Portable respiratory polygraphy monitoring of obese mothers the first night after caesarean section with bupivacaine/morphine/fentanyl spinal anaesthesia

Introduction

In caesarean section (CS), intrathecal morphine (ITM) is associated with better postoperative course with less pain and shorter time to mobilisation¹. Respiratory depression associated with morphine is well known. Dahan et al. described the mechanism for the respiratory effects of opioids, defining the µ-receptor as the key target for the respiratory depressant effect and the sophisticated neuronal interaction in the ventral part of the brain stem². Respiratory depression is the most feared adverse effect from intrathecal opioids, and although rare (0–0.9%), it is the reason for some anaesthesia units to withhold the addition of morphine to spinal anaesthesia for CS^3–5. It has been suggested that ITM in doses <0.3 mg (300 µg) is associated with a lower risk of respiratory depression compared with doses >0.3 mg, and a tendency to a lower risk as compared to systemic opioids^3,6. Palmer et al. found no analgesic benefit to exceed a 100 µg dose of intrathecal morphine, since incidence and severity of side effects increased¹.

Respiratory depression is, however, not well defined and there is still not a general definition of respiratory depression to opioids⁷. George et al. conducted a meta-analysis on the primary outcome risk of respiratory depression and commented that multiple definitions were used in the included studies⁸. Similarly, a review by Ko et al. concerning the “definitions of respiratory depression”, explicitly addressing ITM, found no common description⁹. They also clearly commented the need for further research for defining what is a clinically significant respiratory impairment caused by ITM and how it best can be monitored⁹. Shapiro et al. defined is solely as a respiratory rate (RR) of <10 breaths per minute¹⁰.

Breathing disturbances postoperatively may be caused by several different factors. Residual effects from anaesthesia, abdominal surgery, obesity, immobilisation and opioid analgesics may all contribute to its occurrence^7,11. Postoperative respiratory depression after CS, performed in spinal anaesthesia including neuroaxial morphine, is found in low frequency (0/5036, 8/856, 6/1915)^4,12,13. Studies explicitly assessing the risk of respiratory compromise in obese patients, i.e. mothers with a high BMI (Body Mass Index), have not been previously conducted; apart from a retrospective study of 5036 mothers who had a CS with neuroaxial morphine, where 63% of patients were obese (BMI > 30 kg/m²), and the most commonly used morphine dose was 3 mg epidural and 0.15 mg spinal⁴. Crowgey et al. found no respiratory event with need for naloxone, defined as RR ≤8 breaths/min, oxygen saturation <90% or Richmond Agitation Sedation Scale <-2. Aboulish et al. published in 1991 the results from a study on ITM¹². They found 8/856 cases of respiratory depression in women having intrathecal addition of 0.2 mg morphine in spinal analgesia for CS, all eight cases were obese and naloxone treatment did not reverse the respiratory depression during sleep¹². Carvalho published a review in 2008 addressing the risk, monitoring and prevention of respiratory depression associated with neuroaxial morphine in the obstetric setting³. He summaries the risk and supports the need for observation up to 24 hours following neuraxial morphine, due to the duration of the depressed CO₂ sensitivity. In addition, Carvalho commented on the present lack of efficient, simple and mother-friendly monitoring equipment; he supports RR, saturation and sedation monitoring³.

Portable sleep test equipment and at-home polygraphy monitoring is commonly used for screening for obstructive sleep apnoea. This equipment assesses respiration, actual gas flow, and saturation, and transforms the information into indices: apnoea-hypopnea index (AHI) and oxygen desaturation index (ODI) using defined algorithms.

The aim of this observational study was to explore the use of portable respiratory polygraphy for monitoring of obese mothers for respiratory depression over the first night after CS with bupivacaine/morphine/fentanyl spinal anaesthesia.

Methods

Results

Forty mothers were invited to participate: 27 mothers consented but four of them had an early emergency CS delivery due to contractions, thus 23 mothers were included, but polysomnography registration failed in 3 (see Figure 1). Therefore, 20 mothers were included in analysis.

