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.     

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.     

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.     

Arterial to end-tidal carbon dioxide tension difference during anaesthesia in early pregnancy

Sixteen patients requiring general anaesthesia for termination of pregnancy by dilatation and evacuation of the uterus were studied. Arterial and end-tidal carbon dioxide tensions were determined during anaesthesia. The mean arterial to end-tidal carbon dioxide tension difference was 0.07 kPa (-0.02-0.16, 5-95 per cent confidence limits). These results were similar to those observed during Caesarean section and those during anaesthesia for post-delivery tubal ligations. The physiological changes such as increased cardiac output, haemodilution, and increased blood volume which manifest by 12 weeks of gestation probably result in a reduced (a-E')PCO2 value.     

The effect of increased apparatus dead space and tidal volumes on carbon dioxide elimination and oxygen saturations in a low-flow anesthesia system

STUDY OBJECTIVE: To determine if a large tidal volume (VT), with an unchanged end-tidal carbon dioxide partial pressure (PETco2), could improve arterial carbon dioxide elimination, oxygen saturation (Spo2), and arterial blood oxygenation. DESIGN: Prospective, randomized, clinical study. SETTING: Single university hospital. PATIENTS: 60 ASA physical status I and II patients scheduled for elective urologic or general surgery. INTERVENTIONS: Patients were randomly assigned to one of two treatments: patients in group 1, nondead space (NDS), received a fresh gas flow of 1 L/min without added apparatus dead space volume. Patients in group 2, dead space (DS), received ventilation using an added dead space volume between the Y-piece and tracheal tube. In both groups, patients' lungs were ventilated to a fixed PETco2 value of 33.8 mmHg. Patients in the DS group were ventilated with VTs to maintain an airway plateau pressure (Pplateau) of 0.04 cm H2O/kg over initial plateau pressure. The corrugated tube was then adjusted to maintain a fixed PETco2. MEASUREMENTS: Dead space volumes, PETco2, arterial CO2 tension (Paco2), SpO2, arterial O2 tension (Pao2), VTs, and airway pressures were measured. MAIN RESULTS: Arterial CO2 tension was significantly lower in the DS group, 36 +/- 2.3 mmHg, compared with the NDS group, 37.5 +/- 2.3 mmHg (P < 0.05), and the difference between PETco2 and Paco2 was lower in the DS group than in the NDS group (P < 0.001). Oxygen saturation was 99% +/- 1.0% in the DS group compared with 98.5% +/- 1.5% in the NDS group (P < 0.05). Arterial O2 tension was 13.2 +/- 25.5 mmHg in the DS group and 119.1 +/- 30.2 mmHg in NDS group (not significant). CONCLUSION: Larger VTs, with an unchanged PETCO2 concentration created by an added apparatus dead space volume, improved arterial carbon dioxide elimination.     

Pressure Support Ventilation versus Continuous Positive Airway Pressure with the Laryngeal Mask Airway: A Randomized Crossover Study of Anesthetized Adult Patients

Background: The authors tested the hypothesis that pressure support ventilation (PSV) provides more effective gas exchange than does unassisted ventilation with continuous positive airway pressure (CPAP) in anesthetized adult patients treated using the laryngeal mask airway.

Methods: Forty patients were randomized to two equal-sized crossover groups, and data were collected before surgery. In group 1, patients underwent CPAP, PSV, and CPAP in sequence. In group 2, patients underwent PSV, CPAP, and PSV in sequence. PSV comprised positive end expiratory pressure set at 5 cm H2O and inspiratory pressure support set at 5 cm H2O above positive end expiratory pressure. CPAP was set at 5 cm H2O. Each ventilatory mode was maintained for 10 min. The following data were recorded every minute for the last 5 min of each ventilatory mode and the average reading taken: end tidal carbon dioxide, oxygen saturation, expired tidal volume, leak fraction, respiratory rate, noninvasive mean arterial pressure, and heart rate.

Results: In both groups, PSV showed lower end tidal carbon dioxide (P < 0.001), higher oxygen saturation, (P < 0.001), and higher expired tidal volume (P < 0.001) compared with CPAP. In both groups, PSV had similar leak fraction, respiratory rate, mean arterial pressure, and heart rate compared with CPAP. In group 1, measurements for CPAP were similar before and after PSV. In group 2, measurements for PSV were similar before and after CPAP.     

