Preoxygenation of Pregnant and Nonpregnant Women in the Head-Up Versus Supine Position Anis s. Baraka, MD, FCAnaesth(Hon), Myrna T. Hanna, MD, Samar I. Jabbour, MD, Maud F. Nawfal, MD, Abla A. N. Sibai, MS, Vanda G. Yazbeck, Nawal I. Khoury, MD, and Karam S. Karam, MD
MD,
Departments of Anesthesiology, Obstetrics and Gynecology, and Epidemiology and Biostatistics, American University of Beirut, Beirut, Lebanon
The influence of preoxygenation in the supine (n = 10) versus the 45" head-up (n = 10) position on the duration of apnea leading to a decrease in arterial oxygen saturation to 95%, as monitored by pulse oximetry, was investigated in 20 women undergoing elective cesarean section at term of pregnancy. The results were compared with those obtained in a control group of 20 nonpregnant women. In the supine position, the average time to desaturation to 95% was significantly shorter in the pregnant group (173 2 4.8 s [mean f SD]) than in the control group
P
reoxygenation is mandatory before "rapidsequence" induction of general anesthesia in pregnant women (1,2), because they are at high risk for regurgitation and aspiration of gastric contents. Preoxygenation results in denitrogenation of the lungs (3), leading to an increase in oxygen reserve and delayed onset of hypoxia during apnea. During pregnancy, the functional residual capacity of the lungs (FRC) is decreased, with further reduction occurring in the supine position (4).This decrease of FRC is compounded by the increase in oxygen consumption (5). As a result, the reduction in arterial oxygen tension during apnea is significantly greater in pregnant than in nonpregnant women (1). Raising the awake pregnant woman to the upright position increases FRC (3) and Pao, and reduces Paco, while breathing room air (6). Thus, it is speculated that arterial desaturation during apnea may be delayed in the pregnant patient if induction of anesthesia is accomplished in the head-up position (7,8). We therefore investigated, in parturients about to undergo cesarean section, the effects of the supine Accepted for publication June 2, 1992. Address correspondence to Dr. Baraka, Department of Anesthesiology, American University of Beirut, Beirut, Lebanon. 01992 by the International Anesthesia Research Society OOO3-2999/9Y$5.OO
of nonpregnant women (243 f 7.4 s). Using the head-up position resulted in an increase in the desaturation time in the nonpregnant group (331 f 7.2 s) but had no significant effect in the pregnant group (156 ? 2.8 s). We conclude that pregnant women desaturate their arterial blood of oxygen more rapidly than do nonpregnant women. Furthermore, the head-up position extends the duration of apnea that can take place before desaturation occurs in nonpregnant patients. (Anesth Analg 1992;75757-9)
versus the 45" head-up position on the duration of apnea, which would lead to a decrease in arterial oxygen saturation to 95%.The results were compared with those obtained in a control group of nonpregnant women.
Methods The investigation was approved by the Institutional Research Committee, and informed consent was obtained from all patients. Twenty term-pregnant patients about to undergo elective cesarean section under general anesthesia were enrolled in the investigation. Patients with a history of hypertension, preeclampsia, anemia, diabetes, chronic smoking, or other conditions that might influence respiratory function or oxygen delivery were excluded. Patients with signs of fetal distress or abnormality were also excluded. Preoxygenation, rapid-sequence induction of anesthesia, and tracheal intubation were accomplished in the supine position in 10 patients and in the 45" head-up position in the remaining 10. In all cases, arterial oxygen saturation was monitored continuously, with a pulse oximeter applied to the thumb, beginning while patients were breathing room air. They were then given oxygen (8 L/min) via Anesth Analg 1992;75:757-9
757
758
ANESTH ANALG 1992;75:757-9
OBSTETRIC ANESTHESIA BARAKA ET AL PREOXYGENATION OF THE PARTURIENT
Table 1. Clinical Characteristics of Patients in the Four Study Groups ~
~~
~
Nonpregnant control group Supine Age (yr) Weight (kg) Preop So,% Time to So, 95% ( s )
Cesarean section group
Head-up (n = 10)
Supine (n = 10)
Head-up
( n = 10) 31.2 2 2.9 65.2 f 9.1 98.1 2 1.5 243 t 7.4
32.7 t 5.9 61.9 t 11.6 98.5 2 0.94 331 t 7.Zb
29.5 f 4.5 70.9 2 12.8" 97.5 2 1.3 173 f 4.8"
28.6 2 6.2 72.4 f 7.0" 97.9 f 0.77 156 2 2.8"
(n = 10)
Preop So,%, preoperative arterial oxygen saturation; Time to So, 95%, time of apnea during which So, of 95% was reached Mean 2 SD. "Significantly different from the corresponding value in the control group b$ gnificantly , ' different from supine group.
