Hemodynamic Effects of Oxygen Therapy in Patients With Acute Exacerbations of Chronic Obstructive Pulmonary Disease: Measurements and Calculations
Hemodynamic parameters, Sa02, and mixed-venous oxygen saturation (SvOJ were measured in all patients once they were monitored as described previously (breathing fractional concentration of oxygen in the inspired gas [FIoJ = 0.21). Those values were taken as the baseline. Then oxygen therapy at an FIo2 sufficient to keep Pa02 above 55 mm Hg was started, and measurements were repeated at 30 min and at 24 and 48 h thereafter. When subsequent measurements were made, a steady state was ensured by a change in respiratory’ and heart rates less than 20 percent, systolic blood pressure less than 15 percent, and Sa02 less than 3 percent during the previous hour. All patients survived the ICU admission and were transferred to other medical wards when their condition improved.
Mean arterial pressure, right atrial pressure, pulmonary artery pressure, and pulmonary artery wedge pressure were measured using a stain-gauge transducer and recorded on a multichannel recorder (Siemens-Elema Mingograf 34). The zero reference was set at 4 cm below the sternal angle, with the patient in a semirecumbent position. The values for pressures were averaged for three successive respiratory cycles. Cardiac output was determined by the thermodilution technique, using a cardiac output computer. The mean value of 3 measurements within a range of 5 percent was used for calculation of derived variables. Cardiac index was calculated by dividing cardiac output by body surface area. Arterial and venous oxygen contents were calculated using the standard equation (oxygen content = 1.38 x hemoglobin concentration x02 saturation + 0.0031 x PoJ. Arterial and venous oxygen saturations were estimated from the corresponding PaO, and mixed-venous oxygen pressure (PvOJ values, according to the Severinghaus nomogram, and corrected for pH, temperature, and hematocrit. Oxygen delivery’ was calculated as the product of cardiac output and arterial oxygen content (Ca02) by the following formula: Do2 = 10 x CaO, x cardiac output. Oxygen uptake was calculated using the Fick equation (Vo2 = 10 X cardiac output X [arterial 02 content-venous 02 content]). The oxygen extraction ratio was calculated as the ratio between Vo2 and Do2. Hemoglobin concentration was measured by an automated analyzer (Coulter 5 Plus Analyzer). Blood gas levels were measured for arterial and mixed-venous blood samples (Instrumentation Laboratories Analyzer).
Data were analyzed by a one-way analysis of variance (ANOVA).
А р value less than 0.05 was considered statistically significant.