LVOT velocity–time integral
LVOT VTI · Pulsed Doppler, apical 5- or 3-chamber, 3–5 mm sample volume 5–10 mm proximal to the aortic valve
Normal range & thresholds
Normal 18–22 cm at a normal heart rate. Stroke volume = LVOT cross-sectional area × LVOT VTI = π(D/2)2 × VTI. Stroke volume index 35–55 mL/m²; SVI <35 mL/m² defines the low-flow state that underpins low-gradient aortic stenosis.[5,33]
In critical care, LVOT VTI <15 cm indicates a low cardiac output, and a rise of >10–12% with passive leg raising or a fluid challenge indicates fluid responsiveness.
In critical care, LVOT VTI <15 cm indicates a low cardiac output, and a rise of >10–12% with passive leg raising or a fluid challenge indicates fluid responsiveness.
Pathophysiology
The velocity–time integral is the distance a column of blood travels through the outflow tract during one systole — the ‘stroke distance’. Multiplied by the cross-sectional area of the tract, it is stroke volume; multiplied by heart rate, cardiac output. Because the LVOT diameter changes little within a given patient, serial LVOT VTI is a direct, geometry-free measure of that patient’s stroke volume, which is why it is the workhorse of haemodynamic echocardiography.[33]
Raised by
- Hyperdynamic states — sepsis, anaemia, thyrotoxicosis, pregnancy, arteriovenous fistula, fever
- Significant aortic regurgitation (the regurgitant volume is re-ejected: total stroke volume is high while forward output may be normal or low)
- Bradycardia with a long filling period
- Inotropes, exercise, catecholamine excess
- Athletic conditioning
Lowered by
- Reduced LV systolic function of any cause
- Hypovolaemia and over-diuresis
- Significant mitral regurgitation (a large part of the stroke volume goes backwards)
- Mitral stenosis and any cause of impaired LV filling — restrictive disease, tamponade, constriction
- Tachycardia and tachyarrhythmia (short filling time)
- Severe aortic stenosis with low flow; severe systemic hypertension (afterload mismatch)
- Beta-blockade and negative inotropes
Technique & pitfalls
- Sample volume 5–10 mm proximal to the aortic valve, in the same location where the LVOT diameter was measured — area and velocity must come from the same plane, or the continuity equation is wrong.[5]
- The spectral envelope should be narrow with a well-defined modal velocity and a crisp closing click; a ‘filled-in’ envelope means the sample volume is too close to the valve.
- Trace the modal velocity (the dark, dense edge), not the outer spectral broadening.
- Align within 20°; interrogate from multiple windows and take the highest reproducible value.
- Average 3 beats in sinus rhythm and 5–10 in atrial fibrillation, avoiding post-extrasystolic beats.
- Report the heart rate with the VTI — the same VTI at 50 and at 110 beats per minute means completely different cardiac outputs.
Pseudo-change & artefact
- Sample volume too close to the valve — flow acceleration falsely raises VTI and falsely raises the calculated valve area.
- Angle error — underestimates.
- Post-extrasystolic beats and irregular rhythm.
- Subaortic obstruction, a septal bulge or systolic anterior motion producing a dagger-shaped, late-peaking envelope that must not be traced as LVOT flow.
- Loading and heart rate on the day of the study — especially important when comparing serial studies in aortic stenosis.
- Tracing the spectral broadening rather than the modal velocity — consistently overestimates.
Treatment thresholds
- Stroke volume index <35 mL/m² defines low-flow aortic stenosis, both classical (LVEF <50%) and paradoxical (LVEF ≥50%). This single number changes the diagnostic pathway completely: it mandates low-dose dobutamine stress echocardiography or calcium scoring before an aortic valve area <1.0 cm² with a mean gradient <40 mmHg can be called severe.[5,6,7]
- Dimensionless velocity index = LVOT VTI / aortic VTI; <0.25 indicates severe aortic stenosis and is independent of LVOT diameter — the measurement to fall back on when the diameter is unreliable.[5]
- Prosthetic valve follow-up: a falling DVI and rising gradients identify prosthesis dysfunction or patient–prosthesis mismatch.[20,21]
- Cardiac output monitoring: in shock and in acute heart failure, serial LVOT VTI guides fluid, inotrope and vasopressor decisions; a rise >10–12% with passive leg raise predicts fluid responsiveness.
- Regurgitant and shunt quantification: LVOT stroke volume is the reference flow for volumetric mitral and aortic regurgitant volumes and for Qp:Qs.[4]
Next step
- Next: compute stroke volume index; if <35 mL/m² with a small aortic valve area and a low gradient, go to the low-flow pathway.
- Then: low-dose dobutamine stress echocardiography — flow reserve, projected valve area at normal flow, and contractile reserve.[5,28]
- Then: aortic valve calcium scoring by non-contrast CT — ≥3000 AU in men and ≥1600 AU in women make severe stenosis highly likely and settle most discordant cases.[5,6,80]
- Then: re-measure the LVOT diameter carefully, or use the dimensionless index, before accepting a discordant valve area.
Drugs
- Raised by: inotropes (dobutamine, milrinone, levosimendan), vasodilators in a failing ventricle, fluid resuscitation, and anything that corrects anaemia or thyrotoxicosis.
- Lowered by: beta-blockers, non-dihydropyridine calcium blockers, over-diuresis, general anaesthesia, and vasoconstrictors that raise afterload.
- GDMT raises resting stroke volume over months as the ventricle remodels.
- Note when interpreting a low-gradient aortic stenosis study whether the patient is on high-dose beta-blockade — it contributes to the low-flow state.
Reversibility
Immediately load- and rhythm-dependent, so the acute component is fully reversible — a fluid bolus or an inotrope changes it within minutes. The chronic component follows LV function: it rises in parallel with reverse remodelling on GDMT, after valve intervention (immediately, once the obstruction or regurgitation is relieved), and after successful rate or rhythm control. It falls permanently only when contractile tissue is lost.[33]