Respiratory compensation during spontaneous ventilation with the Bain circuit

The volume of carbon dioxide rebreathed by spontaneously breathing patients under halothane anaesthesia at various fresh gas flow rates (FGF) with the Bain modification of the Mapleson "D" breathing circuit is measured. The effect of rebreathing on a heterogeneous patient population is shown to be unpredictable hypercapnia in those patients who cannot respond adequately to this carbon dioxide challenge. All adults rebreathe significant volumes of carbon dioxide at a FGF rate of 100 ml . kg-1 . min-1. This carbon dioxide load is a potential risk to every patient and this hypercapnia is preventable by using high FGF rates. Rebreathing occurs because the inspired carbon dioxide load is unpredictable in a given patient and the patient's response is uncontrolled. Patients respond to this carbon dioxide challenge by increasing inspiratory flow rate (Vt/Ti), which results in increased rebreathing of carbon dioxide from the expiratory limb of the circuit. To prevent potentially dangerous rebreathing of carbon dioxide in all patients the fresh gas flow rate must be much higher than presently recommended.     

Rebreathing and ventilatory response to different fresh gas flows in the Bain and Lack systems. A clinical study

Thirty-four adults were studied during halothane anaesthesia with spontaneous breathing, while undergoing orthopaedic surgery. They were randomly divided into two groups according to whether the Bain (n = 18) or the Lack (n = 16) system was used. Respiratory flows were recorded and arterial blood gases drawn at different fresh gas flows (VF). The values obtained were compared with those recorded under non-rebreathing conditions (NRC). In the Bain system the proportion of rebreathers was 0.22, 0.25, 0.55 and 0.83 when the VF was 175, 150, 125 and 100 ml X min-1 X kg-1 body weight (b.w.), respectively. In the Lack system these proportions were 0.43, 0.55 and 0.92 at VF of 85, 70 and 55 ml X min-1 kg-1 b.w., respectively. The ventilatory response to rebreathing was an increase in minute ventilation (VE), keeping the partial pressure of arterial carbon dioxide (PACO2) almost unaltered. In the Bain system the VE X kg-1 X b.w. thus increased by 18% and 38% at VF of 125 and 100 ml X min-1 X kg-1 b.w., respectively, when compared to NRC (P less than 0.05). The corresponding increases in the Lack system were 15% and 37% at VF of 70 and 55 ml X min-1 X kg-1 b.w., respectively (P less than 0.01). In the Lack group also the PACO2 increased by 6% when a VF of 55 ml X min-1 X kg-1 b.w. was used compared to the value obtained under NRC (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Fresh gas flow in the Bain circuit during laparoscopy

Twenty-two women were studied during laparoscopy with abdominal insufflation of carbon dioxide. A bain anaesthetic breathing circuit was used with a fresh gas flow (VFG) of 110 ml.min-1.kg-1, and controlled ventilation was applied with a minute ventilation (VE) of 175 ml.min-1.kg-1. Arterial blood gases were analysed at the end of the operation. Nineteen of the women (86 per cent) were found to have a PaCO2 within the range for normocapnia (i.e., 4.7-5.9 kPa (35-45 mmHg), two were hypocapnic with a PaCO2 of 4.4 and 4.5 kPa (33 and 34 mmHg) respectively and one was found to have a PaCO2 of 6.2 kPa (46.5 mmHg). It was concluded that the carbon dioxide absorbed from the abdomen during laparoscopy demands fresh gas flows that are higher than normally used in the Bain circuit if a PaCO2 within the normal range is to be obtained. A simultaneous increase in VFG and VE of about 45 per cent is sufficient to achieve normocapnia.     

