Chapter 11 — Stroke Volume, Cardiac Output and Flow Quantification

📚 Guideline basis

ASE 2019 comprehensive TTE examination; ASE/EACVI 2017 valve stenosis and 2017 native regurgitation documents (continuity equation, volumetric methods); ESICM 2025 haemodynamic monitoring recommendations, which position echocardiography as a first-line haemodynamic assessment tool in shock.

Mechanisms

All echocardiographic flow quantification rests on one relationship: the volume passing a cross-section per beat is the product of that cross-sectional area and the distance the blood column travels (the velocity–time integral).

SV = CSA × VTI

At the left ventricular outflow tract:

CSA_LVOT = π (D/2)² = 0.785 D²

SV = 0.785 D² × VTI_LVOT

CO = (SV × HR) / 1000      CI = CO / BSA      SVI = SV / BSA

Units: D in cm, VTI in cm, SV in mL, CO in L/min, BSA in m².

Body surface area (Du Bois):

BSA = 0.007184 × height(cm)^0.725 × weight(kg)^0.425

Systemic vascular resistance, when a central venous pressure is available:

SVR = ((MAP - CVP) × 80) / CO     (dyn·s·cm⁻⁵)

Normal values

Parameter	Normal range
LVOT diameter (adult)	1.8–2.4 cm
LVOT VTI	18–22 cm (low output < 18 cm; < 15 cm is markedly low)
Stroke volume	60–100 mL
Stroke volume index	35–65 mL/m²; low-flow state defined as < 35 mL/m²
Cardiac output	4–8 L/min
Cardiac index	2.5–4.0 L/min/m²
SVR	800–1200 dyn·s·cm⁻⁵

LVOT VTI is the single most useful number in critical care echocardiography. It requires no diameter measurement, is reproducible within an operator, responds within seconds to interventions, and its change tracks the change in stroke volume exactly — because CSA is constant over the timescale of an ICU intervention.

Acquisition and the two dominant error sources

LVOT diameter

Zoomed parasternal long axis, mid-systole, harmonics off.
Inner edge to inner edge, 0.5–1.0 cm proximal to the aortic annulus, parallel to the annular plane.
Perpendicular measurement in the axial dimension of the image.

Because D is squared, a 10% diameter error produces a 21% stroke volume error. This dwarfs every other error in the calculation.

LVOT VTI

Apical 5-chamber or apical 3-chamber, PW sample volume 3–5 mm.
Placed at the same anatomical level at which the diameter was measured — this matching is the second dominant error source. Sampling at the annulus while measuring diameter 1 cm below produces a systematic mismatch of up to 20%.
Sweep speed 100 mm/s. Trace the modal velocity (the dense inner envelope), not the outer spectral spray.
A crisp aortic valve closure click and a narrow, well-defined envelope indicate good alignment. A broad, "filled-in" envelope indicates the sample volume is too close to the valve or off-axis.
Average ≥3 beats in sinus rhythm, ≥5 and ideally 10 beats in atrial fibrillation.

Serial measurement discipline

For serial monitoring, freeze and store the LVOT diameter once and re-use that value for every subsequent study in that admission. Re-measuring the diameter each time introduces a variance (±10–20% in SV) that is larger than most clinically meaningful changes. Track VTI alone.

Alternative and cross-check sites

Site	CSA source	Use case	Caveat
LVOT	π(D/2)², PLAX	Default	Elliptical LVOT causes 10–20% underestimate vs 3D/CT
RVOT	π(D/2)², PSAX at aortic valve level	LVOT unobtainable; shunt calculation (Qp)	Harder to measure; more angle error
Mitral annulus	π(D/2)² or annular area	Volumetric regurgitant volume calculation	Invalid with significant MR or MS
3D LVOT planimetry	Direct 3D area	Most accurate when available	Requires 3D probe and adequate images

Shunt quantification

Qp/Qs = (CSA_RVOT × VTI_RVOT) / (CSA_LVOT × VTI_LVOT)

Qp/Qs > 1.5 indicates a haemodynamically significant left-to-right shunt; < 1.0 indicates net right-to-left flow.