the circle system of an anesthesia machine and a tight-fitting face mask over a period of 3 min. A rapid-sequence induction of anesthesia was accomplished after application of cricoid pressure using ketamine (1.5 mg/kg). Succinylcholine (1.5 mgkg) was injected intravenously and the trachea intubated. Thereafter, the tracheal tube was left open to room air, and a succinylcholine infusion was administered to maintain apnea. Monitoring of arterial oxygen saturation was continued throughout the period of apnea, and the time to reach 95% saturation (desaturation time) was recorded. For reasons of safety, an arterial oxygen saturation of 95% was considered our endpoint. The endotracheal tube was then connected to the anesthesia circuit and the patient's lungs were ventilated with 100% oxygen until an arterial oxygen saturation of 100% was restored. Thereafter, all patients were placed in the supine position for surgery. Anesthesia was maintained after delivery with nitrous oxide-oxygen (2:1), supplemented with vecuronium. Apgar scores of the newborns were recorded at 1 and 5 min. Oximetry results were compared with those obtained in a control group of 20 nonpregnant women undergoing gynecologic procedures, such as hysterectomy or tuba1 ligation, using similar preoxygenation and anesthetic techniques. Ten of the nonpregnant control patients were studied in the supine position; the other 10 were in the 45" head-up position. (As in the pregnant group, all patients having any disease that might influence respiratory function or oxygen delivery were excluded.) The control oxygen saturation while breathing room air, the peak saturation obtained after preoxygenation, and the mean times of apnea at which 95% saturation were reached were compared. All values were expressed as mean -+ SD. The Student's t-test was used for statistical analysis; P < 0.05 was considered significant.
Results Demographic Data The two groups of patients were similar in age, but the mean body weight of the pregnant patients was significantly greater (Table 1). The control arterial oxygen saturation while breathing room air was not significantly different in pregnant and nonpregnant patients. After the 3 min of preoxygenation, the oxygen saturation increased to 100% in all subjects. During apnea in the supine position, the average time to desaturation to 95% was significantly shorter in the pregnant than in the nonpregnant control patients. Use of the head-up position resulted in a significant increase in the desaturation time in the nonpregnant but not the pregnant women (Table 1). In all patients, subsequent ventilation with 100% oxygen was followed by a transient decrease in saturation to 93%-94% for 5-10 s, with a rapid increase to 100% saturation thereafter. In the cesarean section group, all newborns had an Apgar score 2 7 at 1 min and 2 8 at 5 min.
Discussion The present study shows that desaturation after preoxygenation occurred more rapidly in apneic pregnant than in nonpregnant women, whether positioned in the supine or the 45" head-up tilt position. In the cesarean section group, the mean desaturation time to 95% was 173 t 4.8 s in the supine position and 156 k 2.8 s in the head-up position, a nonsigruficant difference. In contrast, the desaturation time in the nonpregnant women increased from 243 -+ 7.4 s in the supine position to 331 2 7.2 s in the head-up position. During pregnancy, the decrease in FRC (4) results in a reduction in oxygen reserve that is compounded by the increased oxygen consumption (5). During apnea, the oxygen reserve in the FRC will be exhausted more quickly in the pregnant than in the
ANESTH ANALG 1992:75757-9
nonpregnant woman. Similarly, in obese patients arterial desaturation is significantly faster than in normal patients, with a linear correlation between time to desaturation and degree of obesity (9,lO). In obese patients, the FRC is further reduced in the supine position (11). Our data also show that preoxygenation in the head-up position resulted in an increase of the safe duration of apnea in nonpregnant patients but was not associated with any significant change in pregnant patients. These results were unexpected, because a change from the supine to the sitting position has been shown to increase the FRC in both the pregnant (4) and nonpregnant patient (12). It is possible that adopting the 45" head-up rather than the sitting position may not significantly increase the FRC in the pregnant patient at term. In contrast, the FRC may significantly increase with position change in the nonpregnant woman, because most of the alteration in FRC takes place between the horizontal and 60" head-up tilt position (12). In conclusion, the present study shows that after 3 min of preoxygenation, desaturation during subsequent apnea, as monitored by pulse oximetry, is more rapid in pregnant than in nonpregnant patients. Changing from the supine to the 45" head-up position prolongs the desaturation time in the nonpregnant patient but has no effect in the pregnant woman.
