Predicted normocapnea in infants and children using the Bain circuit with controlled ventilation

We have constructed a nomogram for fresh gas flow (VFG) and minute ventilation (VE) for paediatric anaesthesia during controlled ventilation using the Bain coaxial Mapleson D circuit. VFG was based upon the assumption of a high fresh gas utilization because of a low VFG/VE ratio (0.67) and known figures of carbon dioxide elimination. The formulas VFG = 27.8 x VCO2 and VE = 1.5 x VFG were used to calculate the necessary flows to generate normocapnea. The nomogram was evaluated in 59 children (6-62 kg, age 5 months-14 years). PaCO2 (mean +/- s.d.) was 5.0 +/- 0.5 kPa (38 +/- 4 mmHg) with a total range of 3.9-6.3 kPa (29-47 mmHg). Ninety percent of the children had a PaCO2 of 5.7 kPa (43 mmHg) or lower. There was no correlation between body weight and PaCO2. Hence, there was no difference in mean values between children below or above a body weight of 20 kg.     

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.     

High-frequency small-volume ventilation in anesthetized humans

Pulmonary gas exchange during conventional mechanical ventilation (CMV) (tidal volume 10 ml/kg, rate 8-10 breaths/min) was compared with that during high-frequency small-volume ventilation (HFV) in 67 patients undergoing anesthesia for various surgical procedures. HFV was studied at oscillation frequencies ranging from 3 to 18 Hz with stroke volumes of 0.8 to 2.2 ml/kg. Adequate pulmonary gas exchange was achieved with CMV and HFV, and the efficiency of oxygenation, that is, (A-a)DO2, was similar in the two conditions. During HFV, the lung volume was higher than during CMV in most patients. Muscle paralysis did not significantly change either PaCO2 or PaO2. In general, increasing fresh gas flow into the HFV system above approximately 10 1/min resulted in little reduction in PaCO2, but reduction of fresh gas flow below approximately 6 1/min increased PaCO2 progressively. Currently, we do not recommend HFV at 12-18 Hz for routine use during anesthesia for orthopedic or abdominal surgery.     

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 Circle System Without Carbon Dioxide Absorption

An anaesthetic circle system without a carbon dioxide absorber is described. The efficiency of the circle, i.e., the fraction of alveolar gas in the outflow from the circle, was measured in 15 patients during halothane anaesthesia or neurolept analgesia. The fraction ranged from 0.88 to 0.95 (mean 0.91), while the ratio between the alveolar ventilation and the fresh gas inflow ranged from 0.97 to 1.71. The efficiency was not correlated to this ratio. There was no need for hyperventilation if the fresh gas inflow was 10% higher than the alveolar ventilation required to maintain normal PaCO2. The circle was used in 50 patients manually ventilated by nurse anaesthetists. Mean fresh gas inflow was 60 ml/kg. Mean PaCO2 was 5.47 kPa (41 mmHg). In a similar group of 50 other patients, in which the standard circle used in the department was employed, the mean PaCO2 was 4.80 kPa (36 mmHg). The frequency of hypercapnia was equal in the two groups, but hypocapnia was not seen when the circle without absorber was used.     

Is the Bain breathing circuit the future anesthesia system? An evaluation

The Bain breathing circuit, a modified Mapleson D system, was evaluated with regard to oxygenation and CO2 elimination under controlled conditions and compared with the presently popular semiclosed breathing circuit (SCBC) with CO2 absorber. The authors demonstrated that the Bain system compares favorably with the SCBC in regard to oxygenation of manually ventilated patients with fresh gas inflow of 70 ml/kg/min while maintaining a PaCO2 at a mean of 38 torr versus an SCBC mean of 32 torr. The authors were impressed with the clinical simplicity, efficiency, and versatility of the Bain system and believe that it will play a major role in the future of anesthesia-machine design.     