Arterial Carbon Dioxide Tensions during Anaesthesia with Manual Ventilation A Descriptive Study of the Effects of Various Non-Polluting Circuits

In 660 supine, intubated and anaesthetized, healthy patients scheduled for various elective surgical procedures, the distribution of arterial carbon dioxide tension (PaCO2) was investigated during manual non-monitored ventilation. The study comprised six equal groups: group 1: ventilation with a circle circuit absorber system; group 2: ventilation with the Hafnia A circuit using a total fresh gas flow (FGF) of 100 ml . kg-1 . min-1; groups 3-6: ventilation with a Hafnia D circuit with fresh gas flows of 100, 80, 70 and 60 ml . kg-1 . min-1, respectively. The mean PaCO2's of the first three groups were situated in the lower range of normocapnia (the observations in the first group having the greatest total range), whereas the rebreathing (Hafnia A and D) circuits resulted in a clustering of observed data. Employing the rebreathing circuits, protection against hypocapnia can be achieved by lowering the fresh gas flow. The most satisfying result was obtained with the Hafnia D circuit with a fresh gas flow of 70 ml . kg-1 . min-1 resulting in normocapnia with a modest and limited spread towards hypo- and hypercapnia. FGF in excess of this level must be considered as wasted. The study indicates that corrections of fresh gas flows for age are superfluous. Use of relaxants and type of surgery had no influence on the observations.     

A method for producing normocarbia during general anasthesia for Caesarean section

Twenty-six patients were anaesthetised for Caesarean section using the Bain anaesthetic system for intermittent positive pressure ventilation. There was an inverse relationship between maximum end tidal carbon dioxide tension and the fresh gas flow (FGF) to the system. A significant difference existed between the patients receiving 80 ml/kg/min FGF and those receiving 120 ml/kg/min. Estimated carbon dioxide levels in the pregnant term patient were higher at each FGF rate than the levels reported in non-pregnant patients by other workers. In order to maintain maternal arterial carbon dioxide tension at or close to the normally quoted term value of 4.1-4.4 kPa, when using positive pressure ventilation with a Bain system, a fresh gas flow rate of at least 120 ml/kg body weight/minute is required.     

Fresh gas flow in coaxial Mapleson A and D circuits during spontaneous breathing

In a lung model simulating spontaneously breathing halothane anaesthesia, the rebreathing characteristics of the coaxial Mapleson A (Lack circuit) and D (Bain circuit) systems were tested. Using decreasing fresh gas flows (VF), the end-tidal carbon dioxide fraction (FACO2) was monitored and the point of rebreathing (R.P.) detected. The effects of changes in minute volume (VE), dead-space to tidal volume ratio (VD/VT) and carbon dioxide elimination (VCO2) were studied. The effect of increased tidal volumes (VT) on FACO2 was investigated for some different fresh gas flows (VF). The VF/VE ratio for R.P. in the Bain circuit was approximately 2 and in the Lack circuit 0.88. In both circuits an increase in VE and a decrease in the VD/VT ratio resulted in higher demands on VF if rebreathing was to be avoided. The latter effect was much more pronounced in the Lack circuit. In neither system did any changes in VCO2 affect the rebreathing characteristics. The conclusion was drawn that the Lack system is a much better choice concerning the fresh gas flows for anaesthesia with spontaneous breathing than the Bain system. It was also concluded that the fresh gas flows recommended by Humphrey for the Lack system (i.e. 51 ml X min-1 X kg b.w.-1) and by the manufacturers for the Bain system (i.e. 100 ml X min-1 X kg b.w.-1) are inadequate and should be increased if a considerable degree of rebreathing is to be avoided.     

Metabolic Correlates in Infants and Children During Anaesthesia and Surgery

In 58 infants and children with body weights between 2.8 and 20.5 kg carbon dioxide production (VCO2 ml min-1) was measured during halothane anaesthesia for minor surgical procedures. In 22 cases measurements were made during both spontaneous and controlled ventilation during the same operation. A non-rebreathing circuit was used. Expired ventilation volume was measured with a dry gas meter and expired gas collected during 3-5 min in a Douglas bag. The carbon dioxide fraction of exhaled gas was determined with a sampling Gould capnograph. A respiratory quotient (RQ) of 0.8 was used to calculate oxygen consumption (VO2 ml min-1). During spontaneous breathing, regression analysis of the relationship between VCO2 and kg and between VO2 and kg showed high intercepts while corresponding relations to kg3/4 revealed an almost direct proportionality. Thus, VCO2 and VO2 ought to be related to body weight in kg3/4 in spontaneously breathing children. The mean value (+/- 1 s.d.) for VCO2 was 11.4 +/- 3.1 ml kg-3/4 and for VO2 14.2 +/- 3.9 ml kg-3/4. During controlled ventilation, the relationship between kg b.w. showed for VCO2 as well as for VO2 an almost direct proportionality with a mean value (+/- 1 s.d.) for VCO2 of 6.3 +/- 1.6 ml min-1 kg-1 and for VO2 of 7.8 +/- 2.0 ml min-1 kg-1. Prediction of VO2 for infants and children of this size could be based upon 14 X kg3/4 during halothane anaesthesia and surgery.     

