Determination of optimal positive end-expiratory pressure by means of conjunctival oximetry.

A method for determining the optimal level of positive end-expiratory pressure (PEEP) by means of noninvasive conjunctival oxygen (PcjO2) monitoring and arterial blood gas analysis was developed from the pattern of changes in PcjO2 tension, invasive hemodynamic parameters, and oxygen transport variables during PEEP titration in a series of patients with adult respiratory distress syndrome. All patients had radial and pulmonary artery (PA) catheters inserted and blood volume was measured with 125I serum albumin before each study. During progressive increases in the level of PEEP, PcjO2 tensions reflected changes in both PaO2 and cardiac index (CI), depending on whether PEEP produced a significant decrease in CI. In patients with a stable CI, PcjO2 tensions tracked PaO2 values (rw = 0.92); in patients with a greater than a 15% decrease in CI, the conjunctival index, CjI (defined as the PcjO2/PaO2 ratio), tracked CI (rw = 0.87), excluding one patient with high cardiac output-septic shock and severe hypoxemia. PcjO2 correlated with PaO2 in the latter patient (r = 0.99) probably because conjunctival oxygen transport was limited by arterial oxygen content (PaO2 = 34 torr) rather than blood flow (CI greater than 6 L/min X m2). In patients with a greater than a 10% decrease in CI as a result of PEEP, the greater the decrease in CI, the better CjI values correlated with CI. We conclude that PcjO2 monitoring combined with repeated arterial blood gas analysis may be used to titrate PEEP therapy in patients with adult respiratory distress syndrome. In patients whose CjI significantly decreases because of PEEP, PA catheterization and measurement of cardiac output are indicated because of the likelihood of a significant (greater than 15%) decrease in CI. In the absence of a significant decrease in the CjI, optimum PEEP occurs at the level producing maximum PcjO2. It is hoped that by following the described algorithm, many patients will be spared the cost and morbidity of unnecessary PA catheterization.