severely damaged.

On the ICU, 6 hours later:

Blood pressure (BP): 88/60 mmHg → MAP 69 mmHg (seemingly acceptable) | Heart rate (HR): 118 bpm | Cardiac output (CO — the volume of blood the heart pumps per minute): 2.8 L/min (normal 4–8) | Cardiac index (CI — CO adjusted for body size): 1.6 L/min/m² (normal >2.2) | Noradrenaline: 0.35 mcg/kg/min (a vasopressor — a drug that tightens blood vessels to raise blood pressure) | Lactate: 5.8 mmol/L and rising | Urine output: 10 mL/hour (oliguria — dangerously low) | Central venous pressure (CVP — pressure in the large veins entering the heart): 17 mmHg (high) | Capillary refill time (CRT — how quickly skin colour returns after pressing): 6 seconds (very prolonged, normal <2s)

The nurse asks: "His MAP looks OK — why is he getting worse?"

As you work through each tab, you'll understand exactly why David is deteriorating at every level — and what to do about it. The MAP is a lie.

📐 Illustrated Diagram — The Four Levels of Haemodynamic Coupling

Figure 1 — The four interdependent levels of haemodynamic coupling and their failure patterns in cardiogenic vs vasoplegic shock. All four must be assessed and treated simultaneously in refractory shock.

Glossary of Abbreviations

Every abbreviation used in this module — fully explained. Refer back here any time.

Haemodynamic Parameters

MAP

Mean Arterial Pressure — the average blood pressure throughout a full cardiac cycle. Target ≥65 mmHg in shock.

CO

Cardiac Output — the volume of blood the heart pumps per minute (L/min). Normal 4–8 L/min.

CI

Cardiac Index — CO divided by body surface area (L/min/m²). Adjusts for patient size. Normal >2.2.

SV

Stroke Volume — the volume of blood ejected by the heart per beat (mL). Normal 60–100 mL.

SVR

Systemic Vascular Resistance — the resistance the heart pumps against; reflects how tight or relaxed the blood vessels are. High in cardiogenic shock; low in vasoplegic shock.

CVP

Central Venous Pressure — pressure in the large veins entering the right heart. Measured via a central line. Elevated CVP indicates venous congestion.

PCWP

Pulmonary Capillary Wedge Pressure — reflects left heart filling pressure. Measured with a pulmonary artery catheter. Elevated in LV failure.

ScvO₂

Central Venous Oxygen Saturation — oxygen remaining in blood returning to the heart. Low (<70%) = tissues starved. High (>80%) = poor extraction or microvascular shunting.

Coupling-Specific Terms

Ea

Arterial Elastance — a measure of how stiff and resistant the arterial system is to the heart's ejection. Calculated as ~0.9 × systolic BP ÷ stroke volume.

Ees

End-Systolic Elastance — a load-independent measure of the heart muscle's contractile strength at the end of each heartbeat. The slope of the end-systolic pressure-volume relationship.

VAC

Ventriculo-Arterial Coupling — the ratio Ea/Ees. The master index of how well the heart and arteries are matched. Normal 0.6–1.2; uncoupled >1.36.

ESP

End-Systolic Pressure — the blood pressure in the ventricle at the very end of its contraction, when the aortic valve closes.

ESV

End-Systolic Volume — the amount of blood remaining in the ventricle after it has finished contracting. High ESV with low SV = poor ejection.

LVEF

Left Ventricular Ejection Fraction — the percentage of blood the LV pumps out with each beat. Measured by echocardiography. Normal >55%. Important caveat: a normal EF does NOT exclude haemodynamic uncoupling.

PV Loop

Pressure-Volume Loop — a graphical representation of the heart's pressure and volume throughout a complete cardiac cycle. The shape of this loop tells you everything about coupling.

SW

Stroke Work — the work done by the heart per beat, represented as the area within the PV loop. Maximised when Ea/Ees ≈ 1.

Echo & RV Parameters

TAPSE

Tricuspid Annular Plane Systolic Excursion — how much the tricuspid valve ring moves with each heartbeat. A simple echo measure of right ventricular (RV) function. Normal >17 mm.

PASP

Pulmonary Artery Systolic Pressure — the peak pressure in the pulmonary artery. Estimated from the tricuspid regurgitation (TR) jet on echocardiography. Normal <35 mmHg.

RV FAC

Right Ventricular Fractional Area Change — an echo measure of RV systolic function. Normal >35%.

IVS

Interventricular Septum — the wall separating the two ventricles. When the RV is overloaded, it bows into the LV, creating the characteristic "D-sign" on echo.

IVC

Inferior Vena Cava — the large vein returning blood from the lower body to the heart. A dilated, non-collapsing IVC (>21 mm) on echo suggests raised right atrial pressure and venous congestion.

TR jet

Tricuspid Regurgitation jet — backflow through the tricuspid valve. Its velocity allows calculation of pulmonary artery pressure using the Bernoulli equation.

Microcirculation & Organ Perfusion

CRT

Capillary Refill Time — time for skin colour to return after pressing. Normal <2s. Prolonged CRT (>3s) is a validated marker of microcirculatory failure and predicts organ dysfunction.

VExUS

Venous Excess Ultrasound Score — a point-of-care ultrasound grading system (0–3) assessing venous congestion using hepatic vein, portal vein, and intrarenal vein Doppler patterns.

DO₂

Oxygen Delivery — the total amount of oxygen delivered to tissues per minute. = CO × arterial oxygen content. The fundamental target in shock resuscitation.