Effects of a heat and moisture exchanger on carbon dioxide equilibrium during mechanical ventilation with the Bain circuit

The introduction of a heat and moisture exchanger (HME) into the anaesthetic circuit may cause a rise in carbon dioxide (CO2) tension through an increase in dead space. We studied the effects of the Ultipor Pall BB50 filter included 'in series' in the Bain circuit on CO2 equilibrium. Arterial carbon dioxide tension (PaCO2) was measured in 81 patients scheduled for elective surgery before and after the insertion of the filter. Results showed that: females were always more hyperventilated than males when fresh gas flow was set at 70 ml kg-1 ideal body weight; the inclusion of the filter increased the PaCO2 in the group as a whole (the difference was statistically, but not clinically, significant); PaCO2 increased after the application of the filter only in females; the effects of the filter were completely independent of the patient's age. It is concluded that the use of the Ultipor Pall BB50 filter is a safe procedure during mechanical ventilation with the Bain breathing system and there is no need to modify ventilation.     

Does geographical location influence inflow requirements of the Bain breathing system?

A review of publications from various countries, using the Bain system with a fresh gas flow of 70 ml kg-1 min-1 and controlled ventilation, show a range of mean PaCO2 values between 36 and 43 mmHg. It was suggested that these differences could be related to the geographic location of the patient population studied. Anaesthetists from seven institutions in West Germany, England, Sweden, the United States, Australia and Canada collaborated in a preliminary study designed to find out whether these differences could be reduplicated. In 142 patients under a standard anaesthesia with controlled ventilation, PaCO2 values were determined 30 min after the fresh gas flows had been set. For 70 ml kg-1 min-1 the mean PaCO2 values ranged from 33 to 40 mmHg; for 100 ml kg-1 min-1 from 28 to 35 mmHg. Compared to the mean PaCO2 values from Canada, the results from Australia and the USA were not different and all at the lower end of this range; Sweden, West Germany and England reported significantly higher PaCO2 values. In the absence of any other obvious explanation, we suggest that patients in England and Northern Europe could have a higher CO2 output under anaesthesia than North American or Australian patients.     

Arterial to end tidal carbon dioxide tension difference during Caesarean section anaesthesia

The relationship between arterial carbon dioxide tension and end tidal carbon dioxide tension was studied in 19 patients during general anaesthesia for Caesarean section. Thirteen patients scheduled for elective abdominal hysterectomy formed a nonpregnant group. There was significant correlation between arterial and end tidal CO2 tensions in both groups. During Caesarean section, this difference was significantly less than in the nonpregnant group.     

High frequency jet ventilation: use of the Bain system for entrainment

The use of a Bain system to convey anaesthetic gases for entrainment during high frequency jet ventilation (HFJV) was evaluated by examining the effect of varying the fresh gas flow (Vf) on the end-tidal carbon dioxide (PECO2) in 46 ASA physical status I and II patients undergoing extracorporeal shock-wave lithotripsy (ESWL). Anaesthesia was induced with methohexitone (1-2 mg.kg-1), fentanyl (1-1.5 micrograms.kg-1) and vecuronium (0.1 mg.kg-1). After endotracheal intubation with a Mallinckrodt Hi-Lo Jet cuffed endotracheal tube, the patient was immersed in a water bath and HFJV at 150 breaths per minute was instituted with an Acutronic AMS 1000 jet ventilator attached to the side channel of the Hi-Lo tube. A Bain system was attached to the proximal end of the endotracheal tube to provide gases for entrainment. Anaesthesia was maintained with an intravenous infusion of methohexitone (5 mg.kg-1.h-1) and 50% nitrous oxide in oxygen for both the jetted and entrained gases. PECO2 was determined at 5-min intervals by a single-breath technique using a calibrated Engstrom Eliza capnograph. Thirty patients were randomly allocated to receive Vf's of 50 (Group 1), 75 (Group 2) and 100 (Group 3) ml.kg-1.min-1, respectively. A further eight patients (Group 4) received a Vf of 100 ml.kg-1.min-1 for 15 min, 75 ml.kg-1.min-1 for the next 15 min and 50 ml.kg-1.min-1 thereafter. In a further group of eight patients (Group 5), Vf was initially 25 ml.kg-1.min-1 for 10 min and was then switched off for the remainder of the procedure.(ABSTRACT TRUNCATED AT 250 WORDS)     