A new concept in controlled ventilation of children with the Bain anesthetic circuit

The Bain anesthesia circuit was studied as a semi-open or partial rebreathing system during controlled ventilation in 16 children weighing from 7.5 to 48 kg. During anesthesia the lungs were ventilated with a volume ventilator set at three times the calculated alveolar ventilation to provide optimum mixing in the exhalation tube of the Bain circuit. Fresh gas inflow rates initially were set equal to the calculated alveolar ventilation, and after 30 to 45 min, PCO2, PO2 and pH values were measured. At the same time, the fractional concentration of mixed expired carbon dioxide (FECO2) was recorded from a capnograph inserted between the ventilator and the Bain circuit. After initial readings, the fresh gas inflow was varied over a range of 1,400-3,000 ml/m2/min at 20-min intervals, with the arterial blood-gas values and FECO2 recorded at each setting. The results indicate that a lower fresh gas inflow than previously recommended can be used safely in children. When the minute ventilation is three times the fresh gas inflow, values for FECO2 correlate closely with PaCO2 values; with a fresh gas inflow of 2,500 ml/m2/min,PaCO2 values can be maintained near 40 torr.     

Flow requirements for the Bain breathing circuit during anaesthesia for caesarean section

We studied the relationship between arterial carbon dioxide tension (PaCO2) and fresh gas flow (FGF) during use of the Bain breathing circuit for Caesarean section anaesthesia. Thirty-one patients undergoing Caesarean section were anaesthetised using the Bain circuit with intermittent positive pressure ventilation. The PaCO2 were measured at FGF of 70 ml X kg-1 X min-1, 80 ml X kg-1 X min-1, and 100 ml X kg-1 X min-1. The FGF requirement to maintain a given PaCO2 during Caesarean section anaesthesia is the same as the requirements for nonpregnant subjects, despite the increase in carbon dioxide production associated with pregnancy. This is probably because the total FGF determined by body weight and given during Caesarean section anaesthesia is 15-20 per cent higher than nonpregnant levels, due to the weight gain associated with pregnancy. A FGF of 100 ml X kg-1 of pregnant weight/min maintains PaCO2 of 4.44 kPa predelivery, which is in the desirable range of PaCO2 during Caesarean section.     

Progressive hypoxemia, hypercarbia and hyperthermia associated with prolonged anesthesia--a case report

The authors report a case of 66-year-old female patient, 55 kg, ASA I who, under general anesthesia in supine position, developed gradual hypoxemia (from a baseline PaO2 of 250 to 91 mmHg), carbon dioxide build up (from a baseline PaCO2 31 to 41 mmHg) associated with gradual hyperthermia up to 38.3 degrees C over seven hours, intraoperatively. These observations were noted while using a semi-closed carbon dioxide absorption circuit in conjunction with the Hygroster filter at a fresh gas flow of 4 1/min of 50% nitrous oxide in oxygen. While the ventilation pattern was unchanged throughout the procedure, there was a change in exhaled tidal and minute ventilation volume with a net decrease of 28 ml and 0.4 l/min respectively. Findings are probably the result of pulmonary atelecatasis under general anesthesia due to the use of a relatively high-inspired oxygen concentration (50%). In addition, the use of a high humidity and temperature heat moisture exchanger (HME) filter (Hygroster) in conjunction with the circle absorber system may have resulted in over humidification and aggravated the pulmonary atelecatasis over the long operative time.     

Initial clinical experience with a more efficient hollow fiber oxygenator of unique design

The first 90 cardiac surgery cases perfused with a new hollow fiber membrane oxygenator in which the gas flows through the fibers and blood flows around the fibers are reported. The fibers are microporous polypropylene with a pore size of 0.03 microns. Membrane surface area is 2.0 M2 and priming volume is 480 ml, including heat exchanger PaO2 is controlled by FIO2 and PaCO2 by gas flow rate. Patients as large as 2.36 M2 were perfused up to 348 min using hemodilution and hypothermia. The mean PaO2 was 200 mmHg and the mean PaCO2 39.5 mmHg. Oxygen transfer was as high as 230 ml/min. This low prime device transfers large volumes of gas, an efficiency which results from a crossed arrangement of the fibers to break up laminar flow of the blood around them. The low priming volume makes it appropriate for use in all but the smallest patients.     