VO₂

Oxygen Consumption — the amount of oxygen consumed by tissues per minute. In shock, VO₂ may become dependent on DO₂ — the so-called "supply dependency" phenomenon.

AKI

Acute Kidney Injury — sudden deterioration in kidney function. In shock, caused by both low flow (cardiogenic) and venous back-pressure (congestive). Rising creatinine and oliguria are key signs.

MODS

Multi-Organ Dysfunction Syndrome — failure of multiple organs simultaneously. The final common pathway of untreated haemodynamic uncoupling at all levels.

Treatments & Devices

MCS

Mechanical Circulatory Support — devices that assist or replace the heart's pumping function. Includes IABP, Impella, VA-ECMO.

IABP

Intra-Aortic Balloon Pump — a device in the aorta that inflates in diastole and deflates in systole to reduce LV afterload and improve coronary perfusion.

VA-ECMO

Veno-Arterial Extracorporeal Membrane Oxygenation — a heart-lung bypass circuit for severe cardiogenic shock or cardiac arrest. Bypasses both heart and lungs.

iNOS

Inducible Nitric Oxide Synthase — an enzyme activated by inflammation that produces large amounts of nitric oxide (NO), causing vasodilation and vasoplegia in septic shock.

CRRT

Continuous Renal Replacement Therapy — continuous haemofiltration used in ICU for AKI. Also used for fluid removal (decongestion) in heart failure with fluid overload.

GTN / SNP

Glyceryl Trinitrate / Sodium Nitroprusside — intravenous vasodilators used in cardiogenic shock with high SVR to reduce afterload and improve LV ejection.

The Big Picture

Think of the circulation as a series of interlocking gears — each must spin in harmony

🧒 Plain Language Summary

Your heart is a water pump in a theme park, pushing water through pipes of all sizes — from giant motorway pipes down to tiny garden hose trickles reaching every flower. "Coupling" means the pump and the pipes are working together perfectly. In shock, either the pump breaks (cardiogenic shock) or all the pipes go floppy (vasoplegic shock). The damage doesn't stop at the big pipes — it carries all the way down to the tiniest capillaries feeding your organs. And the drainage system (veins) matters too — if the drains back up, even a working pump can't push forward properly.

Normal Coupling

Ea/Ees ≈ 0.6–1.2 Heart & arteries matched for optimal stroke work & efficiency

The LV (left ventricle — main pumping chamber) and aorta behave like a matched engine-gearbox pair. Energy transfer is maximised; the heart doesn't waste effort. Think of driving in the right gear — smooth, efficient, economical.

Uncoupled (Shock)

Ea/Ees > 1.36 Energy transfer fails — organs starve despite apparent BP

Either the arteries become too stiff/floppy, or the heart weakens — the ratio tips. Like driving in the wrong gear: the engine revs but the wheels barely move. David from our case has this problem.

Level 1 — LV↔Aorta

Ees (heart muscle strength at end of contraction) vs Ea (arterial stiffness). The pressure–volume loop tells the story. Ea/Ees is the master ratio.

Level 2 — Macro↔Micro

Good blood pressure and cardiac output at the large-vessel level doesn't guarantee good perfusion at the tissue level. They can uncouple independently.

Level 3 — RV↔Pulmonary

The right heart faces the lung vasculature. TAPSE/PASP (tricuspid excursion / pulmonary artery systolic pressure ratio) is the bedside surrogate. Rises in pulmonary pressure = RV crisis.

Two Shock Phenotypes at a Glance

Parameter	Vasoplegic / Distributive (e.g. Sepsis)	Cardiogenic (e.g. David's case)
Core Problem	Pipes too floppy (↓ SVR, ↓ Ea)	Pump fails (↓ Ees)
CO/CI	↑ or normal (hyperdynamic initially)	↓↓ (low output)
SVR (vascular resistance)	↓↓	↑ (body tries to compensate)
PCWP (LV filling pressure)	Low or normal	↑↑ (LV can't empty properly)
Ea/Ees ratio	↑ (Ees falls with septic myocardial depression)	↑↑ (Ees very low, Ea rises)
Microcirculation	Heterogeneous shunting — some beds flooded, others ischaemic	Low flow, stasis throughout
Skin	Warm, flushed (early)	Cold, mottled, prolonged CRT
Lactate driver	O₂ extraction failure / microvascular shunting	Low oxygen delivery (↓ DO₂)

LV ↔ Systemic Arterial Coupling

The pressure–volume relationship: Ea (Arterial Elastance) vs Ees (End-Systolic Elastance)

🏥 David's Story — LV-Arterial Level

David's LV has been severely damaged by his heart attack. His Ees (the strength of his LV at the end of contraction) has crashed to 0.5 mmHg/mL — his heart muscle is barely squeezing. We've given noradrenaline to keep his MAP above 65, which has raised his SVR (tightened his blood vessels). This has increased his Ea (arterial elastance) to 2.2 mmHg/mL. His Ea/Ees ratio is now 2.2/0.5 = 4.4 — catastrophically uncoupled. The noradrenaline is making his ratio worse, not better. Every extra drop of vasopressor is squeezing more against an engine that can barely fire.

Ea — Arterial Elastance

(How hard it is for the heart to push blood into the aorta)

Ea = 0.9 × SBP / SV SBP = systolic blood pressure; SV = stroke volume

Ea is the spring tension in the pipes. High SVR (tight vessels), tachycardia (fast heart rate), and a stiff aorta all increase Ea. The LV must push harder to eject blood into a high-Ea system.