Figure 1. Patient inclusion, polygraphy first night after CS in spinal anaesthesia with ITM in obese mothers.

In all 20 mothers, the mean age was 35 ± 5 (24–43) years, mean BMI was 35 ± 4 (30–42), and mean ESS 6 ± 3 (0–12). For the ESS grade, 5 mothers had an ESS of <5, 12 had an ESS score between 5 and 10, and 3 scored >10.

Mean bed time during the polygraphic registration was 585 minutes (378–818). The mean registered SpO₂ was 94 ± 1.3 (91–96) and mean nadir SPO₂ 71 ± 10 (49–81). In total, 4 mothers had a mean SpO₂ <94 (91–93).

Mean AHI was 6.6 ± 5.2 (0–18.2) and mean ODI was 4.4 ± 3 (0–10.3). A total of 11 mothers had “normal” (<5) AHI, 7 had an AHI between 5 and 15, and 2 had an AHI 15–30. No mothers had an AHI >30. The longest apnoea duration was (mean) 30 ± 27 seconds, and mean longest hypopnea duration 55 ± 25 seconds.

Mean saturation was 94% (91–96) and four mothers had mean saturation between 90 and 94%, but no had a mean SpO2 < 90%. Nadir saturation was in mean 71% (49 – 81). In total, 11 mothers had an ODI <5, 8 had ODI between 5 to 10, and 1 mother had an ODI of 10.3. The 2 high AHI (15.3 and 18.2) mothers did not show high ODI or signs of hypercapnia on the transcutaneous CO₂ registration.

Mean TcCO₂ was 4.7 ± 0.3 (4.1–5.2) kPa, and mean of max TcCO₂ was 5 ± 0.5 kPa. There were no TcCO₂ >5.9 kPa. The pattern between AHI, ODI, BMI and ESS was overall scattered without correlation; Figure 2 describes the AHI and ODI pattern.

Figure 2. The apnoea-hypopnea index (AHI) and oxygen desaturation index (ODI) measures plotted for each mother.

None of the mothers showed clinical signs or symptoms of severe respiratory depression as assessed by routine clinical monitoring.

Discussion

We found that two out of 20 mothers included in the present study had an AHI of >15 and none had an AHI defined as severe sleep apnoea. Mothers with a high AHI did not show typical high oxygen desaturation index or TcCO₂ elevation. We did see frequent short episodes of oxygen saturation decrease, but we are unfortunately not able to assess whether these events were related to bradypnea or shallow breathing. We did not register any increase in TcCO₂. The TcCO₂ monitoring had a 15-minute averaging algorithm, thus it was not set for the detection of brief episodes of CO₂ elevation. Respiratory depression typically progresses slowly³.

Studies with polygraphy registration sleep apnoea signs, associated with ITM are sparsely performed. The effects of 30 mg oral morphine on patients with mild to moderate obstructive sleep apnoea has been investigated previously by Wang et al.¹⁵. They found that morphine may paradoxically improve sleep apnoea¹⁵. Similarly, Bernards et al. studied the effects of remifentanil infusion, and found a decrease in obstructive events, but a worsening in oxygenation during the infusion¹⁶. Cole et al. studied the respiratory effects of ITM (dose, 300 µg) in prospective randomized fashion among patients having knee replacement in spinal anaesthesia¹⁷. They found similarly to the present study that night time respiratory polygraphy is cumbersome and associated with high patient non-compliance¹⁷. Among the patients studied in that study, the incidence of apnoea and hypopnea episodes was not significantly different compared to the control group of patients that did not receive ITM¹⁷. Median mean oxygen saturation was, however, significantly lower among the ITM patients and the occurrence of “mild and moderate hypoxia” was also high in the ITM group. In another previous study, 45 obese patients undergoing elective bariatric surgery with general anaesthesia were monitored in a similar fashion with portable polygraphy equipment during the first postoperative night on the general ward; only two patients with an AHI >5 and only three with an ODI >5 was found¹⁸. Therefore, our findings, that registration during the first postoperative night following CS with spinal anaesthesia with a modest dose of morphine in obese mothers did not show any high incidence of AHI and ODI, might not be that surprising. It is also in line with a Cochrane review assessing the effects of opioid, hypnotic and sedating medications on sleep-disordered breathing in adults with obstructive sleep apnoea¹⁹. None of the studied drugs in the Cochrane review produced a significant increase in AHI or ODI and two trials have shown a beneficial effect on OSA¹⁹.

