Please see Bibliography of References for a list of abstracts, manuscripts and posters.

StO₂ Proof of Concept

Effects of positive end-expiratory pressure on tissue O2 saturation in an animal model with and without hypoxemia

Lima A, Bikker I, Van Bommel J, Ince C, Bakker J. Int Care Med. 2009;35(Suppl 1):S155.

Introduction: High positive end-expiratory pressure (PEEP) levels during mechanical ventilation have been used to avoid lung collapse and impairment of arterial oxygen saturation. Application of PEEP also decreases venous return and cardiac output, causing secondary poor tissue perfusion. The direct relationship between PEEP and tissue O₂ saturation has not yet been studied.

Objectives: To investigate in an animal model the effects of acute decreases in global blood flow brought by incremental increases in PEEP on peripheral tissue oxygenation (StO₂) assessed by near infrared spectroscopy before and after induction of hypoxemia.

Methods: We performed experiments in five female pigs. Animals were anesthetized and ventilated in a volume control mode. FIO₂ was set to 0.8. Measurements were done in the intact lung condition (model 1) and after induction of hypoxemia (model 2) by injection of oleic acid. PEEP was applied in increments of 5 cmH₂O each 10 min from baseline (0 cmH₂O) to 20 cmH₂O (model 1) or 25 (model 2) after which it was decreased in steps of 5 cmH₂O following the same protocol. Mean arterial pressure (MAP), central venous pressure (CVP), central oxygen saturation (SvO₂) and cardiac output (CO) were continued monitored. StO₂ was continuously measured on the leg using InSpectra Model 650 (Hutchinson Technology Inc.). Differences between groups were tested by Wilcoxon signed test. P values<0.05 were considered statistically significant.

Results: Hemodynamic and oxygenation variables in the different levels of PEEP stratified by lung condition are shown in the following Tables 1, 2.

TABLE 1 INTACT LUNG. A:P<0.05 VS. BASELINE

TABLE 1 PULMONARY EDEMA. A:P<0.05 VS. BASELINE

Elevation of PEEP significantly increased CVP in both models. During manipulation of PEEP, significant changes of StO₂ and SvO₂ were also observed. After the normalization of SaO₂ in the model 2, further increments in PEEP levels did not produce significant changes in StO₂.

Conclusion: Application of high PEEP impairs peripheral tissue oxygenation via its negative effect on macrohemodynamic variables in the intact lung condition. After induction of hypoxemia, the increment of SaO₂ following application of PEEP may outweigh the decline of CO, thereby resulting in higher StO₂.