🧒 Analogy

Ea is how hard it is to blow up a balloon. A stiff balloon (high Ea) needs much more breath to inflate — your heart works harder for the same result.

Ees — End-Systolic Elastance

(How strong the heart muscle actually is, independent of loading conditions)

Ees = ESP / ESV ESP = end-systolic pressure; ESV = end-systolic volume

Ees represents true myocardial contractile strength. Crucially, it is load-independent — it reflects the heart's intrinsic power regardless of how full it is or how hard the vessels are resisting. Inotropes (drugs that strengthen contractions) raise it; ischaemia (lack of blood to heart muscle), sepsis toxins, and myocarditis drop it.

🧒 Analogy

Ees is how strong your hand squeeze is. A weak squeeze (low Ees) can't overcome even a modest balloon — the blood stays stuck inside.

✅ Normal Coupling (Ea/Ees ≈ 1)

Stroke work maximised · Ea/Ees ~1

❌ Cardiogenic (↓Ees, ↑Ea) — David

Tiny loop · ↓ stroke volume · Ea/Ees >>1

📐 Illustrated Diagram — LV–Arterial Coupling: The Gearbox Analogy

Figure 2 — LV–arterial coupling mechanics. Left: matched engine-gearbox (normal). Right: weak engine against stiff gearbox (cardiogenic shock). Vasopressors worsen the mismatch by raising Ea further against a failing Ees.

Shock Type	Ees (Heart strength)	Ea (Arterial stiffness)	Ea/Ees Ratio	What's Happening
Normal	Normal	Normal	~0.6–1.2 ✅	Matched gears — efficient energy transfer
Pure Vasoplegic	↓ (septic depression)	↓↓ (low SVR)	↑ (Ees falls more)	Floppy pipes; heart initially hyperdynamic but still uncoupled
Cardiogenic (David)	↓↓	↑ (reflex vasoconstriction)	↑↑ (often >3)	Weak pump against stiff pipes — worst combination
Mixed CS + Vasoplegia	↓↓	↓ or variable	↑↑ unpredictable	Most lethal phenotype — inflammatory uncoupling

Macro ↔ Microcirculation Coupling

Fixing the MAP and cardiac output doesn't always fix the organs

🏥 David's Story — Macro-Micro Level

We titrate noradrenaline to keep David's MAP at 70. His CO improves slightly with dobutamine (an inotrope) to 3.4 L/min. The numbers look a bit better. But his CRT remains 6 seconds. His lactate is still 5.5. His urine output hasn't improved. Why? Because his microcirculation — the tiny capillary beds feeding his kidneys, gut, and liver — is severely uncoupled from the macrocirculation we've just improved. His skin is mottled. The tissues are not receiving what the monitors are telling us they should be getting.

🧒 City Water Analogy

The macrocirculation is the big underground water mains — we can measure pressure at the stopcock. The microcirculation is the tiny pipes inside each house. You can have perfect pressure at the mains but the house pipes can be blocked, leaking, or bypassed — the taps still run dry. That's macro-micro uncoupling. We call this haemodynamic incoherence — the macro and micro are no longer telling the same story.

Macrocirculation — What We Measure

MAP (mean arterial pressure)
CO/CI (cardiac output / cardiac index)
SVR (systemic vascular resistance)
LVEF (left ventricular ejection fraction)
SV (stroke volume)
CVP (central venous pressure)
ScvO₂ (central venous oxygen saturation)

These are motorway-level measurements — vital, but they don't tell us what's happening in the last mile to the organs.

Microcirculation — What We Assess

CRT (capillary refill time) — >3s = abnormal, a validated resuscitation endpoint (ANDROMEDA-SHOCK trial)
Mottling score (0–5) — patchy skin discolouration from capillary stasis
Lactate and lactate clearance — reflects tissue oxygen debt
Skin temperature gradient (peripheral-to-core difference)
Sublingual SDF/IDF microscopy (research tool — directly visualises capillary flow)

These reflect tissue-level perfusion — the capillary beds feeding your hepatocytes, renal tubules, and enterocytes.

Why Does Micro-Uncoupling Happen? The Mechanisms

Endothelial glycocalyx shedding — the protective sugar-protein lining of capillaries is destroyed by inflammation, causing leak and plug formation
Microvascular thrombosis — platelet activation and fibrin deposition block individual capillaries
Vasoconstrictor/vasodilator imbalance — some capillary beds are clamped shut; others are flooded (shunting)
Interstitial oedema — fluid leaks out of capillaries, increasing the distance oxygen must diffuse to reach cells
RBC rheology changes — red blood cells become stiffer and can't deform to pass through tiny capillaries
Nitric oxide (NO) excess in vasoplegic shock — paradoxically creates heterogeneous perfusion; some beds flooded, others ischaemic

📐 Illustrated Diagram — Haemodynamic Incoherence: Macro OK, Micro Failing

Figure 3 — Haemodynamic incoherence. The large-vessel pressure (MAP 72, CO 5.2) appears acceptable, yet the microvascular beds are simultaneously flooded (shunting), blocked (thrombosis), and starved (ischaemia). CRT, mottling score, and lactate are the clinical windows into this hidden failure.

