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SODIUM NITORPRUSSIDE IN HYPOTENSIVE ANAESTHESIA
BRITISH JOURNAL OF ANAESTHESIA
908
The prediction curves thus constructed (fig. 3) support the clinical impression that the induction-delivery interval plays a significant part in the condition of the infant at birth when a general anaesthetic is employed and the Caesarean section is performed with the patient in the supine position. It would be interesting to repeat this investigation with the patients operated on in the lateral position as described by Waldron and Wood (1971). C
R. CLIMIE; L. MATHER
Sydney REFERENCES
Waldron, K. W., and Wood, C (1971). Cesarean section in the lateral position. Obstet. and Gynec, 37, 706. Williams, G. F., Thomas, R., and Jones, E. C (1971). Anaesthesia for Caesarean section. Correspondence. Brit. J. Anaesth., 43, 863. Marx, Gertie F. (1972). Anaesthesia for Caesarean section. Correspondence. Brit. J. Anaesth., 44, 233. SODIUM NITROPRUSSIDE IN HYPOTENSIVE ANAESTHESIA
10-1 COHPUTERISED PREDICTION CURVES
3-
7-
u
6"
5-
10
15
20
INDUCTION - DELIVERY TIME IK MINUTES
FIG.
3.
O-O
EMERGENCY OPERATIONS
&-&
ELECTIVE OPERATIONS
Computerized prediction curves from data in figures 1 and 2.
Sir,—Three observations of special interest have been made in a personal series of fifty hypotensive anaesthetics using the sequence thiopentone-alcuronium-oxygenmethoxyflurane-nitroprusside-IPPR for a wide variety of operations. Firstly, contrary to previous reports concerning sodium nitroprusside, tachyphylaxis does occur. It has been almost the rule in young patients but exceptional in the elderly. The phenomenon as seen in young patients is an initial fall in arterial pressure followed by a rising pressure if the infusion rate is kept constant. On increasing the infusion rate, pressure falls again then starts to rise once more. Secondly, satisfactory reduction in bleeding has been achieved without resort to the use of head-up tilt. This is an advantage in view of the risks of head-up tilt in this situation, namely progressive pooling of blood in the legs followed eventually by acute cardiovascular decompensation, inadvertent overdose of nitroprusside leading to cardiovascular failure, and air-embolism in operations around the head and neck. Thirdly, it appears that most exponents of hypotensive anaesthesia use an inspired gas mixture containing oxygen at a tension of around 380 mm Hg. In view of the constant worry about maintaining cerebral oxygenation, it would seem that the use of oxygen at an inspired tension of around 750 mm Hg, combined with de-nitrogenation of the patient, offers considerable advantages, namely reduction of the effect of increased pulmonary deadspace during hypotension, an increase in the amount of oxygen carried in the blood, an increase in the blood-tissue oxygen tension gradient, and a reduction of the risk of hypoxia associated with inadvertent interruption of ventilation. In conclusion, it is suggested, in light of the present observations, that an increased safety margin in hypotensive anaesthesia is available through the use of sodium nitroprusside, full tissue oxygenation and exclusion of head-up tilt except for th: occasional resistant case. J. A. LOWSON
Preston, Victoria
Primed id Grdt Britain by John Sberratt & Son Ltd. Pmrk Road. AJtriocham, Cheshire. WA14 5QO
Downloaded from http://bja.oxfordjournals.org/ at University of Bath Library & Learning Centre on July 11, 2015
A record has been kept of all Caesarean sections performed under general anaesthesia at the Royal Hospital for Women over a two-year period 1970-71. The time between the onset of unconsciousness and the delivery of the infant was measured to the nearest half-minute. The Apgar score was estimated one minute after birth by a paediatrician. The total number so recorded was 816. A division was made into elective (341) and emergency (475) groups, but no other factors which may have influenced the condition of the foetus were taken into account. Simple scattergrams of the results (figs. 1 and 2) reveal a range of induction-delivery intervals from two to thirty-four minutes, and Apgar scores from one to ten. In both groups there is, as expected, a concentration in the higher Apgar, shorter time areas. To interpret these results in a more meaningful way the data were processed by computer, using a polynomial regression programme (POLRN, Basser Computer Centre, University of Sydney). The trend was analysed to a three degree polynomial and the Apgar scores were recalculated to the fitted equation.
