Measurements of mean systemic filling pressure in normal subjects during general anesthesia

Mean systemic filling pressure (Psf) is identified as the equilibrium pressure in the systemic circulation when the blood flow is zero. Psf might be an index of the effective filling of the circulatory system [1,2]. Aim of this study was to test a simple method to measure Psf in a population of anesthetized patients under conditions of intact circulation and during mechanical ventilation. Modeling the systemic circulation as a pipe of constant resistance we have assumed the venous return (Qv) to be proportional to the difference between arterial pressure (Pa) and central venous pressure (Pcv). According to Guyton's venous return equation: Qv = (Psf-Pcv)/Rsf. Rsf.d. is the resistance downstream to the site in the circulation where blood pressure is equal to Psf. For the systemic flow, Qs = (Pa–Psf)/Rsf.u (where Qs is the cardiac output and Rsf.u is the resistance upstream). Since Qv = Qs, then Pa = Psf+(1+Rsf.u/Rsf.d)-(Rsf.u/Rsf.d) × Pcv. If Rsf.u. Rsf.d and Psf are constant [2], then Pa is linearly related to Pcv, and Psf can be computed from the regression line Pa versus Pcv, at Pa = Pcv. In eight patients undergoing elective surgery (ASA I-II, weight 68.8 ± 12 kg, age 59.5 ± 14 years), with no previous history of cardiovascular and pulmonary diseases, anesthesia was induced by pentobarbital and maintained with isofluorane (0.4%) and nitrous oxide, while neuromuscular blocking agents were atracurium or pancuronium bromide. The averaged hemodynamic baseline was: HR 85.2 ± 21.6 bpm; Pa 84.6 ± 13.8 mmHg: Pcv 5.26 ± 2.08 mmHg. Airway pressure, flow, Pa and Pcv traces were simultaneously measured, recorded and digitalized on a data acquisition system (Colligo, Elekton, Italy). In order to induce changes in Pcv and consequently in Qv, at least five end inspiratory pauses (IP) were performed after inflation of different tidal volumes (Vt), applied in a random order. The Vt ranged from 8.2 ± 2 ml/kg to 26.0 ± 10 ml/kg. We have measured the fall in Pa and the rise in Pcv caused by IP. At each step Pcv and Pa were measured during the IP, when the fall in Pa reached a plateau, ie after a mean time of 9.1 ± 3.7 s.

Results

(i) In each patient, as shown in the graph, we found a linear relation between Pcv and mean Pa (average R was: 0.93 ± 0.06). (ii) Average Psf was 19.21 ± 3.54 mmHg.

Conclusions

In the anesthetized patients we found a linear relation between Pa and Pcv during the inspiratory hold as in previous laboratory studies, where the Psf measurements were validated utilizing standard venous return curve [2]. Therefore, we conclude that this method is a reliable technique to estimate mean systemic filling pressure.