Treatment: Target the Micro, Not Just the Macro

Target CRT <3s

Use CRT as a resuscitation endpoint — proven non-inferior to lactate in ANDROMEDA-SHOCK

Avoid Excessive Vasopressors

High-dose noradrenaline constricts microvessels — MAP 65 is a ceiling, not a target to exceed

MCS in CS

Impella or VA-ECMO may improve macro but micro improvement is not guaranteed — monitor both

No Systemic NOS Inhibitors

Tilarginine trial failed — blocking systemic NO harms the local micro-NO balance organs need

Capillary ↔ Venous Coupling

The exit side — venous congestion poisons the capillary bed from downstream

🏥 David's Story — Venous Congestion Level

David's CVP is 17 mmHg — very high (normal <8 mmHg). His echo shows a plethoric (dilated, non-collapsing) inferior vena cava (IVC). VExUS score is grade 3. His kidneys, which depend on a low venous pressure to filter blood, are now being squeezed from both sides — low arterial inflow AND high venous back-pressure. This is congestive AKI. We've been pushing fluids to maintain his preload (the filling of the heart before it contracts), but we've created a venous dam that's now killing his kidneys. The answer isn't more fluid — it's gentle decongestion.

🧒 Rubbish Truck Analogy

Imagine the capillaries as a busy market where oxygen is delivered and CO₂ waste is collected. The delivery lorries (arteries) bring goods in; the rubbish trucks (veins) take waste away. If the rubbish trucks are stuck in traffic — venous congestion — the market floods with waste, back-pressure builds, and the delivery lorries can't get in either. The whole market shuts down. This is how high CVP kills kidneys even when blood pressure looks acceptable.

Why High CVP Is an Organ Killer

Renal venous hypertension → AKI even with normal MAP (renal perfusion pressure = MAP − CVP)
Hepatic sinusoidal congestion → hepatic transaminitis, synthetic dysfunction, coagulopathy
Gut mucosal oedema → ileus, impaired absorption, bacterial translocation, increased infection risk
Increased capillary hydrostatic pressure → accelerates glycocalyx leak and oedema
Cardiac tamponade physiology → very high CVP impairs RV filling

CVP >8 mmHg Independently predicts AKI in ICU patients — regardless of MAP

Assessing Venous Congestion: VExUS

Tool	Finding to Look For
IVC diameter	Plethoric >21mm, <50% collapse = high RAP (right atrial pressure)
Hepatic vein Doppler	Reversal of S wave = severe congestion
Portal vein Doppler	Pulsatile flow (normally continuous) = congestion
Intrarenal Doppler	Biphasic or monophasic pattern = severe renal venous HTN
VExUS Score 3	All abnormal = high risk of AKI; decongest urgently

📐 Illustrated Diagram — VExUS: Venous Congestion Grading with Ultrasound

Figure 5 — VExUS grading system. Grade 3 (all three venous Doppler patterns abnormal with dilated IVC) predicts development of AKI with high specificity. In cardiogenic shock, check VExUS before assuming oliguria is purely from low cardiac output.

Decongestion Strategies

Loop Diuretics

Even in cardiogenic shock — evidence supports gentle decongestion when CVP >8 to prevent congestive AKI

CRRT / Ultrafiltration

For AKI with fluid overload — CRRT removes fluid slowly without haemodynamic compromise

GTN / SNP

IV vasodilators reduce LVEDP and pulmonary venous congestion in CS with high SVR — only if MAP allows

Fluid Restriction

The de-resuscitation phase — stopping unnecessary fluids once initial resuscitation is complete

RV ↔ Pulmonary Arterial Coupling

The forgotten ventricle — until it fails catastrophically

🏥 David's Story — RV Level

48 hours in, David develops bilateral pulmonary infiltrates consistent with cardiogenic pulmonary oedema. We intubate and ventilate him with PEEP (positive end-expiratory pressure) of 10 cmH₂O. His echo now shows: TAPSE 11mm, PASP 52 mmHg. TAPSE/PASP = 11/52 = 0.21 mm/mmHg — severely uncoupled. The IVS (interventricular septum) has a D-sign (bowing into the left ventricle). The RV is dilating and failing. The LV is now being mechanically compressed. Both ventricles are failing simultaneously — the biventricular death spiral. His noradrenaline requirements double overnight.

🧒 The Weightlifter Analogy

The right ventricle (RV) is built like a gentle balloon squeezer — it only needs to pump softly because the lungs are normally soft, low-pressure pipes. But when the lungs stiffen (ARDS, pneumonia, high PEEP) or the LV fails and backs up pressure, the pulmonary artery (PA) pressure rises. Suddenly the gentle squeezer has to work like a weightlifter. Its thin walls can't cope. It balloons out. It pushes the dividing wall (IVS) into the LV. Now the LV is being squeezed from the inside. Both pumps fail together — and no amount of vasopressor will fix this without specifically treating the RV.

RV-PA Coupling Ratio — Key Numbers

RV Ees/Ea = SV / ESV Volume-based surrogate — echocardiographic

TAPSE / PASP Bedside surrogate for RV-PA coupling (mm/mmHg)

Note for LV vs RV: For the LV we use Ea/Ees (higher = worse). For the RV the convention is Ees/Ea (lower = worse). Both tell the same story — mismatch between pump strength and vascular load.

Echo Assessment of RV-PA Coupling

Parameter	Normal	Concerning	Critical
TAPSE	>17 mm	14–17 mm	<14 mm
PASP	<35 mmHg	35–50 mmHg	>50 mmHg
TAPSE/PASP	>0.55	0.31–0.55	<0.31
RV/LV ratio	<0.6	0.6–1.0	>1.0 (D-sign)
RV FAC	>35%	25–35%	<25%

📐 Illustrated Diagram — RV–PA Coupling & The Biventricular Death Spiral

Figure 4 — Left: normal A4C echo appearance with straight IVS and crescent-shaped RV. Right: RV failure — dilated RV compresses LV into a D-shape (D-sign), reducing LV filling and CO. This biventricular coupling failure requires specific RV-targeted therapy.