The responsiveness of cerebral blood flow to changes in arterial carbon dioxide is maintained during propofol-nitrous oxide anesthesia in humans.

Because it is common to manipulate PaCO2 during neurosurgery, it is essential to characterize the relationship between cerebral blood flow (CBF) and changes in PaCO2. The purpose of this study was to investigate the effects of propofol-N2O anesthesia on the CBF response to changes in PaCO2 in healthy subjects. In seven patients, anesthesia was induced with propofol 2.0-2.5 mg/kg and then maintained with a propofol infusion of 12 mg.kg-1.h-1 for 10 min and then 9 mg.kg-1.h-1 for 10 min and then was reduced to 3-6 mg.kg-1.h-1 for the remainder of the study. The subjects' lungs were ventilated with N2O in O2 (FIO2 0.3) to the end-tidal CO2 present before anesthesia, and then CBF was measured using intravenous 133Xe and ten scintillation counters, five over each cerebral hemisphere. ETCO2 then was increased to 50 mmHg and CBF measurement repeated; ETCO2 then was reduced to 30 mmHg and CBF measurement repeated. Concurrent with each CBF measurement, arterial blood was sampled for PaCO2 and hemoglobin measurement. CBF at normocapnia (PaCO2 42 +/- 2 mmHg) was 33 +/- 7 ml.100 g-1.min-1, which increased to 58 +/- 10 ml.100 g-1.min-1 and decreased to 19 +/- 4 ml.100 g-1.min-1 on increasing PaCO2 (53 +/- 4 mmHg) and decreasing PaCO2 (31 +/- 2 mmHg), respectively. Both the PaCO2 and CBF values were statistically different from those measured at any other time (CBF P less than 0.002, PaCO2 P less than 0.001). The slope of CBF versus PaCO2 was 1.56 ml.100 g-1.min-1.mmHg.(ABSTRACT TRUNCATED AT 250 WORDS)     

A simple method to reduce the inspiratory oxygen fraction for high pulmonary blood flow patients in an operating room

BACKGROUND: Low inspired oxygen acutely increases pulmonary vascular resistance and decreases pulmonary-systemic blood flow ratio. We present a simple method to lower inspired oxygen fraction (FIO2<0.21) without supplemental nitrogen, during mechanical ventilation by an anesthesia machine. METHODS: After institutional approval, seven healthy adult volunteers and three infants (0-12 month old) scheduled for congenital heart surgery were enrolled in this study. All the infants were diagnosed with congestive heart failure because of high pulmonary blood flow and were thought to benefit from low FIO2. The volunteers performed spontaneous ventilation (fresh air flow rate=10 l.min(-1), tidal volume=600 ml, frequency=10 br.min(-1)). The infants were mechanically ventilated with air (fresh air flow rate=6 l.min(-1), tidal volume=10 ml.kg(-1), 1515%, fresh gas flow rates were increased to adjust FIO2 to 0.21. RESULTS: In all of the seven volunteers and three infants target FIO2 was achieved in <10 min. FIO2 was kept at 0.18+/-0.01 (SD) by calculated fresh air flow rates. In one infant, SpO2 decreased >15% 20 min after lowering FIO2, we had to discontinue this study, and increase fresh gas flow to ventilate the infant with FIO2 0.21. In the other two infants, FIO2 was maintained throughout the study. CONCLUSIONS: This simple and convenient method to decrease FIO2, has a utility in clinical situations, in which pulmonary vascular resistance is to be increased to improve systemic oxygen delivery in patients with high pulmonary blood flow during cardiac surgery.     

