Rationale for using the velocity-time integral and the minute distance for assessing the stroke volume and cardiac...

Stroke volume (SV) and cardiac output (CO) are basic hemodynamic parameters which aid in targeting organ perfusion and oxygen delivery in critically ill patients with hemodynamic instability. While there are several methods for obtaining this data, the use of transthoracic echocardiography (TTE) is gaining acceptance among intensivists and emergency physicians. With TTE, there are several points that practitioners should consider to make estimations of the SV/CO as simplest as possible and avoid confounders.

With TTE, the SV is usually obtained as the product of the left ventricular outflow tract (LVOT) cross-sectional area (CSA) by the LVOT velocity-time integral (LVOT VTI); the CO results as the product of the SV and the heart rate (HR). However, there are important drawbacks, especially when obtaining the LVOT CSA and thus the impaction in the calculated SV and CO. Given that the LVOT CSA is constant, any change in the SV and CO is highly dependent on variations in the LVOT VTI; the HR contributes to CO as well. Therefore, the LVOT VTI aids in monitoring the SV without the need to calculate the LVOT CSA; the minute distance (i.e., SV × HR) aids in monitoring the CO. This approach is useful for ongoing assessment of the CO status and the patient's response to interventions, such as fluid challenges or inotropic stimulation. When the LVOT VTI is not accurate or cannot be obtained, the mitral valve or right ventricular outflow tract VTI can also be used in the same fashion as LVOT VTI. Besides its pivotal role in hemodynamic monitoring, the LVOT VTI has been shown to predict outcomes in selected populations, such as in patients with acute decompensated HF and pulmonary embolism, where a low LVOT VTI is associated with a worse prognosis.

The VTI and minute distance are simple, feasible and reproducible measurements to serially track the SV and CO and thus their high value in the hemodynamic monitoring of critically ill patients in point-of-care settings. In addition, the LVOT VTI is able to predict outcomes in selected populations.