RV Rescue — What to Do

Pulmonary Vasodilators

Inhaled nitric oxide (iNO), inhaled prostacyclin, sildenafil — reduce PA Ea → restore coupling

RV Inotrope Strategy

Noradrenaline maintains RV coronary perfusion (RV perfused in systole AND diastole). Milrinone (inodilator) reduces PA pressure and improves RV contractility simultaneously

Lung-Protective Ventilation

↓ PEEP where tolerable, permissive hypercapnia if possible, prone positioning — all reduce pulmonary Ea

⚠️ Avoid Systemic Vasodilators

↓ SVR → ↓ RV coronary perfusion pressure → RV ischaemia → complete RV collapse. Fatal.

VA-ECMO / RVAD

Mechanical unloading of the RV in refractory failure. Biventricular failure may need BiVAD or total artificial heart support

Shock States Through a Coupling Lens

Two phenotypes, one framework — understanding the mechanisms changes the treatment

Cardiogenic Shock 💔

↓ Ees↑ EaEa/Ees ↑↑↑ CVPTAPSE/PASP ↓

The Story

The pump breaks (↓ Ees). The body panics and clamps the blood vessels (↑ SVR → ↑ Ea) to maintain MAP. But this makes the damaged pump work even harder against stiffened pipes — a vicious circle. CO falls, LV filling pressure (LVEDP) rises, the lungs flood, the RV faces increased back-pressure, venous congestion poisons the kidneys, microcirculation fails. Giving more noradrenaline raises Ea further — worsening the Ea/Ees ratio — while doing nothing for the Ees. This is David's story.

Treatment target: Reduce Ea (vasodilators if MAP allows, or offload with MCS) AND increase Ees (inotropes, mechanical unloading). Goal Ea/Ees → 1.0.

Vasoplegic / Distributive Shock 🌡️

↓ Ea (↓ SVR)↓ Ees (septic depression)Ea/Ees ↑↑ CO (initially)

The Story

The pipes go floppy (↓ Ea). The heart initially compensates — high CO, warm peripheries, bounding pulse. "Hyperdynamic." But septic toxins also depress the myocardium (↓ Ees), so both fall together. Large amounts of nitric oxide flood the microcirculation creating heterogeneous shunting — some capillary beds are hyperaemic, others ischaemic. Approximately 70% of septic shock patients have VA uncoupling despite a normal or even elevated LVEF — because LVEF is load-dependent and Ees is not.

Treatment target: Noradrenaline restores Ea (raises SVR). But titrate against Ea/Ees and CRT — not just MAP. Add vasopressin or angiotensin II to spare noradrenaline dose. Add inotropes if Ees is impaired (dobutamine or levosimendan in septic cardiomyopathy).

Mixed Shock — Cardiogenic + Vasoplegia: The ICU Monster

Occurs when cardiogenic shock evolves with systemic inflammation. Seen in: prolonged low-flow CS, post-cardiac arrest, post-cardiotomy, AMICS (acute myocardial infarction complicated by cardiogenic shock). Inflammation floods the system with iNOS-derived NO and cytokines (TNF-α, IL-6, IL-1β) that simultaneously depress Ees and dilate the vasculature.

Feature	Finding	Clinical Challenge
Ees	↓↓ (pump failing)	Needs inotrope or MCS
Ea	↓ (vasoplegia masking true afterload)	Vasopressors needed but worsen LV coupling
Microcirculation	Severely uncoupled at every level	Macro normalisation doesn't fix micro
RV	Often fails simultaneously	May need biventricular support
Mortality	50–70% in-hospital	Requires invasive haemodynamic monitoring + early MCS consideration

📋 Summary Notes

Everything you need to know — explained simply first, then with clinical depth. Read these after the module to consolidate. Test yourself: can you explain each point without looking?

📖 How to Use These Notes — The Feynman Way

Each summary card starts with a plain-language explanation — the kind you'd give to an intelligent non-doctor. If you can't say it simply, you haven't fully understood it yet. That's not a criticism — it's the entire point. Read the simple version. Close the page. Try to explain it out loud. Come back. Read the clinical detail. Repeat. This is how expertise is built — not by reading, but by retrieval practice and teaching.

1

LV–Arterial Coupling (Ventriculo-Arterial Coupling, VAC)

🧒 Say it simply

The heart and the aorta must be in the same gear. If the heart is weak but the arteries are stiff, it's like a small engine trying to drive uphill — it can't cope. We measure this mismatch with a ratio called Ea/Ees. When it's too high, the heart is failing to deliver, regardless of what the blood pressure shows.

Key equation: VAC = Ea / Ees — Normal 0.6–1.2. Uncoupled >1.36.

Ea (Arterial Elastance) = ~0.9 × systolic BP ÷ stroke volume. Rises with high SVR, tachycardia, aortic stiffness, vasopressors.

Ees (End-Systolic Elastance) = end-systolic pressure ÷ end-systolic volume. Load-independent marker of true myocardial contractility. Reduced by ischaemia, septic toxins, acidosis. Improved by inotropes and mechanical unloading.

