Denis Chemla1, Edmund M T Lau2, Philippe Hervé3, Sandrine Millasseau4, Mabrouk Brahimi5, Kaixian Zhu5, Caroline Sattler6, Gilles Garcia6, Pierre Attal7, Alain Nitenberg5. 1. Service de physiologie, hôpital de Bicêtre, hôpitaux universitaires Paris-Sud, 94275 Le Kremlin-Bicêtre, France; Université Paris-Sud, 94275 Le Kremlin-Bicêtre, France; Inserm UMR_S999, LabEx Lermit, centre chirurgical Marie-Lannelongue, 92350 Le Plessis Robinson, France. Electronic address: denis.chemla@aphp.fr. 2. Sydney medical school, university of Sydney, Camperdown, Australia. 3. Departement de chirurgie thoracique, vasculaire et de transplantation pulmonaire, hôpital Marie-Lannelongue, 92350 Le Plessis Robinson, France. 4. Pulse wave consulting, 95320 Saint-Leu-la-Forêt, France. 5. Service de physiologie, hôpital de Bicêtre, hôpitaux universitaires Paris-Sud, 94275 Le Kremlin-Bicêtre, France; Université Paris-Sud, 94275 Le Kremlin-Bicêtre, France. 6. Service de physiologie, hôpital de Bicêtre, hôpitaux universitaires Paris-Sud, 94275 Le Kremlin-Bicêtre, France; Université Paris-Sud, 94275 Le Kremlin-Bicêtre, France; Inserm UMR_S999, LabEx Lermit, centre chirurgical Marie-Lannelongue, 92350 Le Plessis Robinson, France. 7. Department of otolaryngology, head and neck surgery, Shaare-Zedek medical centre, Hebrew university medical school, Jerusalem, Israel.
Abstract
BACKGROUND: 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. AIMS: To calculate precisely the Tau and P∞ of the Windkessel, and to determine the implications for the indices of systemic arterial load. METHODS: 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). RESULTS: 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. CONCLUSION: 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. Crown
BACKGROUND: 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. AIMS: To calculate precisely the Tau and P∞ of the Windkessel, and to determine the implications for the indices of systemic arterial load. METHODS: 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). RESULTS: 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. CONCLUSION: 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. Crown
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