The Bain anaesthetic circuit.

The Bain co-axial circuit is a recent and versatile addition to the semiclosed anaesthetic breathing systems. The relationship between the patient's arterial carbon dioxide tension (PaCO2) and fresh gas flow during intermittent positive pressure ventilation (IPPV) using this circuit has been reassessed. A mean PaCO2 of 33,4 mmHg for 64 patients was recorded using a fresh gas flow of 100 ml/kg/min and a mean PaCO2 of 37,3 mmHg for 55 patients using a fresh gas flow off 70 ml/kg/min.     

Single lever Humphrey A.D.E. low flow universal anaesthetic breathing system. Part I: Comparison with dual lever A.D.E., Magill and Bain systems in anaesthetized spontaneously breathing adults

The single lever Humphrey A.D.E. anaesthetic system, in both coaxial and parallel (non-coaxial) forms, has recently been introduced. In principle the system offers efficient "universal" function by combining the advantages of Mapleson A, D and E systems. A within-patient comparison of its function in the Mapleson A mode (lever up) in spontaneously-breathing anaesthetized subjects was made to that of the original two lever A.D.E., the Magill (Mapleson A) and the Bain (Mapleson D) systems. The coaxial and parallel single lever A.D.E. systems functioned identically to each other and to the original two lever A.D.E. system, a mean fresh gas flow (FGF) of 51 ml X kg-1 X min-1 causing minimal rebreathing. Under identical conditions, the mean FGF required to just cause rebreathing increased to a mean of 71 ml X kg-1 X min-1 and 150 ml X kg-1 X min-1 with the Magill and the Bain systems respectively. With the single lever system, the switch to its Mapleson E mode for controlled ventilation involves the selection of the only alternative lever position (lever down) without further adjustment. The function and practical advantages in this E mode are presented in Part II.     

Carbon dioxide output in laparoscopic cholecystectomy

In pneumoperitoneum, carbon dioxide eliminated in expired gas (carbon dioxide output) contains both metabolic and absorbed carbon dioxide from the peritoneal cavity. When elimination of carbon dioxide is much higher than carbon dioxide output, storage of tissue carbon dioxide and arterial carbon dioxide concentrations change. Finally, the rate of carbon dioxide eliminated in expired gas is not a match for the real rate of metabolic production and absorbed carbon dioxide from the peritoneal cavity. During and after insufflation of carbon dioxide, changes in carbon dioxide output were elucidated under constant arterial carbon dioxide pressure (PaCO2), the same as the preinduction level. We studied patients undergoing elective laparoscopic cholecystectomy. Carbon dioxide output, oxygen uptake, respiratory exchange ratio (RER), expired minute ventilation (VE), deadspace to tidal volume ratio (VD/VT ratio) and arterial to end-tidal carbon dioxide partial pressure difference (PaCO2-PE'CO2) were determined before induction, and during anaesthesia, pneumoperitoneum and recovery. By controlling ventilatory frequency (f) every 1 min, PaCO2 was adjusted to concentrations before induction. Constant monitoring of end-tidal carbon dioxide partial pressure (PE'CO2) and intermittent measurement of (PaCO2-PE'CO2) (15-min intervals) were conducted to predict PaCO2). Carbon dioxide output and oxygen uptake decreased significantly from mean values of 83.5 (SEM 5.2), 101.6 (5.1) to 68.5 (4.2), 81.1 (4.6) ml min-1 m-2 (ATPS, P < 0.05) with sevoflurane anaesthesia, and RER did not change. During carbon dioxide pneumoperitoneum (intra-abdominal pressure 8 mm Hg), carbon dioxide output increased by 49% (102.4 (5.0) ml min-1 m-2) (P < 0.05) while oxygen uptake remained stable and RER increased from 0.84 (0.02) to 1.16 (0.03) (P < 0.05). It was necessary to increase VE during pneumoperitoneum by 1.54 times that during anaesthesia to maintain individual PaCO2 values constant. After removal of carbon dioxide from the abdominal cavity, the regression equation of excess carbon dioxide output/BSA best fitted a two-compartment model. The time constants of the rapid and slow compartments were 8.2 and 990 min, respectively. Excess carbon dioxide output/BSA was still 5.5 ml min-1 m-2, 30 min after pneumoperitoneum.      

