Noninvasive assessment of cardiac output from arterial pressure profiles during exercise

The stroke volume of the left ventricle (SV) was assessed in nine young men (mean age 22.2, ranging from 20 to 25 years) during cycle ergometer upright exercise at exercise intensities from 60 to 150 W (about 20% to 80% of individual maximal aerobic power). The SV was calculated from noninvasive tracings of the arterial blood pressure, determined from photoplethysmograph records and compared to the SV determined simultaneously by pulsed Doppler echocardiography (PDE). Given the relationshipSV =As·Z−1 in whichAS is the area underneath the systolic pressure profile (in millimetres of mercury and second), andZ (in millimetres of mercury and second per millilitre) is the apparent hydraulic impedance of the circulatory system, a prerequisite for the assessment of SV from the photoplethysmograph tracings is a knowledge of Z. The experimental value of Z (hereafter defined Z*) was calculated by dividing AS (from the finger photoplethysmograph) by SV as obtained by PDE. When the whole group of subjects was considered, Z* was not greatly affected by the exercise intensity: it amounted to 0.089 (SD 0.028;n = 36). The Z was also estimated independently of any parameter other than heart rate (HR), mean (MAP) and pulse (PP) arterial blood pressure obtained from the photoplethysmograph. A computerized statistical method allowed us to interpolate the experimental values ofZ*, HR, PP and MAP by the equationZm = a·(b + c·HR + d·PP + e·MAP)−1, thus obtaining the coefficients a to e. The mean percentage error betweenZm (calculated from the coefficients obtained andZm was 21.8 (SD 14.3)%. However, it was observed that, in a given subject,Z* was significantly affected by the exercise intensity. Therefore, to improve the estimate ofZ a second algorithm was developed to update the experimental value ofZ determined initially at rest (Zin). This updated value (Zcor) ofZ was calculated asZcor =Zin· [(f/(i + g·(HR/HRin) + h·(PP/PPin) + 1· (MAP/MAPin], whereHRin,PPin,MAPin,HR,PP,MAP are the above parameters at rest and during exercise, respectively. Also in this case, the coefficients f to 1 were determined by a computerized statistical method usingZ* as the experimental reference. The values ofZcor so obtained allowed us to calculate SV from arterial pulse contour analysis asSVF =AS·Zcor/−1. The mean percentage error between theSVF obtained and the values simultaneously determined by PDE, was 10.0 (SD 8.7)%. It is concluded that the SV of the left ventricle, and hence cardiac output, can be determined during exercise from photoplethysmograph tracings with reasonable accuracy, provided that an initial estimate of SV at rest is made by means an independent high quality reference method.