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VENTILATION REQUIREMENTS DURING CAESAREAN SECTION
CORRESPONDENCE
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12. Martin D, Tweedle D. Monitoring during sedation for endoscopy. British Medical Journal 1988; 297: 978. 13. Tate N. Monitoring during sedation for endoscopy. British Medical Journal 1988; 297: 561. 14. Carter AS, Coady T, Bell GD, Lee J, Morden A. Monitoring during sedation for endoscopy. British Medical Journal 1989; 298: 114. 15. Colin-Jones DG, Cockel R, Scille KFR. Current endoscopic practice in the U.K. Clinics in Gastroenterology 1978;7: 775-778. 16. Bell GD, Morden A, Bown S, Coady T, Logan RFA. Prevention of hypoxaemia during upper GI endoscopy by means of oxygen via nasal cannulae. Lancet 1987; 1, 1022-1023.
P. W. DUNCAN
Preston REFERENCES 1. Rampton AJ, Mallaiah S, Garrett CPO. Increased ventilation requirements during obstetric general an-
Sir,—Thank you for the opportunity to reply to Dr Duncan's letter. He wonders if we were justified in recommending higher fresh gas flow (FGF) rates before delivery than after delivery. The major importance of the FGF is in regard to the fetal status before delivery. We quoted a post-delivery value (109 ml kg"1 min"1) as stable PE'CO^ and FGF were always obtained, whereas in the much shorter induction-to-delivery interval we obtained a reasonably stable PE'CO, of 4 kPa in only 50 % of the patients, and thus cannot recommend our predelivery FGF (121 ml kg"1 min"1) with such certainty. We were not suggesting that it would be obligatory to alter the FGF after delivery, as small changes in PE'CO, are of little importance to the mother after the baby has been delivered. Maternal hyperventilation may indeed persist for up to 14 days post-partum, but we feel that there may be an immediate reduction in ventilatory requirements after delivery, mainly because of removal of the fetus and placenta, and partly perhaps as a result of a deeper level of anaesthesia. The oxygen consumption of the human fetus at term is estimated to be 8 ml kg"1 min"1, and placental oxygen consumption varies from 10 ml kg"1 min"1 to half that of the fetus [1]. Thus the oxygen consumption of an average weight fetus and placenta at term might be in the region of 33-42 ml min"1, which is a significant proportion of the average oxygen consumption of mothers at term (250-350 ml min"1). A. J. RAMPTON London
REFERENCE 1. Bissonette J. Placental and fetal physiology. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics Normal and Problem Pregnancies. New York: Churchill Livingstone, 1986; 109-136.
Downloaded from http://bja.oxfordjournals.org/ at University of Saskatchewan on June 18, 2015
VENTILATION REQUIREMENTS DURING CAESAREAN SECTION Sir,—I was interested to read the paper by Rampton and colleagues [1] on increased ventilation requirements during obstetric general anaesthesia. Their approach is ingenious for establishing the correct fresh gasflow(FGF) by adjusting this for each patient until the desired /'E'CO, ' S achieved. However, as they observed, this leads to frequently-adjusted flow rates in the short period before delivery. Are they justified in concluding that pre-delivery FGF rates should be higher than the post-delivery FGF? In a similar study using the ADE system during anaesthesia for Caesarean section [2], we used a constant FGF throughout the study period. We had expected to find a change in PE' CO , reflecting a change in FGF requirements following delivery, but on reviewing the hard copy trace of the Pi.'co^ no evidence of an abrupt or gradual reduction was apparent following delivery. This is not surprising if one considers that the increase in ventilatory requirements in pregnancy can persist for as long as 14 days post-partum. Therefore, it would seem unnecessarily complicated to advocate two different FGF during Caesarean section.
aesthesia. British Journal of Anaesthesia 1988; 61: 730737. 2. Duncan PW, Lawes EG, Bland B, Downing JW. Fresh gas flow requirements using the ADE anaesthetic system during late pregnancy. British Journal of Anaesthesia 1987; 59: 360-363.
