Oxygen Cost of Breathing in Postoperative Patients: Results

Measurements and Calculations
Pa02, PaC02 and H+ concentration were measured using standard electrodes. For the measurements of pressures and flow in patient 7, we used pressure transducers, a pneumotachograph (Could), and a four-channel recorder (Could). Esophageal and gastric pressures were measured by two balloon-catheters joined to each other. The esophageal balloon was positioned 10 cm above the cardia. The balloons were 5 cm long and were filled with 0.5 ml air. Pulmonary gas exchange was measured with our mass-spectrometer-microcom-puter system. Details of this procedure and thorough validation have been given in a previous report. The system can be briefly described as follows. Gas samples were drawn from the Y piece of the patients breathing circuit to the mass-spectrometer and analyzed for inspired 02 concentration and C02 wave-form recognition. The latter analysis allowed rejection of artifacted cycles, such as coughing. Then, expired gas was sampled from the outlet of a mixing chamber for the measurement of mixed expired 02 and C02 concentrations. Expired flow was measured by a pneumotachograph (Could). All the signals were collected by the microcomputer which was programmed to reject artifacted respiratory sequences and to compute Vt, RR, Ve, Vo2, Vco2, and RQ at three-minute intervals. For each study period, the mean values of all the measured parameters were calculated. During each period of spontaneous respiration (CPAP and IPSV), Vo*resp was calculated as the difference between Vo2tot of the period, and Vo2tot measured during CV. Results are presented as the mean ± SE and further statistical analysis used two-way analysis of variance with Duncans multiple range follow-up tests. buy mircette birth control
Results
As indicated in Table 2, CV was associated with a slight respiratory alkalosis, which could be accounted for by Vt and Ve values higher than that observed during CPAP and IPSV. There was no difference for Pa02. The Ve was the same for the two modes of spontaneous respiration, but RR was more elevated during CPAP. Figure 1 shows that CPAP resulted in all patients having a significant increase in Vo2 tot. The calculated Vo2 resp was 11.2 ±1.4 percent of Vo2 tot (range 6.6 to 15.6 percent), or 2.6±0.4 ml OJL of ventilation (range 1.7 to 4.2 ml OJh of ventilation). Conversely, there was no difference for Vo2 tot between IPSV and CV. Blood pressure and heart rate remained stable throughout the study. Figure 2 shows the recordings of pressures and flow in the patient 7. During inspiration with IPSV, esophageal pressure decreased, whereas gastric pressure increased, thus indicating a sustained diaphragmatic contraction.
Table 2—Mean Values af Physiologic Parameters for Three Ventilatory Conditions

CV CPAP IPSV
Vt (ml) 746 ±62 480 ±43* 639 ±47*t
Respiratory rate (min) 18 ±1 23 ± 1* 16± It
V (L-min’) 13.2 ±0.6 10.7 ±0.7* 10.3 ±0.5*
Pa02 (mm Hg) 89.3 ±8.5 105 ±10.4 94.5 ±6.1
P&COa (mm Hg) 27.3 ±1.1 34 ±1.8* 33.1 ±1.8*
H+ concentration (n Moles* L) 28.7± 1.6 34.1 ±1.2* 33.8 ±0.9*
Voa tot (ml*min*m) 141 ±7 157 ±9* 140 ±7t
Vo2 resp (ml/*min_1*m“2) 16.2±2.9 — 0.3±1.2t
Vco2 (ml*min~*m~2) 129 ±5 135 ±6* 125 ±5t
RQ 0.91 ±0.01 0.86 ±0.01* 0.89 ±0.02
Systolic blood pressure (mm Hg) 132 ±3 139 ±5 133 ±5
Heart rate (min) 91 ±6 93±6 88±5

Figure 1. Mean Vo2 measured in the seven patients during controlled ventilation (CV), continuous positive airway pressure (CPAP), and inspiratory pressure support ventilation (IPSV).Figure 1. Mean Vo2 measured in the seven patients during controlled ventilation (CV), continuous positive airway pressure (CPAP), and inspiratory pressure support ventilation (IPSV).

Figure 2. Individual recording (patient 7) of gastric pressure (Pga), esophageal pressure (Pes), airway pressure (Paw), and respiratory flow(V) under controlled ventilation (CV), 15 cm H20 inspiratory pressure support ventilation (IPSV) and continuous positive airway pressure (CPAP). One respiratory cycle is depicted for each ventilatory condition. Note that under IPSV, Pes decreases during inspiration, whereas Pga increases.

Figure 2. Individual recording (patient 7) of gastric pressure (Pga), esophageal pressure (Pes), airway pressure (Paw), and respiratory flow(V) under controlled ventilation (CV), 15 cm H20 inspiratory pressure support ventilation (IPSV) and continuous positive airway pressure (CPAP). One respiratory cycle is depicted for each ventilatory condition. Note that under IPSV, Pes decreases during inspiration, whereas Pga increases.