Arterial to end-tidal carbon dioxide tensio difference during laparoscopy Magnitude and effect of anaesthetic technique

The relationship between arterial carbon dioxide tension and end-tidal carbon dioxide tension was studied in 25 patients during laparoscopy. Thirteen patients received general anaesthesia and 12 epidural anaesthesia. The overall mean difference between arterial and end-tidal carbon dioxide tensions was 0.44 kPa (95% confidence intervals 0.28-0.60 kPa) which was significantly less than that reported in studies during other procedures. The reasons for this difference are probably associated with the physiological changes induced by CO2 pneumoperitoneum and steep Trendelenburg positioning. The choice of anaesthetic technique did not affect the arterial to end-tidal carbon dioxide tension difference significantly (p greater than 0.9).     

Lung function, breathing pattern, and gas exchange in interstitial lung disease.

BACKGROUND: The aim of this study was to determine the relation between the severity of abnormalities in ventilatory function tests and tidal breathing pattern and gas exchange indices in interstitial lung disease. METHODS: Pulmonary function, ventilation, carbon dioxide production, oxygen consumption, arterial blood gas tensions, and pH were measured during resting steady state conditions in 60 patients with proved interstitial lung disease. Patients were categorised by forced vital capacity (FVC) (percentage of predicted values) as having a mild, moderate, or severe restrictive defect with means (SD) of 71% (4%), 57% (4%), and 41% (7%) of predicted values, respectively. RESULTS: FVC varied from 29% to 79% of predicted values and from 0.99 l to 4.32 l. The two measurements of FVC correlated strongly with most static lung volumes and with transfer factor for carbon monoxide. Mean respiratory rates (per minute) and tidal volumes (ml) were 17 (4) and 484 (131), 20 (4) and 460 (139), and 23 (5) and 377 (109) in mild, moderate, and severe restrictive defects, respectively. FVC correlated negatively with respiratory rate and positively with tidal volume. Arterial carbon dioxide tension ranged from 30 to 49 mm Hg; only two patients were hypercapnic. Mean arterial oxygen tensions were not significantly different among the three groups, and there were no significant correlations between forced expiratory volume in one second or FVC and arterial carbon dioxide tension or carbon dioxide production. CONCLUSION: Low values of FVC were associated with increased respiratory rate and decreased tidal volume; this pattern of breathing mimics external elastic loading, suggesting that mechanoreceptors may contribute to the rapid and shallow pattern of breathing in interstitial lung disease. Hypercapnia seems to be rare in interstitial lung disease even when functional impairment is severe and tidal volume is small. The increased respiratory rate is important in maintaining adequate ventilation. In the face of a severe restrictive defect carbon dioxide production did not increase, which also contributed to the maintenance of eucapnia.     

End tidal carbon dioxide monitoring in spontaneously breathing, nonintubated patients. A clinical comparison between conventional sidestream and microstream capnometers

BACKGROUND: To evaluate the end tidal carbon dioxide estimation in nonintubated, spontaneously breathing patients using either conventional sidestream or microstream capnometers. METHODS: Patients received a regional anesthesia technique, while the end tidal carbon dioxide partial pressure (EtCO2) was sampled through a nasal cannula (Nasal FilterLine, Nellcor, Plesanton, CA, USA) and measured using either a conventional sidestream capnometer with a 200 ml.min-1 aspiration flow rate, or a microstream capnometer (NBP-75, Nellcor Puritan Bennett, Plesanton, CA, USA) with an aspiration flow rate of 30 ml.min-1. After a 20 min period with stable hemodynamic variables (systolic arterial blood pressure within +/- 20% from baseline values), the EtCO2 was randomly recorded using one of the two capnometer while arterial blood was simultaneously drawn from the radial artery and analyzed for measurement of arterial CO2 partial pressure. Afterwards the nasal cannula was connected to the other capnometer and the procedure repeated. Both the capnometer and arterial blood gas analyzer were calibrated before each studied patient according to the manufacturer instructions. The same procedure was repeated at least two times in each patient. RESULTS: A total of 120 pairs of EtCO2 and PaCO2 measurements were drawn from 30 adults (age: 69 +/- 5 years; weight: 70 +/- 10 kg; height: 160 +/- 10 cm): 60 using the conventional sidestream capnometer and 60 with the microstream one. The median arterial to end tidal CO2 tension difference was 4.4 mmHg (range: 0.28 mmHg) with the microstream capnometer and 7 mm Hg (range: 0-22 mmHg) with the conventional capnometer (p = 0.02). CONCLUSION: The microstream capnometer provides a more accurate end tidal CO2 partial pressure measurement in nonintubated, spontaneously breathing patients than conventional sidestream capnometers, allowing for adequate monitoring of the respiratory function in nonintubated patients.     

