SummaryWe have investigated the effect of oxygen flow rate on pre-oxygenation in pregnant patients at term using a circle system. Twenty patients presenting for elective Caesarean section maintained tidal volume breathing through a standard circle system for 3 min. Subjects were pre-oxygenated using oxygen flow rates of 5 l. Effective pre-oxygenation is a requirement for safe practice of general anaesthesia in pregnancy [1,2]. Increased maternal oxygen consumption combined with decreased functional residual capacity result in the potential for hypoxaemia to develop during apnoea following induction of anaesthesia [3,4]. This may be further compounded by pregnancy associated airway changes, which can contribute to delay in securing a definitive airway [3]. Ensuring that pre-oxygenation is performed to a high standard minimises the risk of hypoxaemia developing and prolongs the safe duration of apnoea following induction of anaesthesia. The aim of this study was to establish how the circle breathing system and tidal volume breathing technique could be employed to provide optimal pre-oxygenation in term pregnant patients. It is our clinical experience that the circle system has become commonplace in obstetric theatres in the United Kingdom. Previous studies investigating the use of the circle system in parturients have used fixed oxygen flow rates for tidal volume breathing pre-oxygenation [2,5]. We proposed that higher fresh gas oxygen flow rates would increase the efficiency of denitrogenation within the circle system, thereby optimising pre-oxygenation. Our study design incorporated a commonly employed breathing system and routine preoxygenation technique with the intention that the results would be applicable to everyday clinical situations encountered by anaesthetists in obstetric theatres. MethodsFollowing approval by the North Sheffield Research Ethics Committee, 20 women presenting for elective Caesarean section under regional anaesthesia gave informed, written consent to participate in the study. Subject had completed 36 weeks of gestation. Exclusion criteria included pregnancy related complications, suspected fetal compromise and pulmonary disease.The age, antenatal booking weight, height and gestation of the subjects were recorded. Pre-oxygenation was performed in a supine position with left uterine displacement by means of a 15°firm, rubber wedge placed under the right hip. Subjects maintained tidal volume breathing through a facemask for 3 min. For our study, we used the circle system integral to the Aestiva ⁄ 5 anaesthetic machine (Datex-Ohmeda, Stirling, UK) with a 2 l

1. The action of angiotensin II on lung vessels was compared in dogs, cats, ferrets and rats using isolated perfused lungs and lobes of lungs perfused in vivo, both at constant flow, so that increases in inflow pressure at constant outflow pressure indicated increases in resistance. 2. Angiotensin II caused some increases in resistance in dogs, cats and rats but not ferrets. The increases were small compared with changes in the systemic circulation. Larger increases could be obtained in isolated lungs where there was no interference from rises in left atrial pressure. 3. All four species showed pulmonary vasoconstriction during hypoxia and ferret lungs were especially responsive. Thus angiotensin II cannot be the mediator of hypoxic vasoconstriction in all these species. 4. Angiotensin II significantly increased the response to hypoxia in isolated cat and rat lungs while also increasing baseline inflow pressure; the response to hypoxia was not altered in ferrets. Angiotensin II may therefore enhance reactivity of pulmonary vessels. 5. Verapamil, an inhibitor of calcium transport across cell membranes, reduced the response to hypoxia in rats and ferrets more than it reduced the response to angiotensin II in rats or prostaglandin F2 alpha in ferrets. Thus hypoxic pulmonary vasoconstriction probably involves transport of calcium.

