Stroke volume and cardiac output (LVOT VTI)

Clinical background

Doppler-based calculation of stroke volume and cardiac output through the left ventricular outflow tract (LVOT) is the standard non-invasive method for flow measurement at echocardiography. The method serves two principal decisions: assessing cardiac output during haemodynamic evaluation, and providing the stroke volume index on which the grading of low-flow aortic stenosis rests. Without a reliable stroke volume calculation, the continuity equation cannot be used for valve area, and low-flow phenotypes of aortic stenosis can neither be identified nor distinguished from moderate stenosis.

Calculating stroke volume and cardiac output

The method rests on the assumption that the LVOT is circular and that the flow profile is flat. The cross-sectional area is calculated from the LVOT diameter, and stroke volume is obtained by multiplying the area by the velocity–time integral (VTI) over one cardiac cycle:

$$A_{\text{LVOT}} = \pi \times \left(\frac{d_{\text{LVOT}}}{2}\right)^{2}$$

$$SV = A_{\text{LVOT}} \times \text{VTI}_{\text{LVOT}}$$

$$CO = SV \times HR$$

$$CI = \frac{CO}{BSA}$$

where $d_{\text{LVOT}}$ is the LVOT diameter in cm, $\text{VTI}_{\text{LVOT}}$ is the velocity–time integral in the LVOT in cm, $HR$ is the heart rate in beats/min and $BSA$ is the body surface area in m².

The LVOT diameter is measured in the parasternal long-axis view in mid-systole, 5 to 10 mm proximal to the aortic valve, and should represent the smallest diameter of the outflow tract [1]. The LVOT VTI is measured with pulsed-wave Doppler in the apical five-chamber view with the sample volume placed at the same level at which the diameter was measured. Three consecutive beats should be averaged, particularly in atrial fibrillation.

The method was standardised in its present form by the joint ASE and EACVI guidelines for echocardiographic examination, most recently revised in 2019 [1]. The guidelines define measurement technique, choice of view and requirements for image quality, but do not derive the measure from a specific cohort, since the method is a physical calculation and not a risk model.

Interpretation in practice

A stroke volume index below 35 mL/m² is the established threshold for low-flow aortic stenosis and determines whether a patient with a low gradient and preserved ejection fraction should be classified as having paradoxical low-flow stenosis or non-severe stenosis. In the grading of aortic stenosis, it is this value that determines whether the continuity-equation valve area should be interpreted as truly severe or as an artefact of low flow.

For haemodynamic monitoring alone, for example in the intensive care unit, the method is most valuable for trending over time rather than for absolute measurement. Since the LVOT diameter is anatomically fixed in adults, it does not need to be measured on every occasion, and changes in LVOT VTI then directly reflect changes in stroke volume.

Validation and performance

Agreement of the method with thermodilution has been studied in several settings. In a prospective study of 167 patients undergoing coronary artery bypass surgery, cardiac output was measured with the LVOT VTI by transoesophageal echocardiography and by thermodilution via a pulmonary artery catheter under stable haemodynamic conditions [2]. Median CO was 3.64 L/min (IQR 1.59) with the LVOT VTI and 3.90 L/min (IQR 1.6) with thermodilution, indicating a systematic underestimation of approximately 0.25 L/min with the Doppler method. The correlation between the two methods was, however, only moderate (r = 0.28), reflecting wide limits of agreement at the individual level.

A study of intensive care patients in whom the LVOT diameter could not be measured in approximately 27% of patients illustrates the practical limitation [3]. When body surface area was used as a surrogate for the LVOT diameter, the error was very large: the percentage error was 58% with a modified BSA method and 91% with untransformed BSA as the surrogate. Even a machine learning model based on patient characteristics gave a percentage error of 33% compared with expert measurement. The conclusion was that body surface area should not be used in place of a measured LVOT diameter.

The reproducibility of the VTI measurement has been examined in a study with 25 operators and 20 patients with aortic stenosis [4]. The coefficient of variation for the LVOT VTI with pulsed-wave Doppler was 18.0% (intra-operator 11.9%, inter-operator 12.9%), compared with 10.1% for peak velocity measured on the same trace. The main reason for the poorer reproducibility of the VTI was the steep slopes at the beginning and end of the Doppler envelope, which are difficult to trace consistently. This means that a change in LVOT VTI of less than approximately 28% in an individual patient cannot be distinguished from measurement error with 95% confidence.

Limitations

The LVOT diameter is squared and is therefore the dominant source of error. A measurement error of 2 mm at a diameter of 20 mm produces an error in cross-sectional area of approximately 20%. The measurement should be made in the parasternal long-axis view in mid-systole using an inner-edge-to-inner-edge technique at the base of the aortic valve cusps. Incorrect placement of the sample volume for the VTI measurement, too far proximal or distal to the level of the diameter, introduces further error.

The method presupposes a circular LVOT geometry. In patients with marked concentric hypertrophy or a sigmoid septum, the cross-section may be oval, which leads to systematic error in the area estimate. This is particularly relevant in aortic stenosis with marked ventricular hypertrophy.

Body surface area must not be used as a surrogate for the LVOT diameter [3]. Individual variation in LVOT diameter is too great for BSA-based estimates to be clinically acceptable.

The method does not apply with a mechanical aortic valve prosthesis, in severe aortic stenosis with calcification in the LVOT, or in hypertrophic cardiomyopathy with dynamic outflow obstruction, since the flow profile is then not flat and the Doppler traces become difficult to interpret. With an irregular rhythm, particularly atrial fibrillation, averaging over several beats is required, and with large beat-to-beat variation the calculation becomes unreliable.

The high variability of the VTI measurement means that the method is more useful at group level and for trending than for absolute determination in the individual patient [4]. Where clinical decisions rest on a single measurement — for example the grading of low-flow aortic stenosis — the measurement should therefore be repeated and averaged over several beats by an experienced operator.

References

1. Mitchell C et al. Guidelines for Performing a Comprehensive Transthoracic Echocardiographic Examination in Adults: Recommendations from the American Society of Echocardiography. J Am Soc Echocardiogr 2019. PMID: 30282592
2. Komanek T et al. Quantification of left ventricular ejection fraction and cardiac output using a novel semi-automated echocardiographic method: a prospective observational study in coronary artery bypass patients. BMC Anesthesiol 2023. PMID: 36855077
3. Aligholizadeh E et al. A novel method of calculating stroke volume using point-of-care echocardiography. Cardiovasc Ultrasound 2020. PMID: 32819371
4. Sacchi S et al. Doppler assessment of aortic stenosis: a 25-operator study demonstrating why reading the peak velocity is superior to velocity time integral. Eur Heart J Cardiovasc Imaging 2018. PMID: 29346531