Response of cerebral blood flow to changes in carbon dioxide tension during hypothermic cardiopulmonary bypass

Changes in cerebral blood flow (CBF) in response to changes in PaCO2 were measured by intraaortic injection of 133Xe in 12 patients during hypothermic (23-30 degrees C) cardiopulmonary bypass. In each patient, CBF was determined at two randomly ordered levels of PaCO2 obtained by varying the rate of gas inflow into the pump oxygenator (Group I, n = 6) or by varying the percentage of CO2 added to the gas inflow (Group II, n = 6). Nasopharyngeal temperature, mean arterial pressure, pump-oxygenator flow, and hematocrit were maintained within a narrow range. In group I, a PaCO2 (uncorrected for body temperature) of 36 +/- 4 mmHg (mean +/- SD) was associated with a CBF of 13 +/- 5 ml X 100 g-1 X min-1, while a PaCO2 of 42 +/- 4 mmHg was associated with a CBF of 19 +/- 10 ml X 100 g-1 X min-1. In group II, a PaCO2 of 47 +/- 3 mmHg was associated with a CBF of 20 +/- 8 ml X 100 g-1 X min-1, and a PaCO2 of 53 +/- 3 mmHg was associated with a CBF of 26 +/- 9 ml X 100 g-1 X min-1. Within group I, the difference in CBF was significant (P less than 0.05); within group II, the difference in CBF was significant at the P less than 0.002 level. All CBF measurements were lower than those reported for normothermic, unanesthetized subjects of similar age.(ABSTRACT TRUNCATED AT 250 WORDS)     

How rebreathing anaesthetic systems control PaCO2: studies with a mechanical and a mathematical model

Results from a proposed equation for rebreathing systems, (formula: see text), were compared with results from a mechanical model "lung" ventilated either with a Bain Breathing Circuit, or a circle system (Eger-Ethans type A) without soda lime. When values for carbon dioxide excretion (VCO2), dead spacetidal volume ratio (VD/VT), minute volume ventilation (VE), and fresh gas flow (VF) were varied over a wide range, the model and the equation yielded identical values of PaCO2. When VCO2 = 2.25 ml/kg, VD/VT = 0.5, and VE = 140 ml/kg, then fresh gas flows (VF) with both the equation and the model produced values of PaCO2 which were very close to those found by Bain and Spoerel in anaesthetized, artificially ventilated humans. It is concluded that the equation is an accurate mathematical representation of how rebreathing anaesthetic systems control PaCO2. It is expected that the equation will be useful in the clinical application of rebreathing anaesthetic systems, allowing the selection of minute volumes and fresh gas flows which will yield predictable PaCO2 values.     

An experimental study of the influence of positive end-expiratory pressure for a bidirectional cavopulmonary shunt

In 8 mongrel dogs (weight 9-13 kg), we created a bidirectional cavopulmonary shunt through 4th intercostal thoracotomy. Positive end-expiratory pressure (PEEP) was added from 0cmH2O to 16cmH2O at the steps of 2cmH2O. The heart rate (HR), central venous pressure (CVP), pulmonary artery pressure (PAP), femoral artery pressure (FAP), pulmonary vascular resistance index (PVRI), and systemic vascular resistance index (SVRI) were measured as parameters of hemodynamics. Cardiac output (CO), pulmonary artery flow at proximal and distal site of this shunt (D-SF, P-SF) were measured using a magnetic flow meter. Blood gas analysis (PH, PaO2, PaCO2, HCO3-) were performed at the same time. HR had no significant change. CVP, PAP, PVRI, SVRI increased significantly (p less than 0.05, p less than 0.05, p less than 0.05, p less than 0.05) at 2cmH2O (9.2 +/- 2.5 mmHg), 10cmH2O), (29.3 +/- 5.5 mmHg), 4cmH2O (287 +2- 56 dyne.sec.cm-5.m2), and 8cmH2O (1298 +/- 156 dyne.sec.cm-5.m2) compared with 0cmH2O (87.3 +/- 2.6 mmHg, 26.8 +/- 3.4 mmHg, 240 +/- 29 dyne.sec.cm-5.m2, 1136 +/- 176 dyne.sec.cm-5.m2). FAP, CO, D-SF, P-SF decreased significantly (p less than 0.01, p less than 0.05, p less than 0.01, p less than 0.05) at 6cmH2O (129 +/- 7 mmHg), 2cmH2O(0.44 +/- 0.05 L/min), 2cmH2O(449 +/- 47 ml/min), and 8cmH2O(105 +/- 17 ml/min) compared with 0cmH2O(148 +/- 11 mmHg, 048 +/- 0.06 L/min, 471 +/- 44 ml/min, 132 +/- 19 ml/min). On blood gas analysis, PaO2 increased significantly (p less than 0.05) from 2cmH2O PEEP except PH, PaCO2, HCO3-. A mechanism for decline in D-SF was considered of being a secondary effect due to increase in CVP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiac output measurement using the transesophageal Doppler method is less accurate than the thermodilution method when changing PaCO2

