Oxygenation and ventilation characteristics of thoracic surgical patients the night before surgery

Studies of the respiratory effects of perioperative analgesic and anesthetic agents are complicated by evidence that healthy subjects and those with COPD may have abnormalities of oxygenation and ventilation during sleep. This report thus characterizes respiration during sleep preoperatively in a population frequently examined in postoperative analgesia studies. Sixty-two thoracic surgical patients were monitored by continuous respiratory inductive plethysmography, 49 of whom also had continuous pulse oximetry and bi-hourly blood gas measurements. The mean respiratory rate (RR) at each hour during sleep was similar to the awake RR (P > 0.06). The minimum RR during sleep was 7.6 ± 2.1 (2.6-9.8; median 7.9). At least 1 apnea (tidal volume < 100 mL for >- 15 sec) occurred in 77% of subjects, and 32% had >-1 slow respiratory rate (SRR; 5-min RR < 10 beats/min) episode. The mean SpO₂ for hours 2 to 4 was significantly less than while awake (P < 0.0015), but the differences were ≤1.1%. The minimum SpO₂ was 88 ± 7% (62-97; 89) and 40% of subjects had >-1 desaturation (DESAT90; SpO₂ < 90% for >- 15 sec) episode. The pH at hours 2 and 4 was significantly (P < 0.0009) less than while awake, but the differences were only 0.02. Other bi-hourly pH, PaCO₂, and PaO₂ values were similar to the awake values (P > 0.02). Interpatient variability was high, especially for episodic abnormalities. The percentage ideal body weight correlated with the number of DESAT90 episodes, and the awake RR correlated with the number of SRR and apneic episodes. Factors associated with minimum SpO₂ were age, V25, V50, DCO, awake PaO₂ and SpO₂ The highest coefficient for any correlation was 0.55. In summary, a high percentage of subjects had >- 1 episode of desaturation, SRR, and/or apnea; no single set of respiratory parameters defined a "typical" thoracic surgical patient. Eight demographic and awake respiratory factors correlated with various respiratory events during sleep, but their predictive value was low.