A reduced capacity of the diaphragm to produce inspiratory pressure (diaphragm dysfunction) is frequent on ICU admission. It is associated with sepsis and disease severity, suggesting that it may represent another form of organ failure. It is associated with a poor prognosis. Clinical trial registered with www.clinicaltrials.gov (NCT 00786526).

On the basis of the windkessel model, the stroke volume-to-aortic pulse pressure ratio (SV/PP) has been proposed as an estimate of total arterial compliance, but recent studies have questioned this approximation. Aortic pressure was obtained at rest in 31 adults undergoing cardiac catheterization (47 ± 14 yr): controls ( n = 7), patients with dilated cardiomyopathy ( n = 10), and patients with other cardiac diseases ( n = 14). We calculated PP, mean aortic pressure (MAoP), heart period ( T), SV (thermodilution cardiac output/heart rate), total peripheral resistance ( R), total arterial compliance estimated by area method ( C area), and the time constant of aortic pressure decay in diastole ( RC area). In the overall population ( n = 31), there was no significant difference between SV/PP and C area. SV/PP was linearly related to C area (SV/PP = 0.99 C area + 0.05; r = 0.98; P < 0.001); the slope and intercept did not differ from unity and zero, respectively. Similar results were obtained in the three subgroups. These results implied that PP/MAoP and T/ RC areawere proportionally related ( T/ RC area= 1.18PP/MAoP − 0.07; r = 0.96; P < 0.001). We conclude that for humans at rest 1) SV/PP gave a reliable estimate of C area, and 2) T normalized by the time constant of aortic pressure decay in diastole was proportionally related to PP/MAoP. This last relationship could be considered an aspect of the coupling between the left ventricle and its load.