[Continuous monitoring of spontaneous respiration in the postoperative phase. 4. The effect of postoperative pain therapy on cutaneous oxygen and carbon dioxide partial pressure following gynecologic surgery with neuroleptanalgesia].

METHODS

Cutaneous O2 and CO2 pressures were monitored for 16 h in 55 female patients recovering from major gynaecological surgery performed under neurolept anaesthesia. Postoperative pain was managed either with an antipyretic analgesic (i.m. or i.v. metamizol up to 2.5 g/4 h; group NLA) or with i.v. patient-controlled analgesia using fentanyl (demand dose 34 micrograms, infusion rate 4 micrograms/h, hourly maximum dose 0.25 mg, lock-out time 1 min; group NLA/PCA). In addition, 11 patients received a single i.v. bolus injection of 150 mg amiphenazole, a respiratory stimulant, at the beginning of PCA treatment (group NLA/PCA/AMI). Data were collected and stored by a personal computer, using the TCM3 system with a combination electrode for simultaneous measurement of cutaneous oxygen and carbon dioxide partial pressures (TINA, Radiometer) at 30-s intervals. The overall observation period was four times 240 min; patients from the NLA group who required additional opioids were excluded from the analysis. Means and standard deviations were calculated for individual data and data pooled for 15- or 60-min intervals. Groups were compared by means of the chi-square test, Student's t-test or analysis of variance (level of significance, P < or = 0.05).

RESULTS

The 55 patients were classified as ASA I-II. The study groups were comparable with respect to demographic and anaesthesiological data, except that those in the NLA group were younger and had received less intraoperative fentanyl (Table 1). Mean PCA fentanyl consumption was 0.6-0.7 mg in the 16-h observation period (Table 2). In all groups, pctO2 levels were decreased and pctCO2 levels elevated in the first observation hours and slowly returned to normal within the first observation period (Figs. 1, 2, Tables 3, 4). Episodes of hypercapnia (pct-CO2 > 50 or > 55 mm Hg) were frequent in the first 2 h (8-29% of individual values for pctCO2 > 50, up to 5% of values recorded for pctCO2 > 55; Table 4). There were no statistically significant differences between patients treated with metamizol and those treated with fentanyl. Amiphenazole did not significantly improve postoperative respiration. PCA patients had occasional episodes of hypercapnia (up to 19% of all values for pctCO2 > 50, up to 5% for pctCO2 > 55) even in the last observation period (13-16 h after surgery), indicating the need for close monitoring of spontaneous ventilation during PCA following neurolept anaesthesia.

DISCUSSION AND CONCLUSIONS

The present study confirmed that spontaneous respiration in the early postoperative period can be monitored non-invasively by measuring cutaneous partial pressures of carbon dioxide and, less precisely owing to wide individual variations, oxygen. It showed that spontaneous respiration is less effective immediately after termination of surgery under neurolept anaesthesia and recovers slowly over the next 4 h. During the first observation period, ventilation was no worse with i.v. PCA using fentanyl than with conventional pain management using the antipyretic analgesic metamizol, confirming the hypothesis that opioid-induced respiratory depression occurs only at overdosage (which is not a problem with individualized dose titration using PCA). Since all patients in the NLA group required additional opioids after the first observation period and had to be excluded from further analysis, it cannot be decided from the present data whether late hypercapnia was due to PCA or to residual effects of surgery and anaesthesia. The respiratory stimulant amiphenazole (150 mg i.v.) was not helpful in improving ventilation; there was no indication of analgesic effects or interactions of amiphenazole.