VAC 0.6–1.2 = normal

VAC >1.36 = uncoupled

Ea = 0.9×SBP/SV

Optimal efficiency at Ea/Ees ≈ 1

Clinical pearl: A patient with cardiogenic shock on high noradrenaline with rising lactate despite adequate MAP → noradrenaline is increasing Ea against a low Ees → VAC worsening → less stroke volume → apparent haemodynamic stability masking deterioration. The answer is not more vasopressor — it's inotrope and/or mechanical unloading.

2

Macro–Microcirculation Coupling

🧒 Say it simply

Good blood pressure doesn't mean the organs are getting oxygen. The big pipes (macrocirculation) and the tiny pipes (microcirculation) can become disconnected. You can fix the MAP and the cardiac output and still have organs dying — because the last mile of circulation is blocked, leaking, or bypassed. CRT and mottling are your windows into this hidden world.

Haemodynamic incoherence = macrohaemodynamics normalised but microhaemodynamics persistently impaired. Proven to occur in up to 50% of ICU patients resuscitated to macro targets.

CRT >3 seconds = microcirculatory failure (ANDROMEDA-SHOCK: CRT-guided resuscitation non-inferior to lactate-guided)
Mottling score ≥3 = independent predictor of 14-day mortality
Sublingual microvascular density on SDF/IDF imaging: low proportion of perfused vessels (PPV <80%) = organ failure risk

CRT <2s = normal

CRT >3s = micro failure

Mottling ≥3 = high mortality

Clinical pearl: If MAP is 72, CO is 5, but CRT is 5 seconds and lactate is 4.8 — this is haemodynamic incoherence. Do not give more fluids or more vasopressors blindly. Assess VExUS, reassess vasoactive agents, consider microcirculation-targeted strategy. Check CVP — if >8, consider decongestion.

3

Capillary–Venous Coupling (Venous Congestion)

🧒 Say it simply

The organs need blood to flow through them — in from the arteries, out through the veins. If the venous drains are blocked (high CVP), blood backs up, the organs swell, and they stop working — even if the heart is pumping reasonably well. High CVP is not just a number — it's your kidneys drowning from the back.

Renal perfusion pressure = MAP − CVP. A patient with MAP 70 and CVP 18 has a renal perfusion pressure of only 52 mmHg — insufficient for normal glomerular filtration. Raising MAP further with vasopressors does not help if CVP remains high.

CVP >8 mmHg: independent predictor of AKI in ICU (Legrand et al., JAMA 2013)
VExUS grade 3 (abnormal hepatic, portal AND renal vein Doppler): strongly predicts poor renal outcomes within 72 hours
Management: diuretics, CRRT ultrafiltration, vasodilators (if MAP allows), fluid restriction

Renal Perf P = MAP − CVP

CVP target <8 mmHg

VExUS grade 3 = urgent decongest

Clinical pearl: In cardiogenic shock with AKI — before blaming low CO for the renal failure, check the CVP and VExUS score. If venous congestion is the dominant problem, gentle diuresis or ultrafiltration may recover the kidneys more effectively than increasing inotropes.

4

RV–Pulmonary Arterial (PA) Coupling

🧒 Say it simply

The right ventricle is a thin-walled, gentle pump — built for a low-pressure world. When the lungs get sick or the left heart fails, the pulmonary artery pressure rises, and suddenly the right ventricle has to do a job it was never designed for. It dilates, it squashes the left ventricle, and both pumps fail together. TAPSE/PASP below 0.31 is your early warning signal — catch it before the spiral begins.

TAPSE/PASP (Tricuspid Annular Plane Systolic Excursion ÷ Pulmonary Artery Systolic Pressure): a bedside echocardiographic surrogate for RV-PA coupling. Below 0.31 mm/mmHg = uncoupled.

RV coronary perfusion occurs in both systole and diastole (unlike the LV) — make it unique and vulnerable to hypotension
D-sign (paradoxical IVS movement) = RV dilation causing LV underfilling — biventricular failure
Causes of RV-PA uncoupling in ICU: LV failure (back-pressure), ARDS, high PEEP, PE, hypoxia, acidosis
Do NOT give systemic vasodilators in RV failure — will drop RV coronary perfusion pressure and cause RV infarction

TAPSE >17mm = normal

PASP <35mmHg = normal

TAPSE/PASP <0.31 = uncoupled

RV/LV >1.0 = D-sign

Clinical pearl: Any ICU patient deteriorating on vasopressors — do a bedside echo. TAPSE 12, PASP 52: TAPSE/PASP = 0.23. RV uncoupled. Treatment: iNO, noradrenaline for RV coronary perfusion, milrinone for pulmonary vasodilation, optimise ventilator (↓ PEEP, consider prone). Do NOT add a vasodilator. Consider RVAD if deteriorating.

★

The Integration — All Four Levels Together

🧒 The whole story simply

In severe shock, all four systems fail together. The heart can't pump (↓ Ees), the vessels are wrong (↑ or ↓ Ea), the tiny capillaries stop delivering oxygen to tissues, the veins back up and drown the organs, and the right heart gives out under the pressure from the flooded lungs. Treating only one layer — just giving vasopressors for blood pressure — is like fixing a leak in one pipe while the rest of the plumbing is collapsing. Precision haemodynamics means seeing and treating all four levels simultaneously.