OBSTETRIC ANESTHESIA BARAKA ET AL. PREOXYGENATION OF THE PARTURIENT
759
References 1. Archer GW, Marx GF. Arterial oxygen tension during apnoea in parturient women. Br J Anaesth 1974;46:35MO. 2. Norris MC, Dewan DM. Preoxygenation for cesarean section: a comparison of two techniques. Anesthesiology 1985;62:827-9. 3. Carmichael FJ, Cruise CJE, Crago RR, Paluck S. Preoxygenation: a study of denitrogenation. Anesth Analg 1989;68:406-9. 4. Russell IF, Chambers WA. Closing volume in normal pregnancy. Br J Anaesth 1981;53:1043-7. 5. Pernoll ML, Metcalf J, Schlenker TL, Welch JE, Matsumoto JA. Oxygen consumption at rest and during excercise in pregnancy. Respir Physiol 1975;25:2&93. 6. Ang CK, Tan TH, Walters WAW, Wood C. Postural influence on maternal capillary oxygen and carbon dioxide tension. Br Med J 1969;4:2013. 7. Snow RG, Nunn JF. Induction of anaesthesia in the foot-down position for patients with a full stomach. Br J Anaesth 1959;31: 493-7. 8. Hodges RJH, TunstaIl ME, Bennett JR. Vomiting and the head-up position. Br J Anaesth 1960;32:619-20. 9. Jense HG, Dubin SA, Silverstein PJ, OLeary-Escolas U.Effect of obesity on safe duration of apnea in anesthetized humans. Anesth Analg 1991;72:89-93. 10. Berthoud MC, Peacock JE, Reilly CS. Effectiveness of preoxygenation in morbidly obese patients. Br J Anaesth 1991;67 464-6. 11. Damia RW, Mascheroni D, Croci M, Tarenzi L. Preoperative changes in functional residual capacity in morbidly obese patients. Br J Anaesth 1988;66:574-8. 12. Nunn JF. Elastic forces and lung volumes. In: Nunn JF, ed. Applied respiratory physiology. 3rd ed. London: Butterworths, 19873940.
MD,
Departments of Anesthesiology, Obstetrics and Gynecology, and Epidemiology and Biostatistics, American University of Beirut, Beirut, Lebanon
The influence of preoxygenation in the supine (n = 10) versus the 45" head-up (n = 10) position on the duration of apnea leading to a decrease in arterial oxygen saturation to 95%, as monitored by pulse oximetry, was investigated in 20 women undergoing elective cesarean section at term of pregnancy. The results were compared with those obtained in a control group of 20 nonpregnant women. In the supine position, the average time to desaturation to 95% was significantly shorter in the pregnant group (173 2 4.8 s [mean f SD]) than in the control group
P
reoxygenation is mandatory before "rapidsequence" induction of general anesthesia in pregnant women (1,2), because they are at high risk for regurgitation and aspiration of gastric contents. Preoxygenation results in denitrogenation of the lungs (3), leading to an increase in oxygen reserve and delayed onset of hypoxia during apnea. During pregnancy, the functional residual capacity of the lungs (FRC) is decreased, with further reduction occurring in the supine position (4).This decrease of FRC is compounded by the increase in oxygen consumption (5). As a result, the reduction in arterial oxygen tension during apnea is significantly greater in pregnant than in nonpregnant women (1). Raising the awake pregnant woman to the upright position increases FRC (3) and Pao, and reduces Paco, while breathing room air (6). Thus, it is speculated that arterial desaturation during apnea may be delayed in the pregnant patient if induction of anesthesia is accomplished in the head-up position (7,8). We therefore investigated, in parturients about to undergo cesarean section, the effects of the supine Accepted for publication June 2, 1992. Address correspondence to Dr. Baraka, Department of Anesthesiology, American University of Beirut, Beirut, Lebanon. 01992 by the International Anesthesia Research Society OOO3-2999/9Y$5.OO
of nonpregnant women (243 f 7.4 s). Using the head-up position resulted in an increase in the desaturation time in the nonpregnant group (331 f 7.2 s) but had no significant effect in the pregnant group (156 ? 2.8 s). We conclude that pregnant women desaturate their arterial blood of oxygen more rapidly than do nonpregnant women. Furthermore, the head-up position extends the duration of apnea that can take place before desaturation occurs in nonpregnant patients. (Anesth Analg 1992;75757-9)
versus the 45" head-up position on the duration of apnea, which would lead to a decrease in arterial oxygen saturation to 95%.The results were compared with those obtained in a control group of nonpregnant women.
