British Journal of Obstetrics and Gynaecology January 1977. Vol 84. pp 48-50
THE EFFECT OF DOXAPRAM ON FETAL BREATHING IN THE SHEEP BY
M. I. J. HOGG Department of Anaesthetics, Welsh National School of Medicine Heath Park, Cardif, CF4 4XN
R . H . GOLDING Department of Obstetrics and Gynaecology, Royal Gwent Hospital, Newport AND
M . ROSEN Department of Anaesthetics, Welsh National School of Medicine Heath Park, Cardif, CF4 4XN
Summary In five ewes of 105 to 120 days gestation intravenous doxapram (1 * 8 to 3.85 mg/kg) administered to the mother produced a dose-related increase in fetal breathing. This effect of doxapram might prove useful as a test of human fetal health. Fetal breathing movements could be a sensitive indication of the effects of drugs on the human fetal respiratory system.
the fetal head and neck were delivered. The trachea, a carotid artery and jugular vein were catheterized. A catheter was also placed in the amniotic sac to distinguish other movements from fetal breathing activity. Benzyl penicillin sodium (1 mega unit) and streptomycin (500 mg) were injected into the amniotic sac, the membranes were sutured and the uterine wound was closed. The ewe was left in a single pen for at least three days to ensure full recovery. Each day, the maternal and fetal intravascular lines were filled with heparin (250 unitslml). The ewe was treated daily with penicillin and streptomycin, and the fetus withchloramphenicol (125 mg) administered through the jugular vein catheter. For at least two hours prior to the injection of doxapram the fetal blood pressure, tracheal and amniotic pressures were continuously recorded. Fetal breathing movements were
SPONTANEOUS breathing movements occur in fetal lambs from 40 days gestation onwards (Dawes et al: 1970). It has been demonstrated that pentobarbitone and chloralose depress fetal breathing movements for about one hour (Dawes et al, 1972; Boddy et al, 1974). In man, using an ultrasonic A-scan detector to measure fetal breathing activity (Boddy and Robinson, 1971), cigarette smoking has been shown to reduce that activity for between 35 to 60 minutes (Manning et al, 1975). This study in sheep describes the effects on fetal breathing movements of a respiratory stimulant, doxapram. METHOD Welsh mountain ewes (mean weight 40 kg) of between 105 and 120 days gestation were anaesthetized using thiopentone (1 . 5 mg/kg) and suxamethonium (1 mg/kg) prior to intubation and maintained with a halothaneoxygen-air mixture. After incising the uterus, 48
DOXAPRAM
analyzed by measuring the percentage of each 5 minute period that fetal breathing was present for at least one hour before and after each injection. Doxapram was infused into a maternal vein over 2 to 5 minutes in doses of between 1 -8mg/kg and 3.85 mg/kg of maternal weight. For the first three experiments doxapram was given as 14C labelled drug (A. H. Robins Ltd). After establishing that there was sufficient activity for measurements of plasma levels a 1:2 mixture of doxapram I4C and doxapram was used. Blood samples of 2 ml were taken from maternal and fetal jugular veins at 2, 4, 6, 10, 20, 30, 45, 60 and 90 minutes and the concentration of 14C in plasma was measured by scintillation counting. Since metabolites of doxapram were not separated, the results represent levels of doxapram and metabolites. In two animals, further plasma samples were taken from the ewe and fetus at 90 minutes for analysis of doxapram metabolites by thin layer chromatography. RESULTS Fetal breathing increased in all ewes after doxapram administration (Table I). Figure 1 illustrates this effect. The mean percentage time of fetal breathing after doxapram increased from 9.1 per cent to 41.2 per cent (P = 0.06).
% fetal breathing in each five mimute perlod
49
Doxaprom Infusion 2 76 mg/Kg in 3 5 minutes
I
'0°1
+n
751
0 Time
- minutes
FIG.1 The effect of maternally administered doxapram on fetal breathing (sheep No. 2).
When the variation in dose of doxapram is taken into account by dividing the increase in fetal breathing by the dose of drug (mg/kg) the mean increase in the time of fetal breathing (10.56 per cent) is statistically significant (P = 0.025). Doxapram 14C plasma levels from the ewe and the fetus were measured for 90 minutes after the start of the infusion. The maternal plasma level declined rapidly at first and then more slowly after 0.2 to 0 - 5 hours. Figure 2 shows that the fetal plasma levels rose to a peak between 0.08 and 0.17 hours after the start of the infusion and then fell. The mean
TABLE I Increase in fetal breathing with varying doses of doxapram ___
-
Ewe No.