The four-level checklist in every refractory shock patient:

🔴 LV-Arterial: Calculate Ea/Ees. If >1.36 — consider inotrope, reduce vasopressor, consider MCS
🔴 Macro-Micro: Check CRT, mottling score. If CRT >3s with normalised macro — haemodynamic incoherence; target micro
🔴 Capillary-Venous: Check CVP, VExUS. If congested — decongest; calculate renal perfusion pressure (MAP − CVP)
🔴 RV-PA: Echo TAPSE/PASP. If <0.31, D-sign — RV rescue protocol; NO systemic vasodilators

David's outcome: We recognised all four levels of uncoupling. We added dobutamine to raise Ees. We reduced noradrenaline carefully. We started iNO for the RV. We began furosemide infusion for decongestion (CVP fell to 10). We targeted CRT <3s. 72 hours later: lactate 1.8, CRT 2s, urine output 60 mL/hr, TAPSE/PASP 0.38. David survived to discharge.

🖼️ Echo Visual Reference

Illustrated schematic echo findings — what to look for at the bedside. Based on published echo criteria. Annotated for learning, not clinical decision-making.

📌 How to Use This Tab

These are schematic illustrations of echocardiographic findings — drawn to teach pattern recognition, not reproduced from clinical images. Each card shows the key echo view, what normal looks like, what the pathological finding looks like, and what measurement to take. For real echo images from peer-reviewed open-access papers, links are provided under each card.

Normal A4C View

BASELINE

What you're seeing: Four cardiac chambers. LV is the large left chamber. RV is the crescent-shaped right chamber. Straight IVS (interventricular septum) means balanced biventricular pressures.

RV/LV ratio<0.6 = normal RV size

IVS shapeStraight = normal biventricular pressures

LVEF (visual)Walls contract inward >50% — "good squeeze"

Reference: Lancellotti P et al. — EACVI recommendations for use of echocardiography in acute cardiovascular care. Open Access →

Cardiogenic Shock — Severe LV Dysfunction

CARDIOGENIC

What you're seeing: Massively dilated LV with barely moving walls. The LV occupies most of the frame. LVEF ~15–20%. Left atrium dilated from chronically raised filling pressure.

LVEF~15–20% — severely reduced. Eyeball: walls barely moving

LV cavityDilated — end-diastolic diameter >5.5 cm

LVOT VTILow (<15 cm) — calculate CO; SV = LVOT VTI × π(d/2)²

E/e' ratio>15 = elevated filling pressures (↑ LVEDP)

Reference: Bansal M et al. — Echocardiography in the Management of Cardiogenic Shock. Full Text →

RV Failure — D-Sign (Pressure Overload)

RV-PA COUPLING

What you're seeing: Parasternal short axis view. Normally the LV is a perfect circle. In RV pressure overload, the IVS bows leftward — flattening the LV into a D-shape. The RV is much larger than the LV (normal RV/LV <0.6).

D-signIVS flattening in systole = pressure overload; diastole = volume overload

RV/LV ratio>1.0 = severe RV dilation (in A4C view)

TAPSEM-mode at lateral tricuspid annulus. <14mm = severe RV dysfunction

PASP4 × (TR velocity)² + RAP (estimated from IVC). >50 = severe

Reference: Vieillard-Baron A et al. — Acute cor pulmonale in ARDS. DOI → · Tello K et al. — TAPSE/PASP clinical relevance. Free Full Text →

IVC Assessment — Preload & Congestion

CAPILLARY–VENOUS

What you're seeing: Subcostal long-axis view of the inferior vena cava entering the right atrium. IVC diameter and respiratory collapsibility estimate right atrial pressure and guide fluid management decisions.

IVC <21mm, >50% collapseRAP ~5 mmHg → may be fluid-responsive

IVC >21mm, <50% collapseRAP >10–15 mmHg → venous congestion, do not fluid-load

IVC plethoric in shockProceed to VExUS Doppler (hepatic, portal, renal veins)

Reference: Beaubien-Souligny W et al. — VExUS grading system. Open Access →

TAPSE — M-Mode Measurement

RV FUNCTION

What you're seeing: M-mode trace through the lateral tricuspid annulus in A4C view. The vertical amplitude of movement per beat = TAPSE. Normal >17mm. Combined with PASP from TR jet, gives TAPSE/PASP ratio.

TAPSE >17mmNormal RV longitudinal function

TAPSE 14–17mmMildly impaired — monitor

TAPSE <14mmSevere RV dysfunction — assess TAPSE/PASP, look for D-sign

TAPSE/PASP <0.31RV-PA uncoupled — escalate RV management

Reference: Tello K et al. — TAPSE/PASP as RV-PA coupling surrogate. Free Full Text →

Vasoplegic Shock — Hyperdynamic LV

VASOPLEGIC

What you're seeing: A hyperdynamic LV in vasoplegic/septic shock — small cavity, walls nearly touching in systole, EF >70%. The trap: this looks reassuring but ~70% of these patients have VA uncoupling because Ees is also depressed by septic toxins. LVEF is load-dependent and misleads here.

EF >70%Hyperdynamic — do NOT assume good coupling. Check Ea/Ees.

Cavity obliterationSevere hypovolaemia or very low afterload — not necessarily good contractility

Small LASuggests low filling pressures (vs cardiogenic shock where LA is dilated)

ManagementNoradrenaline restores Ea; add inotrope (dobutamine/levo) if Ees depressed

Reference: Guarracino F et al. — Ventriculo-arterial decoupling in human septic shock. Open Access →

🔗 Open-Access Echo Image Libraries

For real clinical echo images and video loops (not illustrated schematics), these open-access resources are recommended:

Echo in Shock — Critical Care

Vignon P et al. — Full text with echo images. PMC open access.