Arterial to end-tidal carbon dioxide tension difference during anaesthesia for tubal ligation

Twenty-nine patients scheduled for postnatal tubal ligation by minilaparotomy under general anaesthesia were studied. Arterial and end-tidal carbon dioxide tensions were determined during anaesthesia. The mean arterial to end-tidal carbon dioxide tension difference was 0.08 kPa (SEM 0.05). Thirty-one percent of the patients had negative values. These results were similar to those observed during Caesarean section. The physiological changes responsible for reduced arterial to end-tidal carbon dioxide values, persist into the postnatal period. It is predicted from the regression analysis of the time between delivery and anaesthesia for tubal ligation and arterial to end-tidal CO2 difference, that the values might return to normal nonpregnant levels by 8 days following delivery.     

Cerebral Blood Flow and CO2 Reactivity Is Similar during Remifentanil/N2 O and Fentanyl/N2 O Anesthesia

Background: Remifentanil, a rapidly metabolized [micro sign]-opioid agonist, may offer advantages for neurosurgical procedures in which prolonged anesthetic effects can delay assessment of the patient. This study compared the effects of remifentanil-nitrous oxide on cerebral blood flow (CBF) and carbon dioxide reactivity with those of fentanyl-nitrous oxide anesthesia during craniotomy.

Methods: After institutional approval and informed patient consent were obtained, 23 patients scheduled to undergo supratentorial tumor surgery were randomly assigned to remifentanil or fentanyl infusion groups in a double-blinded manner. Midazolam, thiopental, and pancuronium induction was followed by equipotent narcotic loading infusions of remifentanil (1 [micro sign]g [middle dot] kg-1 [middle dot] min-1) or fentanyl (2 [micro sign]g [middle dot] kg-1 [middle dot] min-1) for 5-10 min. Patients were ventilated with 2:1 nitrous oxide-oxygen, and opioid rates were reduced and then titrated to a stable hemodynamic effect. After dural exposure, CBF was measured by the intravenous133 xenon technique at normocapnia and hypocapnia. Reactivity of CBF to carbon dioxide was calculated as the absolute increase in CBF per millimeters of mercury increase in the partial pressure of carbon dioxide (PaCO2). Data were analyzed by repeated-measures analysis of variance, unpaired Student's t tests, or contingency analysis.

Results: In the remifentanil group (n = 10), CBF decreased from 36 +/- 11 to 27 +/- 8 ml [middle dot] 100 g-1 [middle dot] min-1 as PaCO2 decreased from 33 +/- 5 to 25 +/- 2 mmHg. In the fentanyl group (n = 8), CBF decreased from 37 +/- 11 to 25 +/- 6 ml [middle dot] 100 g-1 [middle dot] min-1 as PaCO2 decreased from 34 +/- 3 to 25 +/- 3 mmHg. Absolute carbon dioxide reactivity was preserved with both agents: 1 +/- 1.2 ml [middle dot] 100 g-1 [middle dot] min-1 [middle dot] mmHg-1 for remifentanil and 1.5 +/- 0.5 ml [middle dot] 100 g-1 [middle dot] min-1 [middle dot] mmHg-1 for fentanyl (P = 0.318).     

