Episodes of hypoxaemia occur in healthy women during painful labour (1). Hyperventilation during a contraction reduces maternal arterial PC02 and so reduces ventilatory drive. Subsequent hypoventilation or apnoea between contractions may cause maternal hypoxaemia (2). Systemic opioids increase the incidence of maternal hypoxaemia whilst epidurals using local anaesthetic alone are associated with a reduced incidence compared with pethidine (3), 'Entonox' (4) or no analgesia (5, 6). Effective analgesia promotes more regular respiration and the need for systemic opioids or 'Entonox' (nitrous oxide:oxygen 50:50 mixture) is avoided (Chapter 33). The recent addition of opioids to epidurals reduces the dose of local anaesthetic used and lessens the degree of motor block (Chapter 60, Chapter 88). However, epidural infusions of bupivacaine with fentanyl increase the incidence of maternal hypoxaemia during the second stage of labour compared with infusions of bupivacaine alone (6) (Figure 87.1, Table 87.1). The reasons for this are unclear but hypoxaemia associated with breath holding during pushing may be corrected more slowly between pushes due to opioid-induced respiratory depression. (Chapter 90)
Effects on the fetus
Maternal hypoxaemic episodes may cause a reduction in fetal oxygenation (2) but there is little evidence that the baby is adversely affected (3, 6) (Chapter 35). However, a sub-set of 'at risk' babies may be more susceptible (eg. some cases of preeclampsia, eclampsia, maternal respiratory or cardiac disease, intra-uterine growth retardation or fetal distress) (Table 3.1) (Chapter 25). Pulse oximetry and/or oxygen therapy may be of benefit in these cases.
Caesarean section
During regional blockade for caesarean section, the sensory block T4 - T5 required to provide adequate anaesthesia is accompanied by paralysis of the abdominal muscles and a number of intercostal muscles. Breathing becomes predominantly diaphragmatic and is limited by the gravid uterus and the supine position. The administration of oxygen to the mother before delivery has been recommended (7) to maximise fetal oxygenation (Chapter 76). The ability to cough is also impaired (8) and persists for some time post-operatively. Whilst this degree of motor block may not be dangerous for most women, in those with respiratory or musculoskeletal disease it may pose a clinically significant risk to ventilation. Epidural and intrathecal opioids, given during surgery or for post-operative analgesia, also have the potential to cause respiratory depression (9) and reduce maternal oxygenation (Chapter 99).
References:
1. Porter KB, Goldhamer R, Mankad A, Peevy K, Gaddy J, Spinnato JA. Evaluation of arterial oxygen saturation in pregnant patients and their newborns. Obstetrics & Gynecology 1988; 71:354-357.
2. Huch A, Huch R, Schneider H, Rooth G. Continuous transcutaneous monitoring of fetal oxygen tension during labour. British Journal of Obstetrics and Gynaecology 1977; 84 (Supp. 1): 1-39.
3. Minnich ME, Brown M, Clark RB, Miller FC, Thompson DS. Oxygen desaturation in women in labor. Journal of Reproductive Medicine 1990; 35:693-696.
4. Arfeen Z, Armstrong PJ, Whitfield A. The effects of Entonox and epidural analgesia on arterial oxygen saturation of women in labour. Anaesthesia 1994; 49:32-34.
5. Curtis J, Shnider SM, Saitto C, Orezzi C, Volante I, Bertini L, Cosmi EV. The effects of painful uterine contractions, position, and epidural anesthesia on maternal transcutaneous oxygen tension (tcP02). Anesthesiology 1980; 53:S315.
6. Griffin RP, Reynolds F. Maternal hypoxaemia during labour and delivery: the influence of analgesia and effect on neonatal outcome. Anaesthesia 1995; 50:151-156.
7. Ramanathan S, Gandhi S, Arismendy J, Chalon J, Turndorf H. Oxygen transfer from mother to fetus during cesarean section under epidural anesthesia. Anesthesia and Analgesia 1982; 61:576-581.
8. Kirby SA, Kendrick AH, Anderson M, Williams AB, Thomas TA. Dynamic respiratory functions and expiratory mouth pressures in obese and non-obese patients during sub-arachnoid (SAB) anaesthesia for caesarean section. International Journal of Obstetric Anesthesia 1995; 4:260-261.
9. Gustafsson LL, Schildt B, Jacobsen K. Adverse effects of extradural and intrathecal opiates: Report of a nationwide survey in Sweden. British Journal of Anaesthesia 1982; 54:479-486.