Free Full Text →

EACVI Echo in Acute Care

Lancellotti P et al. — Comprehensive acute echo guidelines with clinical images.

DOI →

Bedside Echo in CS (2025)

Jaramillo-Restrepo V et al. — CC BY 4.0 open access with real echo images.

Open Access →

PV Loop Physiology — PMC

Brener MI et al. — Case-based PV loop teaching with simulation figures.

Free Full Text →

References & Further Reading

Peer-reviewed evidence underpinning this module — open-access articles linked where available

LV–Arterial Coupling

1

Ventriculo-arterial coupling in cardiogenic shock (Guarracino et al., 2017)

Critical Care · Foundational paper establishing VAC as a key prognostic and therapeutic target in cardiogenic shock. Demonstrated that ~70% of septic shock patients have VA uncoupling despite preserved LVEF.

→ Open Access DOI

2

Sunagawa K et al. — Optimal arterial resistance for the maximal stroke work studied in isolated canine left ventricle (1985)

Circulation Research · The original landmark paper establishing the theoretical and experimental basis for Ea/Ees coupling and optimal stroke work at Ea/Ees ≈ 1.

→ DOI

3

Chen CH et al. — Estimation of central aortic pressure waveform by mathematical transformation of radial tonometry pressure (1997)

Circulation · Validated single-beat method for bedside Ees estimation — making coupling assessment practical in the ICU without invasive pressure-volume catheters.

→ DOI

Macrocirculation–Microcirculation Coupling

4

Hernandez G et al. — Effect of a Resuscitation Strategy Targeting Peripheral Perfusion Status vs Serum Lactate Levels on 28-Day Mortality Among Patients With Septic Shock (ANDROMEDA-SHOCK, 2019)

JAMA · Landmark RCT. CRT-guided resuscitation was non-inferior to lactate-guided in 28-day mortality, with signal toward less organ dysfunction. Validated CRT as a clinically actionable microcirculatory endpoint.

→ Open Access DOI

5

Ince C et al. — The endothelium in sepsis (2016)

Shock · Comprehensive review of glycocalyx pathology, microvascular thrombosis, and endothelial dysfunction as mechanisms of macro-micro uncoupling. Essential background reading.

→ DOI

6

Merdji H et al. — Microcirculatory dysfunction in cardiogenic shock (2022)

European Heart Journal: Acute Cardiovascular Care · Reviews how microvascular failure in CS is distinct from and additive to macrovascular failure. Critical reading for understanding why MCS alone doesn't always rescue organs.

→ DOI

Venous Congestion & VExUS

7

Legrand M et al. — Association between systemic hemodynamics and septic acute kidney injury in critically ill patients (2013)

JAMA · Demonstrated that elevated CVP >8 mmHg is independently associated with AKI in ICU patients, regardless of MAP and CO. Foundational evidence for decongestion as a therapeutic target.

→ DOI

8

Beaubien-Souligny W et al. — Quantifying systemic congestion with Point-Of-Care ultrasound: development of the Venous Excess Ultrasound Grading System (VExUS, 2020)

CHEST · Original derivation and validation study for the VExUS scoring system. Grade 3 VExUS predicts AKI development with high specificity. Essential for anyone using POCUS in the ICU.

→ Open Access DOI

RV–PA Coupling

9

Vieillard-Baron A et al. — Acute cor pulmonale in ARDS submitted to protective ventilation (2013)

Intensive Care Medicine · Demonstrated that acute cor pulmonale (RV failure from pulmonary hypertension) occurs in up to 25% of ARDS patients on protective ventilation. Defined echocardiographic criteria for RV failure monitoring.

→ DOI

10

Tello K et al. — Relevance of right ventricular–arterial coupling in clinical practice (2021)

European Respiratory Review · Comprehensive review of TAPSE/PASP as a clinical surrogate for RV-PA coupling. Validates the 0.31 mm/mmHg threshold and reviews its prognostic value across cardiac, pulmonary, and critical care populations.

→ Open Access DOI

11

Price LC et al. — Rescue therapy for severe right ventricular failure (2010)

Intensive Care Medicine · Describes pharmacological and mechanical rescue strategies for refractory RV failure. Covers vasopressor strategy for RV coronary perfusion and rationale for avoiding systemic vasodilators.

→ DOI

When Do We Use This Framework?

What "Coupling" Actually Means

Haemodynamic Parameters

Coupling-Specific Terms

Echo & RV Parameters

Microcirculation & Organ Perfusion

Treatments & Devices

Normal Coupling

Uncoupled (Shock)

Level 1 — LV↔Aorta

Level 2 — Macro↔Micro

Level 3 — RV↔Pulmonary

Two Shock Phenotypes at a Glance

Ea — Arterial Elastance

Ees — End-Systolic Elastance

Macrocirculation — What We Measure

Microcirculation — What We Assess

Why Does Micro-Uncoupling Happen? The Mechanisms

Treatment: Target the Micro, Not Just the Macro

Why High CVP Is an Organ Killer

Assessing Venous Congestion: VExUS

Decongestion Strategies

RV-PA Coupling Ratio — Key Numbers

Echo Assessment of RV-PA Coupling

RV Rescue — What to Do

Cardiogenic Shock 💔

Vasoplegic / Distributive Shock 🌡️

Mixed Shock — Cardiogenic + Vasoplegia: The ICU Monster

🔗 Open-Access Echo Image Libraries

LV–Arterial Coupling

Macrocirculation–Microcirculation Coupling

Venous Congestion & VExUS

RV–PA Coupling

Recommended Open-Access Reading