908
The prediction curves thus constructed (fig. 3) support the clinical impression that the induction-delivery interval plays a significant part in the condition of the infant at birth when a general anaesthetic is employed and the Caesarean section is performed with the patient in the supine position. It would be interesting to repeat this investigation with the patients operated on in the lateral position as described by Waldron and Wood (1971). C
R. CLIMIE; L. MATHER
Sydney REFERENCES
Waldron, K. W., and Wood, C (1971). Cesarean section in the lateral position. Obstet. and Gynec, 37, 706. Williams, G. F., Thomas, R., and Jones, E. C (1971). Anaesthesia for Caesarean section. Correspondence. Brit. J. Anaesth., 43, 863. Marx, Gertie F. (1972). Anaesthesia for Caesarean section. Correspondence. Brit. J. Anaesth., 44, 233. SODIUM NITROPRUSSIDE IN HYPOTENSIVE ANAESTHESIA
10-1 COHPUTERISED PREDICTION CURVES
3-
7-
u
6"
5-
10
15
20
INDUCTION - DELIVERY TIME IK MINUTES
FIG.
3.
O-O
EMERGENCY OPERATIONS
&-&
ELECTIVE OPERATIONS
Computerized prediction curves from data in figures 1 and 2.
Sir,—Three observations of special interest have been made in a personal series of fifty hypotensive anaesthetics using the sequence thiopentone-alcuronium-oxygenmethoxyflurane-nitroprusside-IPPR for a wide variety of operations. Firstly, contrary to previous reports concerning sodium nitroprusside, tachyphylaxis does occur. It has been almost the rule in young patients but exceptional in the elderly. The phenomenon as seen in young patients is an initial fall in arterial pressure followed by a rising pressure if the infusion rate is kept constant. On increasing the infusion rate, pressure falls again then starts to rise once more. Secondly, satisfactory reduction in bleeding has been achieved without resort to the use of head-up tilt. This is an advantage in view of the risks of head-up tilt in this situation, namely progressive pooling of blood in the legs followed eventually by acute cardiovascular decompensation, inadvertent overdose of nitroprusside leading to cardiovascular failure, and air-embolism in operations around the head and neck. Thirdly, it appears that most exponents of hypotensive anaesthesia use an inspired gas mixture containing oxygen at a tension of around 380 mm Hg. In view of the constant worry about maintaining cerebral oxygenation, it would seem that the use of oxygen at an inspired tension of around 750 mm Hg, combined with de-nitrogenation of the patient, offers considerable advantages, namely reduction of the effect of increased pulmonary deadspace during hypotension, an increase in the amount of oxygen carried in the blood, an increase in the blood-tissue oxygen tension gradient, and a reduction of the risk of hypoxia associated with inadvertent interruption of ventilation. In conclusion, it is suggested, in light of the present observations, that an increased safety margin in hypotensive anaesthesia is available through the use of sodium nitroprusside, full tissue oxygenation and exclusion of head-up tilt except for th: occasional resistant case. J. A. LOWSON
Preston, Victoria
Primed id Grdt Britain by John Sberratt & Son Ltd. Pmrk Road. AJtriocham, Cheshire. WA14 5QO
Downloaded from http://bja.oxfordjournals.org/ at University of Bath Library & Learning Centre on July 11, 2015
A record has been kept of all Caesarean sections performed under general anaesthesia at the Royal Hospital for Women over a two-year period 1970-71. The time between the onset of unconsciousness and the delivery of the infant was measured to the nearest half-minute. The Apgar score was estimated one minute after birth by a paediatrician. The total number so recorded was 816. A division was made into elective (341) and emergency (475) groups, but no other factors which may have influenced the condition of the foetus were taken into account. Simple scattergrams of the results (figs. 1 and 2) reveal a range of induction-delivery intervals from two to thirty-four minutes, and Apgar scores from one to ten. In both groups there is, as expected, a concentration in the higher Apgar, shorter time areas. To interpret these results in a more meaningful way the data were processed by computer, using a polynomial regression programme (POLRN, Basser Computer Centre, University of Sydney). The trend was analysed to a three degree polynomial and the Apgar scores were recalculated to the fitted equation.