Methods The investigation was approved by the Institutional Research Committee, and informed consent was obtained from all patients. Twenty term-pregnant patients about to undergo elective cesarean section under general anesthesia were enrolled in the investigation. Patients with a history of hypertension, preeclampsia, anemia, diabetes, chronic smoking, or other conditions that might influence respiratory function or oxygen delivery were excluded. Patients with signs of fetal distress or abnormality were also excluded. Preoxygenation, rapid-sequence induction of anesthesia, and tracheal intubation were accomplished in the supine position in 10 patients and in the 45" head-up position in the remaining 10. In all cases, arterial oxygen saturation was monitored continuously, with a pulse oximeter applied to the thumb, beginning while patients were breathing room air. They were then given oxygen (8 L/min) via Anesth Analg 1992;75:757-9
757
758
ANESTH ANALG 1992;75:757-9
OBSTETRIC ANESTHESIA BARAKA ET AL PREOXYGENATION OF THE PARTURIENT
Table 1. Clinical Characteristics of Patients in the Four Study Groups ~
~~
~
Nonpregnant control group Supine Age (yr) Weight (kg) Preop So,% Time to So, 95% ( s )
Cesarean section group
Head-up (n = 10)
Supine (n = 10)
Head-up
( n = 10) 31.2 2 2.9 65.2 f 9.1 98.1 2 1.5 243 t 7.4
32.7 t 5.9 61.9 t 11.6 98.5 2 0.94 331 t 7.Zb
29.5 f 4.5 70.9 2 12.8" 97.5 2 1.3 173 f 4.8"
28.6 2 6.2 72.4 f 7.0" 97.9 f 0.77 156 2 2.8"
(n = 10)
Preop So,%, preoperative arterial oxygen saturation; Time to So, 95%, time of apnea during which So, of 95% was reached Mean 2 SD. "Significantly different from the corresponding value in the control group b$ gnificantly , ' different from supine group.
the circle system of an anesthesia machine and a tight-fitting face mask over a period of 3 min. A rapid-sequence induction of anesthesia was accomplished after application of cricoid pressure using ketamine (1.5 mg/kg). Succinylcholine (1.5 mgkg) was injected intravenously and the trachea intubated. Thereafter, the tracheal tube was left open to room air, and a succinylcholine infusion was administered to maintain apnea. Monitoring of arterial oxygen saturation was continued throughout the period of apnea, and the time to reach 95% saturation (desaturation time) was recorded. For reasons of safety, an arterial oxygen saturation of 95% was considered our endpoint. The endotracheal tube was then connected to the anesthesia circuit and the patient's lungs were ventilated with 100% oxygen until an arterial oxygen saturation of 100% was restored. Thereafter, all patients were placed in the supine position for surgery. Anesthesia was maintained after delivery with nitrous oxide-oxygen (2:1), supplemented with vecuronium. Apgar scores of the newborns were recorded at 1 and 5 min. Oximetry results were compared with those obtained in a control group of 20 nonpregnant women undergoing gynecologic procedures, such as hysterectomy or tuba1 ligation, using similar preoxygenation and anesthetic techniques. Ten of the nonpregnant control patients were studied in the supine position; the other 10 were in the 45" head-up position. (As in the pregnant group, all patients having any disease that might influence respiratory function or oxygen delivery were excluded.) The control oxygen saturation while breathing room air, the peak saturation obtained after preoxygenation, and the mean times of apnea at which 95% saturation were reached were compared. All values were expressed as mean -+ SD. The Student's t-test was used for statistical analysis; P < 0.05 was considered significant.