Pulmonary vasomotor actions of histamine and the possible relationship of histamine to hypoxic pulmonary vasconstriction were studied in anaesthetized cats with one lobe of lung perfused at constant flow and in isolated perfused rat and ferret lungs. In the cat histamine caused dilatation, biphasic responses and constriction with increasing doses. Histamine induced dilatation was better demonstrated during hypoxic vasoconstriction and was reduced by an H2 histamine antagonist; constriction with histamine was abolished by an H1 antagonist. kistamine also caused both vasodilatation and vasoconstriction in ferret lungs. A mast cell stabilizing agent had no effect on hypoxic pulmonary vasoconstriction in cats or rats. This response was unaffected in cats but greatly reduced in rats and ferrets by cyproheptadine, a combined histamine and 5-hydroxytryptamine inhibitor. It was unaffected in cats but abolished in ferrets by an H1 histamine inhibitor. It was again unaffected in cats but greatly reduced in rats and ferrets bv an H2 histamine inhibitor. These species differences may reflect differences in mechanism but more probably reflect nonspecific effects of the inhibitors in certain circumstances. However, when drugs nearly abolished hypoxic vasoconstriction, ATP still caused vasoconstriction.Histamine constricts pulmonary vascular and alveolar duct smooth muscle. It may be released from mast cells and discharged into pulmonary veins during hypoxia [Haas and Bergofsky, 1972]. In rats the pulmonary vasoconstriction caused by hypoxia was modified by drugs inhibiting, releasing or enhancing histamine [Hauge, 1968]. It has therefore been proposed that histamine might be a transmitter responsible for hypoxic vasoconstriction. The hypothesis cannot be sustained in its original form. Histamine has been shown to cause pulmonary vasodilatation as well as vasoconstriction [Dawes and Mott, 1962;Barer, 1966;Shaw, 1971;Silove and Simcha, 1973;Howard, Barer, Thompson, Warren, Abbott and Mungall, 1975;Thompson, Barer and Shaw, 1976], and the drugs affecting hypoxic vasoconstriction in the rat are ineffective in other species [Barer and McCurrie, 1969;Howard et al, 1975;Tucker, Weir, Reeves and Grover, 1976].Our objective was first to clarify the two actions of histamine in terms of dose-response relationships and by the use of antagonistic drugs to see whether they are attributable to the two types of histamine receptor [Ash and Schild, 1966]. Second, we tried to clarify the relation ofhistamine to hypoxic pulmonary vasoconstriction. We used three species. We compared the cat in which H1 histamine receptor antagonists have not abolished hypoxic vasoconstriction

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A dequate preoxygenation must be performed to a high standard to minimize the risk of hypoxemia and to prolong the safe duration of apnea after general anesthesia is induced in pregnant patients. This study examined how the circle breathing system and tidal volume breathing technique could be used to provide optimal preoxygenation in term pregnant women. This study looked at the efficacy of different fresh gas flow rates to increase the efficiency of denitrogenation within the circle system and to optimize maternal preoxygenation.Twenty women at the authors' institution who were at 36-week gestation or greater and presented for elective cesarean section under regional anesthesia were included. Prior to each period of preoxygenation, oxygen was allowed to flow through the circle system until an FiO 2 of 1.0 was achieved. Patients were preoxygenated 3 consecutive times using flow rates of 5 L/min, 10 L/min, and 15 L/min in random order. Baseline fractional end-tidal oxygen (F E O 2 ) and fractional end-tidal carbon dioxide (F E CO 2 ) were obtained before preoxygenation and then were recorded every 10 seconds during preoxygenation using a gas analyzer. The tidal capnography trace was observed by the investigator during the study to monitor adequacy of mask seal. Between each preoxygenation session, baseline values were recorded with the patient in an upright position breathing room air for 5 minutes at the end of which F E O 2 and F E CO 2 were recorded.The patients had a mean age of 30.3 ± 5.2 years and gestational age of 38.9 ± 1.0 week. The mean fractional endtidal oxygen values at the 5, 10, and 15-L/min flow rates were 0.86, 0.92, and 0.90, respectively. Compared with the flow rate at 5 L/min, the values at flow rates of 10 and 15 L/min values were significantly higher (P<0.001 respectively), although preoxygenation was not improved by increasing the flow from 10 to 15 L/min. Entrainment of air occurred on 4 occasions each at flow rates of 5 and 10 L/min and 5 occasions at 15 L/min. In 4 of these instances, a minimal change in fractional end-tidal carbon dioxide occurred.Oxygen flow rates Z10 L/min within the circle breathing system can provide optimal preoxygenation to parturients who maintain tidal volume breathing for 3 minutes. Entrainment of air occurred in a surprisingly high percentage of cases.