Chapter 28 — Undifferentiated Shock

📚 Guideline basis

ESICM 2025 recommendations on haemodynamic monitoring in shock (echocardiography positioned as first-line haemodynamic assessment); Surviving Sepsis Campaign 2021; Vieillard-Baron et al. Intensive Care Med 2019 (PMID 30877351); Champion et al. Can J Cardiol 2020 on dynamic LVOT obstruction (PMID 33173359); Lichtenstein et al. Intensive Care Med 2022 on integrated lung–heart assessment (PMID 35187694).

The clinical problem

Shock is circulatory failure producing cellular oxygen utilisation failure. The four classical categories — hypovolaemic, cardiogenic, obstructive, distributive — are useful only insofar as they map to distinct therapies. In practice, shock in the ICU is usually mixed: the septic patient with pre-existing cardiomyopathy, the haemorrhaging patient with a right ventricle failing from massive transfusion, the post-arrest patient with both stunning and vasoplegia. The purpose of echocardiography is not to assign a label but to quantify which mechanism is dominant at this moment, and to re-quantify after each intervention.

Diagnostic Synthesis — the four-question protocol

Answer these four questions in order. The examination is complete when all four are answered.

Q1. Is there an obstructive cause requiring immediate intervention?

Note that RA collapse lasting more than one-third of the cardiac cycle is the more specific form of that sign, and that tamponade is a physiological diagnosis: a small effusion under high pressure (post-cardiotomy, loculated) can be tamponading with a benign-looking 2D image, and a large chronic effusion may not.

Q2. What is the flow state?

Measure LVOT VTI and heart rate. This single measurement partitions the differential more effectively than any qualitative assessment.

• VTI ≥ 18 cm with hypotension → high- or normal-flow shock: the deficit is arterial (distributive). Vasopressor, not inotrope, not fluid alone.
• VTI < 15 cm → low-flow shock: the deficit is cardiac output. Determine whether the cause is filling (preload), pump (contractility), or obstruction.

Q3. Which ventricle, and is it a pump or a filling problem?

The LV in isolated RV failure looks identical to the LV in hypovolaemia — small, underfilled, hyperdynamic. The RV and the IVC distinguish them, and the therapeutic paths are opposite. Always assess the right heart before giving fluid to a small, hyperdynamic LV.

Q4. Is fluid indicated, permitted, and tolerated?

Apply Chapter 12. Passive leg raise with LVOT VTI is the default test. Assess tolerance simultaneously: lung B-lines, E/e′, RV size, hepatic and portal venous Doppler.

Ventriculo-arterial coupling in shock

The elastance framework (Chapter 10) sharpens the vasoplegia-versus-pump distinction:

Ea = (0.9 × SBP) / SV

• Low Ea (< ~1.5 mmHg/mL) with hypotension → arterial failure dominates. Vasopressor.
• High Ea with low Ees (Ea/Ees > 1.5–2) → decoupling. Inotrope, afterload reduction, or mechanical support.
• Ea/Ees normal with low SV and small chambers → preload deficit.

The characteristic sequence in septic shock: a hyperdynamic-appearing ventricle with high EF and very low Ea; noradrenaline restores Ea; the previously masked reduction in Ees now manifests as a falling EF and stroke volume. This is unmasking, not vasopressor toxicity, and the correct response is to add inotropic support if perfusion remains inadequate.

Dynamic LVOT obstruction

An under-diagnosed and iatrogenically driven cause of refractory shock. It requires three conditions that critical illness supplies simultaneously: hypovolaemia (small LV cavity), hypercontractility (endogenous or exogenous catecholamines), and a susceptible geometry (basal septal hypertrophy, sigmoid septum, small LV, post-mitral-repair, Takotsubo, or post-TAVI).

Findings: systolic anterior motion of the anterior mitral leaflet; a late-peaking, concave-upward "dagger" CW envelope in the LVOT (distinguishing it from the earlier-peaking, symmetrical envelope of fixed aortic stenosis); a posteriorly directed eccentric mitral regurgitant jet (SAM pulls the leaflet away from coaptation); and a gradient that increases with inotropes and decreases with volume.

The clinical signature is a patient who becomes more hypotensive as the dobutamine or adrenaline dose is increased. Management inverts standard shock therapy: stop inotropes, volume load, use a pure vasoconstrictor (phenylephrine or noradrenaline for its alpha effect), consider beta-blockade, and slow the heart rate to lengthen diastolic filling.

Integrated lung–heart assessment

Cardiac findings become far more specific when combined with lung ultrasound:

An A-line profile with a collapsing IVC and a small hyperdynamic ventricle is the strongest available bedside indication for fluid; the same cardiac picture with diffuse B-lines is a contraindication.

Therapeutic Logic

Re-scan after every major intervention. The value of echocardiography in shock lies in the paired measurement, not the initial snapshot.

🛑 Critical pitfall: Giving fluid to a small, hyperdynamic left ventricle without looking at the right ventricle. In RV failure this worsens septal shift and lowers cardiac output.

🛑 Critical pitfall: Escalating inotropes in a hypotensive patient with a hyperdynamic ventricle. If a dynamic LVOT gradient is present, every dose increment worsens the obstruction.

🛑 Critical pitfall: Concluding "hyperdynamic — this is sepsis" and stopping. Hyperdynamic function is also the signature of hypovolaemia, dynamic obstruction, and isolated RV failure, and all four require different treatment.

• 💡 Clinical pearl: LVOT VTI plus RV:LV ratio plus IVC, in that order, resolves the majority of undifferentiated shock in under three minutes.
• 💡 Clinical pearl: If the patient deteriorates as the inotrope is increased, put the CW cursor through the LVOT before doing anything else.
• 💡 Clinical pearl: Repeat the study. A single scan describes a moment in a process that changes hourly; serial scanning is what changes outcomes in practice.

References

1. Vieillard-Baron A, Millington SJ, Sanfilippo F, et al. A decade of progress in critical care echocardiography. Intensive Care Med 2019;45:770–88. PMID 30877351.
2. Muller et al. Fluid responsiveness assessment. J Am Soc Echocardiogr 2020. PMID 32147001.
3. Champion S, et al. Dynamic left ventricular outflow tract obstruction in the critically ill. Can J Cardiol 2020. PMID 33173359.
4. Lichtenstein D, et al. Lung and cardiac ultrasound integration. Intensive Care Med 2022. PMID 35187694.
5. Evans L, Rhodes A, Alhazzani W, et al. Surviving Sepsis Campaign 2021. Crit Care Med 2021;49:e1063–143.