Anesthesia for cesarean section and excision of pheochromocytoma caused by Sipple's syndrome]

A case of a 23-year-old primigravida with a tumour of the left adrenal gland and a medullary thyroid carcinoma is reported. Her blood pressure remained at about 100/60 mmHg throughout pregnancy. She was scheduled for elective Caesarean section combined with removal of both adrenal glands. Anaesthesia was carried out using 10 micrograms.kg-1 alfentanil, 5 mg.kg-1 thiopentone, 1.5 mg.kg-1 succinylcholine and 0.5 vol % enflurane. A single hypertensive crisis (190/100 mmHg) occurred intraoperatively, during dissection of the left adrenal gland. This responded well to 1 mg.min-1 phentolamine. The postoperative course was uneventful for both the mother and the child. Total thyroidectomy with block dissection of the lymph nodes was to be carried out within three weeks after the Caesarean section. Only two similar cases of multiple endocrine neoplasia associated with pregnancy have previously been published.     

Alfentanil infusions: relationship between pharmacokinetics and pharmacodynamics in man

Fourteen fit young patients undergoing body surface surgery received an infusion of alfentanil at either 50 or 100 micrograms kg-1 hr-1 to supplement nitrous oxide anaesthesia. The alfentanil infusion was continued for two hours post-operatively at the lower rate of 20 micrograms kg-1 hr-1. Resting ventilation, carbon dioxide responsiveness and pain scores were measured post-operatively. Values for clearance and elimination half life were similar to data following single doses of alfentanil but showed considerable interindividual variation. However, there was a greater systemic clearance (P = 0.02) when determined using the post-infusion decay data compared with that calculated during anaesthesia (527 ml min-1 compared with 434 ml min-1). This is in accord with observations for other intravenous drugs.     

Disposition of propofol infusions for caesarean section

The disposition of propofol was studied in women undergoing elective Caesarean section. Indices of maternal recovery and neonatal assessment were correlated with venous concentrations of propofol. After induction of anaesthesia with propofol 2.0 mg.kg-1, ten patients received propofol 6 mg.kg-1.hr-1 with nitrous oxide 50 per cent in oxygen (low group) and nine were given propofol 9 mg.kg-1.hr-1 with oxygen 100 per cent (high group). Pharmacokinetic variables were similar between the groups. The mean +/- SD Vss = 2.38 +/- 1.16 L.kg-1, Cl = 39.2 +/- 9.75 ml.min-1.kg-1 and t1/2 beta = 126 +/- 68.7 min. At the time of delivery (8-16 min), the concentration of propofol ranged from 1.91-3.82 micrograms.ml-1 in the maternal vein (MV), 1.00-2.00 micrograms.ml-1 in the umbilical vein (UV) and 0.53-1.66 micrograms.ml-1 in the umbilical artery (UA). Neonates with high UV concentrations of propofol at delivery had lower neurologic and adaptive capacity scores 15 minutes later. The concentrations of propofol were similar between groups during the infusion but they declined at a faster rate in the low group postoperatively. Maternal recovery times did not depend on the total dose of propofol but the concentration of propofol at the time of eye opening was greater in the high group than the low group (1.74 +/- 0.51 vs 1.24 +/- 0.32 micrograms.ml-1, P less than 0.01). The rapid placental transfer of propofol during Caesarean section requires propofol infusions to be given cautiously, especially when induction to delivery times are long.     

Predictable Paco2, with two different flow settings using the Mapleson D system

Two different settings of fresh gas flow (VFG) and minute ventilation (VE) used with the coaxial Mapleson D system (Bain), were evaluated in 59 adults (ASA I-III) during controlled ventilation and different types of surgical procedures. The two flow settings (alternatives A and B) were VFG of 75 and 110 ml.min-1.kg-1 and VE of 150 and 175 ml.min-1.kg-1, aiming to generate normocapnea and mild hypocapnea, respectively. The PaCO2 obtained with alternative A was 5.5 +/- 0.5 kPa (mean +/- s.d.), with 92% of the patients within the range 4.7-6.1 kPa. With alternative B, the PaCO2 was 4.4 +/- 0.5 kPa, with 82% of the patients within the range 3.5-4.9 kPa. It is concluded that these two flow regimes are suitable for clinical use when either normocapnea or mild hypocapnea is desired.