Cardiac output (CO) determination using transesophageal Doppler is based on the measurement of descending aortic blood flow. Because cerebral blood flow is dependent on PaCO2, an increase in PaCO2 would result in an increase of CO because of the increase in cerebral blood flow and vice versa. We enrolled 30 patients undergoing off-pump coronary artery graft surgery in the study. The CO was determined by both transesophageal Doppler and thermodilution while PaCO2 was maintained at either 30 mmHg or 40 mmHg in random order. The CO by thermodilution was significantly higher at PaCO2 of 40 mmHg (4.17 +/- 0.94 L/min) than at 30 mmHg (3.78 +/- 0.85 L/min). On the other hand, there were no significant differences in CO by transesophageal Doppler: 3.85 +/- 0.76 L/min at PaCO2 of 40 mmHg and 3.77 +/- 0.74 at 30 mmHg. Bland-Altman analysis yielded bias and precision of -0.32 and 0.49 L/min at PaCO2 of 40 mmHg, and -0.01 and 0.34 L/min at 30 mmHg. These results indicate that both methods of CO measurement are in agreement at 30 mmHg of PaCO2, but the thermodilution method provides higher values at 40 mmHg of PaCO2.     

Effect of nitrogen on carbon dioxide elimination during continuous flow apneic ventilation in dogs

Continuous endobronchial insufflation of air in paralyzed animals (continuous flow apneic ventilation - CFAV) has been shown to maintain adequate oxygenation and carbon dioxide removal. CFAV in patients using oxygen resulted in adequate oxygenation but a mean rise in PaCO2 of 0.6 mmHg/min (0.08 kPa/min). This experiment compared carbon dioxide removal in dogs with air and oxygen. Ten dogs were anesthetized and paralyzed, and CFAV was used for 1 h with either air or oxygen in a randomized fashion. Adequate oxygenation was obtained with air and oxygen. Normal PaCO2 levels were obtained with air; however, in the animals where oxygen was used, PaCO2 levels rose to a mean of 6.45 +/- s.e. mean 0.4 kPa (48.5 +/- s.e.mean 3.2 mmHg).     

Respiratory waveform and rebreathing in T-piece circuits: a comparison of enflurane and halothane waveforms

The effects of respiratory waveform on rebreathing in a modified Mapleson D circuit were studied in 18 healthy adult patients anesthetized with either enflurane or halothane. At high fresh gas flow (FGF) rate, when no rebreathing of CO2 occurred, the duration of inspiration (Ti) with enflurane was 41 per cent greater than that with halothane. With enflurane there was a characteristic long end-expiratory pause, 0.69 s, whereas with halothane it was only 0.196 s. The mean inspiratory flow rate (Vt/Ti) was higher (224 ml/s) when halothane was used than with enflurane (187 ml/s). When the FGF rate was reduced to 100 ml/kg/min in the modified Mapleson D circuit, patients breathing halothane had increases in minute volumes (VE) in response to increases of 53-75 per cent in inspired volumes of CO2. The increases in VE resulted fro increases in Vt/Ti of 34-38 per cent. The volume of CO2 inspired when enflurane was used did not increase until FGF rate was as low as 70 ml/kg/min. The reduced rebreathing was related to the respiratory waveform. The advantage of reduced rebreathing with enflurane is counter-balanced by the more profound respiratory depression it causes. The FGF needed to abolish rebreathing of CO2 is highly variable, and is dependent on respiratory waveform.     

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.