We found that almost half of the women who underwent cesarean section with 0.1 mg intrathecal morphine developed sustained bradypnea that was detected by the acoustic respiration sensor; however, the rate of cumulative sustained bradypnea time was as low 0.7% and without any related factors. The first episode of sustained bradypnea generally occurred several hours after spinal anesthesia. Immediate bradypnea and hypoxemia, including both sustained and immediate episodes, also commonly occurred but none of them required naloxone or a rapid response team. Furthermore, none of the factors were related to bradypnea and hypoxemia.
Capnography is the gold standard in monitoring respiratory rate; however, it requires sampling end-tidal carbon dioxide and is especially troublesome for patients without oxygen administration. Therefore, in this study, an adhesive acoustic respiration sensor was used to measure the respiratory rate. Additionally, although previous studies that used continuous monitoring reported a high incidence of respiratory depression, the total time of respiratory depression during the monitoring time was not reported. We believe that sustained and repeated respiratory depression is a more important adverse event rather than transient depression. Therefore, we primarily aimed to assess the rate of cumulative sustained bradypnea time. 8, 11, 14
In our cohort, the incidence of sustained bradypnea was as high as 48%, which is similar to the reported incidence of 53% in a previous study where end-tidal carbon dioxide < 5 mmHg for 30–120 consecutive seconds (respiratory rate < 2 bpm) was defined as apnea; similarly, the very low rate of cumulative sustained bradypnea time was comparable with the reported rate of 29 apnea events during an average of 8.5 hours of monitoring.11 Furthermore, the incidence of hypoxemia was higher than those in some studies that used scheduled intermittent monitoring, although it was comparable with those of previous reports that performed continuous monitoring using pulse oximetry. 7, 8 However, considering that there were no clinically relevant episodes of respiratory depression although the impacts of alarm sound on sleep and awaking was not assessed, postoperative routine continuous respiratory monitoring might be excessive in healthy women who undergo cesarean section with low-dose intrathecal morphine.
None of the factors were related to the rate of cumulative sustained bradypnea time. Similarly, the multiple logistic regression analyses did not identify any factors related to bradypnea and hypoxemia. In contrast, one study revealed that obstructive sleep apnea, screened using the Berlin questionnaire, was associated with desaturation events with SpO2 < 90% for 30 seconds.8 This could be explained by the small sample size in the multiple logistic regression analysis because, although the calibration of the model and the descriptive tool for measuring model bias were relatively valid, because our sample size was calculated for multiple regression analysis. Additionally, the small number of women with body mass index ≥ 30 kg/m2 and positive Berlin questionnaire might have contributed to these results; therefore, further studies that can include women at high risk of respiratory depression are needed. Interestingly, previous studies have reported that women without any risks developed the highest number of apnea events, which were defined as episodes of end-tidal carbon dioxide < 5 mmHg for 30–120 consecutive seconds.11 Although the exact reason remains unknown, this can be attributed to factors that have not been evaluated yet, such as opioid sensitivity.
Our study has several limitations. First, this was a single-center study in Japan; therefore, the generalizability of our findings may be limited. Second, our cohort included a small number of women with risk factors of respiratory depression following cesarean section, such as obesity and hypertension. Therefore, our results should be interpreted with caution as they are from a limited population with low risk factors. Third, our sample size was too small to estimate clinically relevant episodes of respiratory depression; therefore, further studies in a larger number of women are required.
In conclusion, we conducted this prospective observational study to assess postoperative bradypnea in women who underwent cesarean section with 0.1 mg intrathecal morphine. Approximately half of the women developed episodes of sustained bradypnea and this high prevalence should be recognized by all medical staff involved in maternal care, including obstetricians, nurses, and anesthesiologists; however, these episodes constituted only a small portion of the total respiratory monitoring time with no related factors. The incidence of bradypnea was as high as those reported previously using continuous monitoring; however, no related factors were identified. Continuous respiratory monitoring in relatively healthy women who undergo cesarean section with 0.1 mg intrathecal morphine requires careful patient selection.