Influence of critical closing pressure on systemic vascular resistance and total arterial compliance: A clinical invasive study

Systemic vascular resistance (SVR) and total arterial compliance (TAC) modulate systemic arterial load, and their product is the time constant (Tau) of the Windkessel. Previous studies have assumed that aortic pressure decays towards a pressure asymptote (P∞) close to 0mmHg, as right atrial pressure is considered the outflow pressure. Using these assumptions, aortic Tau values of ∼1.5seconds have been documented. However, a zero P∞ may not be physiological because of the high critical closing pressure previously documented in vivo. To calculate precisely the Tau and P∞ of the Windkessel, and to determine the implications for the indices of systemic arterial load. Aortic pressure decay was analysed using high-fidelity recordings in 16 subjects. Tau was calculated assuming P∞=0mmHg, and by two methods that make no assumptions regarding P∞ (the derivative and best-fit methods). Assuming P∞=0mmHg, we documented a Tau value of 1372±308ms, with only 29% of Windkessel function manifested by end-diastole. In contrast, Tau values of 306±109 and 353±106ms were found from the derivative and best-fit methods, with P∞ values of 75±12 and 71±12mmHg, and with ∼80% completion of Windkessel function. The “effective” resistance and compliance were ∼70% and ∼40% less than SVR and TAC (area method), respectively. We did not challenge the Windkessel model, but rather the estimation technique of model variables (Tau, SVR, TAC) that assumes P∞=0. The study favoured a shorter Tau of the Windkessel and a higher P∞ compared with previous studies. This calls for a reappraisal of the quantification of systemic arterial load. La résistance vasculaire systémique (RVS) et la compliance artérielle totale (CAT) modulent la charge artérielle. Leur produit est la constante de temps (Tau) de la décroissance de la pression aortique diastolique selon le modèle du Windkessel. Les travaux antérieurs font l’hypothèse d’une asymptote de pression en débit nul (P∞) égale à 0mmHg, aboutissant à des valeurs de Tau d’environ 1,5 secondes. Une valeur nulle de P∞ n’est cependant pas physiologique. Calculer précisément Tau et P∞ à l’aide de nouvelles méthodes numériques. La pression aortique a été acquise par sondes haute-fidélité chez 16 patients. Tau a été d’abord calculée par la méthode semi-logarithmique en faisant l’hypothèse que P∞=0mmHg, puis par deux méthodes ne faisant pas d’hypothèse sur P∞ : la méthode des dérivées et la méthode du meilleur ajustement. En faisant l'hypotthèse P∞=0mmHg, la valeu