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12. Martin D, Tweedle D. Monitoring during sedation for endoscopy. British Medical Journal 1988; 297: 978. 13. Tate N. Monitoring during sedation for endoscopy. British Medical Journal 1988; 297: 561. 14. Carter AS, Coady T, Bell GD, Lee J, Morden A. Monitoring during sedation for endoscopy. British Medical Journal 1989; 298: 114. 15. Colin-Jones DG, Cockel R, Scille KFR. Current endoscopic practice in the U.K. Clinics in Gastroenterology 1978;7: 775-778. 16. Bell GD, Morden A, Bown S, Coady T, Logan RFA. Prevention of hypoxaemia during upper GI endoscopy by means of oxygen via nasal cannulae. Lancet 1987; 1, 1022-1023.
P. W. DUNCAN
Preston REFERENCES 1. Rampton AJ, Mallaiah S, Garrett CPO. Increased ventilation requirements during obstetric general an-
Sir,—Thank you for the opportunity to reply to Dr Duncan's letter. He wonders if we were justified in recommending higher fresh gas flow (FGF) rates before delivery than after delivery. The major importance of the FGF is in regard to the fetal status before delivery. We quoted a post-delivery value (109 ml kg"1 min"1) as stable PE'CO^ and FGF were always obtained, whereas in the much shorter induction-to-delivery interval we obtained a reasonably stable PE'CO, of 4 kPa in only 50 % of the patients, and thus cannot recommend our predelivery FGF (121 ml kg"1 min"1) with such certainty. We were not suggesting that it would be obligatory to alter the FGF after delivery, as small changes in PE'CO, are of little importance to the mother after the baby has been delivered. Maternal hyperventilation may indeed persist for up to 14 days post-partum, but we feel that there may be an immediate reduction in ventilatory requirements after delivery, mainly because of removal of the fetus and placenta, and partly perhaps as a result of a deeper level of anaesthesia. The oxygen consumption of the human fetus at term is estimated to be 8 ml kg"1 min"1, and placental oxygen consumption varies from 10 ml kg"1 min"1 to half that of the fetus [1]. Thus the oxygen consumption of an average weight fetus and placenta at term might be in the region of 33-42 ml min"1, which is a significant proportion of the average oxygen consumption of mothers at term (250-350 ml min"1). A. J. RAMPTON London
REFERENCE 1. Bissonette J. Placental and fetal physiology. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics Normal and Problem Pregnancies. New York: Churchill Livingstone, 1986; 109-136.
Downloaded from http://bja.oxfordjournals.org/ at University of Saskatchewan on June 18, 2015
VENTILATION REQUIREMENTS DURING CAESAREAN SECTION Sir,—I was interested to read the paper by Rampton and colleagues [1] on increased ventilation requirements during obstetric general anaesthesia. Their approach is ingenious for establishing the correct fresh gasflow(FGF) by adjusting this for each patient until the desired /'E'CO, ' S achieved. However, as they observed, this leads to frequently-adjusted flow rates in the short period before delivery. Are they justified in concluding that pre-delivery FGF rates should be higher than the post-delivery FGF? In a similar study using the ADE system during anaesthesia for Caesarean section [2], we used a constant FGF throughout the study period. We had expected to find a change in PE' CO , reflecting a change in FGF requirements following delivery, but on reviewing the hard copy trace of the Pi.'co^ no evidence of an abrupt or gradual reduction was apparent following delivery. This is not surprising if one considers that the increase in ventilatory requirements in pregnancy can persist for as long as 14 days post-partum. Therefore, it would seem unnecessarily complicated to advocate two different FGF during Caesarean section.
aesthesia. British Journal of Anaesthesia 1988; 61: 730737. 2. Duncan PW, Lawes EG, Bland B, Downing JW. Fresh gas flow requirements using the ADE anaesthetic system during late pregnancy. British Journal of Anaesthesia 1987; 59: 360-363.