Weight (kg)
Fetal breathing per cent of time
Gestational age (days)
Dose (dkg)
1 hour
before
':Er
Difference
__1
40
116
1.80
0.10
15.33
+15.23
2
37
116
2.76
10.63
42.90
1-32.27
3
52
I19
2.81
29.75
61.25
+31.50
-
~~
4
35
110
2.93
1.19
5.08
+3.89
5
40
119
3.85
3.92
81.33
+77.41
Mean
40.8
116
2.83
9.11
41.18
f32.07
50
HOGG, GOLDING AND ROSEN
Fetal Doxapram cmcentration ymol/l
h
No I
.-----A‘
05
10
15
Time hours
FIG.2 Fetal plasma concentrations of doxapram following administration by slow intravenous injection to the ewe.
fetal-maternal ratio of plasma concentrations was low (less than 0.3) except in a few samples taken at the start. The regression of the log/dose of doxapram (mg/kg) on the increase in fetal breathing was calculated (y = 262 log x--88.3) (p = 0.1). Omitting the animal (No. 4) which had the least increase (less than 5 per cent) in fetal breathing hardly alters the slope but greatly improves the significance of the relationship (y = 273 log X--86) (p = 0.005).
DISCUSSION Boddy and Dawes (1975) stimulated fetal breathing activity by the direct infusion of
catecholamine into the ,fetus. In this study breathing was stimulated by the administration of a drug to the mother. Although the plasma levels of radioactive doxapram included metabolic products, approximately 55 per cent of the 14C activity at 90 minutes was doxapram, therefore the activity in earlier samples must have been mainly due to unmetabolized doxapram. There is also a relationship between the dose of doxapram and the fetal response. Since this is so, this stimulant effect of doxapram on the healthy sheep fetus might be of value in testing human fetal health. Since fetal hypoxia depresses regular respiratory movements (Boddy and Dawes, 1975), doxapram administered to the mother might reveal a change in fetal breathing response, indicating a depressed fetal respiratory mechanism. It is possible that fetal breathing movements could prove a sensitive, and non-invasive, means of studying the effects of drugs which act upon the respiratory system of the human fetus.
ACKNOWLEDGEMENTS We thank South Glamorgan Area Health Authority for facilities at Sully Research Laboratory and A. H. Robins for a grant.
REFERENCES Boddy, K., and Robinson, J. S. (1971): Lancet, 2, 1231. Boddy, K., Dawes, G . S., Fisher, R., Pinter, S., and Robinson, J. S. (1974): Journal of Physiology (London), 243, 599. Boddy, K., and Dawes, G. S. (1975): British Medical Bulletin, 31, 3. Dawes, G. S., Fox, H. E., Leduc, B. M., Liggins, G. C., and Richards, R. T. (1970): Journal of Physiology (London), 210, 47. Dawes, G. S., Fox, H. E., Leduc, B. M., Liggins, G. C., and Richards, R. T. (1972): Journal of Physiology (London), 220, 119. Manning, F., Wyn Pugh, E., and Boddy, K. (1975): British Medical Journal, 1, 552.
THE EFFECT OF DOXAPRAM ON FETAL BREATHING IN THE SHEEP BY
M. I. J. HOGG Department of Anaesthetics, Welsh National School of Medicine Heath Park, Cardif, CF4 4XN
R . H . GOLDING Department of Obstetrics and Gynaecology, Royal Gwent Hospital, Newport AND
M . ROSEN Department of Anaesthetics, Welsh National School of Medicine Heath Park, Cardif, CF4 4XN
Summary In five ewes of 105 to 120 days gestation intravenous doxapram (1 * 8 to 3.85 mg/kg) administered to the mother produced a dose-related increase in fetal breathing. This effect of doxapram might prove useful as a test of human fetal health. Fetal breathing movements could be a sensitive indication of the effects of drugs on the human fetal respiratory system.
the fetal head and neck were delivered. The trachea, a carotid artery and jugular vein were catheterized. A catheter was also placed in the amniotic sac to distinguish other movements from fetal breathing activity. Benzyl penicillin sodium (1 mega unit) and streptomycin (500 mg) were injected into the amniotic sac, the membranes were sutured and the uterine wound was closed. The ewe was left in a single pen for at least three days to ensure full recovery. Each day, the maternal and fetal intravascular lines were filled with heparin (250 unitslml). The ewe was treated daily with penicillin and streptomycin, and the fetus withchloramphenicol (125 mg) administered through the jugular vein catheter. For at least two hours prior to the injection of doxapram the fetal blood pressure, tracheal and amniotic pressures were continuously recorded. Fetal breathing movements were
SPONTANEOUS breathing movements occur in fetal lambs from 40 days gestation onwards (Dawes et al: 1970). It has been demonstrated that pentobarbitone and chloralose depress fetal breathing movements for about one hour (Dawes et al, 1972; Boddy et al, 1974). In man, using an ultrasonic A-scan detector to measure fetal breathing activity (Boddy and Robinson, 1971), cigarette smoking has been shown to reduce that activity for between 35 to 60 minutes (Manning et al, 1975). This study in sheep describes the effects on fetal breathing movements of a respiratory stimulant, doxapram. METHOD Welsh mountain ewes (mean weight 40 kg) of between 105 and 120 days gestation were anaesthetized using thiopentone (1 . 5 mg/kg) and suxamethonium (1 mg/kg) prior to intubation and maintained with a halothaneoxygen-air mixture. After incising the uterus, 48
DOXAPRAM
analyzed by measuring the percentage of each 5 minute period that fetal breathing was present for at least one hour before and after each injection. Doxapram was infused into a maternal vein over 2 to 5 minutes in doses of between 1 -8mg/kg and 3.85 mg/kg of maternal weight. For the first three experiments doxapram was given as 14C labelled drug (A. H. Robins Ltd). After establishing that there was sufficient activity for measurements of plasma levels a 1:2 mixture of doxapram I4C and doxapram was used. Blood samples of 2 ml were taken from maternal and fetal jugular veins at 2, 4, 6, 10, 20, 30, 45, 60 and 90 minutes and the concentration of 14C in plasma was measured by scintillation counting. Since metabolites of doxapram were not separated, the results represent levels of doxapram and metabolites. In two animals, further plasma samples were taken from the ewe and fetus at 90 minutes for analysis of doxapram metabolites by thin layer chromatography. RESULTS Fetal breathing increased in all ewes after doxapram administration (Table I). Figure 1 illustrates this effect. The mean percentage time of fetal breathing after doxapram increased from 9.1 per cent to 41.2 per cent (P = 0.06).