Regurgitant volume by the volumetric method

RegVol = SV(through regurgitant valve) - SV(through competent valve)

RF (%) = (RegVol / SV through regurgitant valve) × 100

Requires that exactly one valve is regurgitant; fails when two are.

Validity against thermodilution

Doppler cardiac output correlates well with thermodilution in stable patients, with limits of agreement of roughly ±1–1.5 L/min in most comparative studies — clinically acceptable for trend monitoring, not interchangeable for absolute values. The comparison degrades in: significant tricuspid regurgitation (which biases thermodilution), atrial fibrillation, and low-output states.

Practical position: treat Doppler CO as a trend instrument with excellent within-patient precision and modest between-patient accuracy. That is precisely the property required for guiding an intervention.

ICU-Specific Limitations

Confounder	Effect on SV/CO	Response
Positive-pressure ventilation	Cyclical variation in VTI by design (this is the signal in Chapter 12)	Average across the respiratory cycle for absolute CO; measure the variation deliberately for responsiveness
Atrial fibrillation	Beat-to-beat SV variation of 20–50%	Average ≥5–10 beats
Dynamic LVOT obstruction	Dagger-shaped, late-peaking envelope; SV overestimated if the obstructed envelope is traced	Recognise the waveform shape; move the sample volume proximally; treat the obstruction
Aortic stenosis	LVOT velocity remains valid; do not trace the transvalvular jet	Confirm the envelope is LVOT, not AV
Significant aortic regurgitation	LVOT SV includes the regurgitant volume; it is total not forward SV	Use the mitral or RVOT site for forward flow
VA-ECMO	Native LVOT VTI reflects only native ejection, not total systemic flow	Total flow = circuit flow + native SV × HR (Chapter 35)
IABP	1:1 augmentation produces alternating beat morphology	Measure across a consistent set of beats and note the ratio
Tachycardia > 130/min	Shortened ejection, reduced VTI, CO may still be normal or high	Always report HR alongside VTI

🛑 Critical pitfall: Reporting a low VTI as "low cardiac output" without the heart rate. VTI 14 cm at 140/min gives a cardiac output near 6 L/min; VTI 14 cm at 60/min gives 2.6 L/min. VTI is stroke distance, not flow.

🛑 Critical pitfall: Tracing the aortic stenosis jet when intending to measure LVOT VTI. The envelope is denser, later-peaking, and much higher; the resulting "cardiac output" is grossly inflated. The LVOT envelope in AS should peak in early systole and be substantially lower in velocity.

🛑 Critical pitfall: Re-measuring LVOT diameter on the follow-up study. The apparent change in cardiac output will be an artefact of the new diameter.

💡 Clinical pearl: Learn to eyeball the LVOT VTI envelope. Roughly, VTI ≥ 18 cm with a normal heart rate is an adequate flow state; ≤ 15 cm is a low-flow state that demands an explanation.
💡 Clinical pearl: A stroke volume index below 35 mL/m² is the accepted definition of a low-flow state and is the trigger for the low-flow, low-gradient aortic stenosis pathway (Chapter 18).
💡 Clinical pearl: In a patient on VA-ECMO with a closed aortic valve, LVOT VTI approaches zero while systemic flow is normal. Never report "cardiac arrest physiology" from an LVOT trace in a supported circulation.

References

Mitchell C, Rahko PS, Blauwet LA, et al. Guidelines for performing a comprehensive TTE examination in adults. J Am Soc Echocardiogr 2019;32:1–64.
Baumgartner H, Hung J, Bermejo J, et al. Recommendations on the echocardiographic assessment of aortic valve stenosis. J Am Soc Echocardiogr 2017;30:372–92.
Zoghbi WA, Adams D, Bonow RO, et al. Recommendations for noninvasive evaluation of native valvular regurgitation. J Am Soc Echocardiogr 2017;30:303–71.
Muller et al. Fluid responsiveness and stroke volume assessment. J Am Soc Echocardiogr 2020. PMID 32147001.