However, Subramani et al. describes, in a recent paper in the British Journal of Anaesthesia, catastrophic events in patients related to obesity and sleep apnoea, stating that ‘Morbid obesity, male sex, undiagnosed OSA, partially treated/untreated OSA, opioids, sedatives, and lack of monitoring are risk factors for death or near-death events’²⁰.

We cannot further comment on how and how long obese patients having ITM should be monitored. It seems still of importance to monitor respiration in patients at risk. Monitoring of AHI and ODI seems, however, not to be of major help. It may be that simple RR monitoring, TcCO₂ measure and SpO₂ are more feasible techniques^21,22. Kopka et al. suggest that TcCO₂ may be more effective in detecting respiratory depression compared to SpO₂ when patients receive supplementary oxygen²³. Ladha et al. studied oximetry after CS having 150 µg morphine intrathecal²⁴. They found frequent mild desaturation events and increased risk in patients with obesity. Bauchat et al. studied TcCO₂ tension and found that hypercapnia events (>50 mm Hg for ≥2-minute duration) occurred frequently in women receiving 150 μg ITM for post-caesarean analgesia; higher baseline TcCO₂ readings were observed in women who had hypercapnia events²⁵. Dalchow et al. studied both TcCO₂ and SpO₂ and found more frequent changes, hypercapnia as compared to desaturation²⁶. They concluded, ‘The incidence of opioid-induced respiratory depression detected by TOSCA is higher than previously reported by other monitoring methods. TOSCA may have a role in detecting subclinical respiratory depression in the obstetric population. Further studies with a control population are needed’²⁶. Patients at risk should of course be assessed prior to surgery/anaesthesia. Optimal screening method is however not well defined¹¹.

There are several limitations with our study. It is merely an observational study, and we could only include 23 mothers. We had a high number of mothers that declined to participate after having been informed about the monitoring techniques. The portable polygraphy is intended for use in the home for sleep apnoea screening instead of being in-hospital for a full polysomnography. The equipment involves straps around the thorax and abdomen, nasal prongs and a pulseoximetry probe all connected with cables to the monitoring unit. We included mothers with a BMI between 30 and 42 (mean 35) and none of our mothers had a known sleep apnoea. Merely three had an ESS of more than 10. Higher BMI and higher number of patients, possibly with more signs and symptoms of sleep apnoea would have been of interest. We are not able to assess sleep time, whether the mothers studied were asleep or merely rested. A full polysomnography would be needed for further in depth analysis. One may however strongly question whether that is ethical in a mothers’ first night after caesarean section.

In conclusion, we found in this explorative study that portable polygraphy is cumbersome and many mothers decline its use. It seems also reasonable to conclude that although episodes of oxygen saturation decrease were not infrequently noticed, upper airway collapse, obstructive hypo/apnoea, role as risk factor for respiratory depression during the first night after caesarean section in spinal anaesthesia with addition of low dose intrathecal morphine even in obese mothers seems minor. However, further studies with a combination of RR monitoring, TcCO₂ monitor and SpO₂ seems warranted, especially in high risk mothers. Preoperative screening in obese patients, at risk for sleep breathing disorder, is of course of value.

Data availability

Dataset 1: Raw data for the study polygraphy on the first night after caesarean section in spinal anaesthesia with morphine in obese mothers by Hein et al. The three patients who were excluded from analysis are highlighted. doi, 10.5256/f1000research.13206.d185388²⁷