% fetal breathing in each five mimute perlod
49
Doxaprom Infusion 2 76 mg/Kg in 3 5 minutes
I
'0°1
+n
751
0 Time
- minutes
FIG.1 The effect of maternally administered doxapram on fetal breathing (sheep No. 2).
When the variation in dose of doxapram is taken into account by dividing the increase in fetal breathing by the dose of drug (mg/kg) the mean increase in the time of fetal breathing (10.56 per cent) is statistically significant (P = 0.025). Doxapram 14C plasma levels from the ewe and the fetus were measured for 90 minutes after the start of the infusion. The maternal plasma level declined rapidly at first and then more slowly after 0.2 to 0 - 5 hours. Figure 2 shows that the fetal plasma levels rose to a peak between 0.08 and 0.17 hours after the start of the infusion and then fell. The mean
TABLE I Increase in fetal breathing with varying doses of doxapram ___
-
Ewe No.
Weight (kg)
Fetal breathing per cent of time
Gestational age (days)
Dose (dkg)
1 hour
before
':Er
Difference
__1
40
116
1.80
0.10
15.33
+15.23
2
37
116
2.76
10.63
42.90
1-32.27
3
52
I19
2.81
29.75
61.25
+31.50
-
~~
4
35
110
2.93
1.19
5.08
+3.89
5
40
119
3.85
3.92
81.33
+77.41
Mean
40.8
116
2.83
9.11
41.18
f32.07
50
HOGG, GOLDING AND ROSEN
Fetal Doxapram cmcentration ymol/l
h
No I
.-----A‘
05
10
15
Time hours
FIG.2 Fetal plasma concentrations of doxapram following administration by slow intravenous injection to the ewe.
fetal-maternal ratio of plasma concentrations was low (less than 0.3) except in a few samples taken at the start. The regression of the log/dose of doxapram (mg/kg) on the increase in fetal breathing was calculated (y = 262 log x--88.3) (p = 0.1). Omitting the animal (No. 4) which had the least increase (less than 5 per cent) in fetal breathing hardly alters the slope but greatly improves the significance of the relationship (y = 273 log X--86) (p = 0.005).
DISCUSSION Boddy and Dawes (1975) stimulated fetal breathing activity by the direct infusion of
catecholamine into the ,fetus. In this study breathing was stimulated by the administration of a drug to the mother. Although the plasma levels of radioactive doxapram included metabolic products, approximately 55 per cent of the 14C activity at 90 minutes was doxapram, therefore the activity in earlier samples must have been mainly due to unmetabolized doxapram. There is also a relationship between the dose of doxapram and the fetal response. Since this is so, this stimulant effect of doxapram on the healthy sheep fetus might be of value in testing human fetal health. Since fetal hypoxia depresses regular respiratory movements (Boddy and Dawes, 1975), doxapram administered to the mother might reveal a change in fetal breathing response, indicating a depressed fetal respiratory mechanism. It is possible that fetal breathing movements could prove a sensitive, and non-invasive, means of studying the effects of drugs which act upon the respiratory system of the human fetus.
ACKNOWLEDGEMENTS We thank South Glamorgan Area Health Authority for facilities at Sully Research Laboratory and A. H. Robins for a grant.
REFERENCES Boddy, K., and Robinson, J. S. (1971): Lancet, 2, 1231. Boddy, K., Dawes, G . S., Fisher, R., Pinter, S., and Robinson, J. S. (1974): Journal of Physiology (London), 243, 599. Boddy, K., and Dawes, G. S. (1975): British Medical Bulletin, 31, 3. Dawes, G. S., Fox, H. E., Leduc, B. M., Liggins, G. C., and Richards, R. T. (1970): Journal of Physiology (London), 210, 47. Dawes, G. S., Fox, H. E., Leduc, B. M., Liggins, G. C., and Richards, R. T. (1972): Journal of Physiology (London), 220, 119. Manning, F., Wyn Pugh, E., and Boddy, K. (1975): British Medical Journal, 1, 552.
