Systemic Vascular Resistance By Echo Doppler

Abstract & Commentary

Synopsis: Doppler echocardiography can estimate SVR, and is particularly accurate at identifying a low SVR.

Source: Abbas AE, et al. J Am Soc Echocardiogr. 2004; 17:834-838.

Systemic vascular resistance (SVR) is a useful measure for managing critically ill patients, but often invasive procedures to measure this parameter are not done. Thus, Abbas and colleagues suggested that Doppler echocardiography may be able to estimate SVR by measuring left ventricular (LV) pressure and flow. Since peak mitral valve regurgitation velocity (MRV) is directly proportional to LV pressure, and LV outflow tract time velocity integral (TVI) is directly proportional to flow, Abbas et al hypothesized that MRV/TVI would correlate with SVR. They screened 41 patients with pulmonary artery catheters, and 8 were excluded either because they had no detectable mitral regurgitation (MR) or they had other significant valvular disease. In the 33 remaining patients, invasive and non-invasive measures were made within 45 minutes of each other. MRV/TVI correlated well with SVR (r = 0.84). Receiver operating characteristics showed that a MRV/TVI > 0.27 had a 70% sensitivity and a 77% specificity for SVR > 14 Woods Units (normal 10-14 WU). MRV/TVI < 0.2 had a 92% sensitivity and a 88% specificity for SVR < 10 Woods Units. Abbas et al concluded that Doppler echocardiography can estimate SVR, and is particularly accurate at identifying a low SVR.

Comment by Michael H. Crawford, MD

The choice of appropriate therapy—volume, cardiac inotropes, vasoactive agents—in critically ill patients, often hinges on an accurate hemodynamic assessment, but few intensive care unit (ICU) patients today have right heart catheterization performed for fear of complications and data showing little mortality gain. Thus, echocardiography has become the most frequently used technique to resolve these issues. Echo Doppler techniques can estimate LV size and function, right atrial pressure, pulmonary artery pressure, and LV filling pressure.

This new technique adds SVR to the list. The estimation of SVR was most accurate for low SVRs.

This is extremely helpful because a low SVR suggests distributive shock, eg, septic shock. A high SVR is less useful since it can be caused by hypovolemia, which is easily corrected, or cardiac failure, which is not. If the SVR is not low, then other clinical or echo parameters need to be used to determine the mechanism of shock.

There are limitations to this technique. In the ICU setting, 7% (3/41) had no measurable MR. Patients with other conditions that would affect the measurement variables, resulted in 5 exclusions (12%). Thus, in almost 20% of patients, the technique could not be used. Also, right atrial and left atrial pressures occasionally are markedly different; this technique assumes that they are nearly equal. Since low atrial pressures are more likely to be similar than high ones, perhaps this is why their method worked best in low SVR patients. Although this technique will not be useful in everyone, it is a simple, easy addition to current echo Doppler parameters, which should increase our certainty about ICU patients’ hemodynamics, and improve their care.

Dr. Crawford, Professor of Medicine, Associate Chief of Cardiology for Clinical Programs University of California San Francisco, is Editor of Clinical Cardiology Alert.