Results Demographic Data The two groups of patients were similar in age, but the mean body weight of the pregnant patients was significantly greater (Table 1). The control arterial oxygen saturation while breathing room air was not significantly different in pregnant and nonpregnant patients. After the 3 min of preoxygenation, the oxygen saturation increased to 100% in all subjects. During apnea in the supine position, the average time to desaturation to 95% was significantly shorter in the pregnant than in the nonpregnant control patients. Use of the head-up position resulted in a significant increase in the desaturation time in the nonpregnant but not the pregnant women (Table 1). In all patients, subsequent ventilation with 100% oxygen was followed by a transient decrease in saturation to 93%-94% for 5-10 s, with a rapid increase to 100% saturation thereafter. In the cesarean section group, all newborns had an Apgar score 2 7 at 1 min and 2 8 at 5 min.
Discussion The present study shows that desaturation after preoxygenation occurred more rapidly in apneic pregnant than in nonpregnant women, whether positioned in the supine or the 45" head-up tilt position. In the cesarean section group, the mean desaturation time to 95% was 173 t 4.8 s in the supine position and 156 k 2.8 s in the head-up position, a nonsigruficant difference. In contrast, the desaturation time in the nonpregnant women increased from 243 -+ 7.4 s in the supine position to 331 2 7.2 s in the head-up position. During pregnancy, the decrease in FRC (4) results in a reduction in oxygen reserve that is compounded by the increased oxygen consumption (5). During apnea, the oxygen reserve in the FRC will be exhausted more quickly in the pregnant than in the
ANESTH ANALG 1992:75757-9
nonpregnant woman. Similarly, in obese patients arterial desaturation is significantly faster than in normal patients, with a linear correlation between time to desaturation and degree of obesity (9,lO). In obese patients, the FRC is further reduced in the supine position (11). Our data also show that preoxygenation in the head-up position resulted in an increase of the safe duration of apnea in nonpregnant patients but was not associated with any significant change in pregnant patients. These results were unexpected, because a change from the supine to the sitting position has been shown to increase the FRC in both the pregnant (4) and nonpregnant patient (12). It is possible that adopting the 45" head-up rather than the sitting position may not significantly increase the FRC in the pregnant patient at term. In contrast, the FRC may significantly increase with position change in the nonpregnant woman, because most of the alteration in FRC takes place between the horizontal and 60" head-up tilt position (12). In conclusion, the present study shows that after 3 min of preoxygenation, desaturation during subsequent apnea, as monitored by pulse oximetry, is more rapid in pregnant than in nonpregnant patients. Changing from the supine to the 45" head-up position prolongs the desaturation time in the nonpregnant patient but has no effect in the pregnant woman.
OBSTETRIC ANESTHESIA BARAKA ET AL. PREOXYGENATION OF THE PARTURIENT
759
References 1. Archer GW, Marx GF. Arterial oxygen tension during apnoea in parturient women. Br J Anaesth 1974;46:35MO. 2. Norris MC, Dewan DM. Preoxygenation for cesarean section: a comparison of two techniques. Anesthesiology 1985;62:827-9. 3. Carmichael FJ, Cruise CJE, Crago RR, Paluck S. Preoxygenation: a study of denitrogenation. Anesth Analg 1989;68:406-9. 4. Russell IF, Chambers WA. Closing volume in normal pregnancy. Br J Anaesth 1981;53:1043-7. 5. Pernoll ML, Metcalf J, Schlenker TL, Welch JE, Matsumoto JA. Oxygen consumption at rest and during excercise in pregnancy. Respir Physiol 1975;25:2&93. 6. Ang CK, Tan TH, Walters WAW, Wood C. Postural influence on maternal capillary oxygen and carbon dioxide tension. Br Med J 1969;4:2013. 7. Snow RG, Nunn JF. Induction of anaesthesia in the foot-down position for patients with a full stomach. Br J Anaesth 1959;31: 493-7. 8. Hodges RJH, TunstaIl ME, Bennett JR. Vomiting and the head-up position. Br J Anaesth 1960;32:619-20. 9. Jense HG, Dubin SA, Silverstein PJ, OLeary-Escolas U.Effect of obesity on safe duration of apnea in anesthetized humans. Anesth Analg 1991;72:89-93. 10. Berthoud MC, Peacock JE, Reilly CS. Effectiveness of preoxygenation in morbidly obese patients. Br J Anaesth 1991;67 464-6. 11. Damia RW, Mascheroni D, Croci M, Tarenzi L. Preoperative changes in functional residual capacity in morbidly obese patients. Br J Anaesth 1988;66:574-8. 12. Nunn JF. Elastic forces and lung volumes. In: Nunn JF, ed. Applied respiratory physiology. 3rd ed. London: Butterworths, 19873940.
