Mean outflow resistance as a marker of left ventricular afterload

The changes of mean outflow resistance (MOR) (defined as the ratio between mean aortic ejection pressure and mean ejection rate) and of total peripheral resistance (TPR) together with their relations to some circulatory parameters and to each other were studied under control (NaCl) and six different loading conditions on anesthetized closed chest dogs. Phenylephrine increased both MOR and TPR (ca. 160%, max), pacing increased MOR (p less than 0.01) but not TPR. Isoproterenol and dextran infusion decreased TPR (p less than 0.001) but not MOR. Calcium chloride decreased MOR (p less than 0.05) but increased TPR (NS). The relationship between MOR and TPR was linear in each case (r from 0.63 to 0.91). Both MOR and TPR showed either positive or negative linear correlations with volume and pressure parameters although MOR appeared to be more sensitive to the administered interventions. The present results suggest that, when referring to the pulsatile and discrete nature of the blood-flow-pressure output from the left ventricle, MOR seems to be a more useful measure than TPR in the assessment of the left ventricular afterload.