Cerebral and peripheral oxygen saturation during red cell transfusion

BACKGROUND: Changes in regional hemoglobin oxygen saturation occur in response to blood transfusion and can be measured by near infrared spectroscopy. PATIENTS AND METHODS: Cerebral (CsO2) and peripheral (PsO2) oxygen saturation were monitored with an INVOS 4100 near infrared spectroscopy oximeter in 29 patients undergoing 84 intraoperative blood transfusions during aortic or spinal surgery. Hemoglobin concentration was measured before and after transfusion. Mean arterial pressure, end tidal carbon dioxide tension, and arterial oxygen saturation were also monitored. RESULTS: Mean arterial pressure, arterial oxygen saturation and end tidal carbon dioxide tension remained stable during transfusion, while CsO2 rose by a mean (95% CI) of 4.2 (3.2-5.2%; P = 0.001) and PsO2 rose by a mean (95% CI) of 1.6 (0.3-2.8%; P = 0.016). The rise in CsO2 correlated well with the rise in hemoglobin (r = 0.59, P < 0.001) and with the volume transfused (r = 0.58, P < 0.001). PsO2 correlated with the volume transfused (r = 0.35, P = 0.019) but not with hemoglobin concentration (r = 0.08, P = 0.47). CONCLUSIONS: Near infrared spectroscopy detected significant rises in tissue oxygenation in response to blood transfusion, particularly in the cerebral cortex. CsO2 may be developed into a blood loss monitor if further research confirms our findings.     

Alveolar partial pressures of carbon dioxide and oxygen measured by a helium washout technique.

A non-invasive technique was developed for measuring alveolar carbon dioxide and oxygen tension during tidal breathing. This was achieved by solving the Bohr equations for mean alveolar carbon dioxide and oxygen tensions (PACO2, PAO2) from known values of the dead-space:tidal volume ratio measured by helium washout, and from the mixed expired partial pressure of carbon dioxide and oxygen. The derived values of wPACO2 and wPAO2 were compared with PaCO2 obtained from arterial gas analysis and PAO2 calculated from the ideal air equation. Four normal subjects and 58 patients were studied. Calculated and measured PCO2 values agreed closely with a difference in mean values (wPACO2 - PaCO2) of 0.01 kPa; the SD of the differences was 0.7 kPa. The difference in mean values between wPAO2 and PAO2 was 0.02 kPa; the SD of the differences was 0.93 kPa. The method is simple and not time consuming, and requires no special cooperation from the patients. It can be applied in the laboratory or at the bedside to any subject breathing tidally. Physiological deadspace:tidal volume ratio, PAO2 and PACO2, static lung volumes, respiratory exchange ratio, carbon dioxide production, oxygen uptake, tidal volume, and total ventilation can be measured with acceptable accuracy and reproducibility in one test. An arterial blood sample is needed initially to provide an independent measure of PaCO2 and for measurement of the alveolar-arterial PO2 difference. Subsequently, PaCO2 can be estimated from wPACO2 sufficiently well for clinical purposes and PaO2 or SaO2 can be monitored by non-invasive methods.     

Pressure support ventilation versus continuous positive airway pressure with the laryngeal mask airway: a randomized crossover study of anesthetized adult patients

BACKGROUND: The authors tested the hypothesis that pressure support ventilation (PSV) provides more effective gas exchange than does unassisted ventilation with continuous positive airway pressure (CPAP) in anesthetized adult patients treated using the laryngeal mask airway. METHODS: Forty patients were randomized to two equal-sized crossover groups, and data were collected before surgery. In group 1, patients underwent CPAP, PSV, and CPAP in sequence. In group 2, patients underwent PSV, CPAP, and PSV in sequence. PSV comprised positive end expiratory pressure set at 5 cm H2O and inspiratory pressure support set at 5 cm H2O above positive end expiratory pressure. CPAP was set at 5 cm H2O. Each ventilatory mode was maintained for 10 min. The following data were recorded every minute for the last 5 min of each ventilatory mode and the average reading taken: end tidal carbon dioxide, oxygen saturation, expired tidal volume, leak fraction, respiratory rate, noninvasive mean arterial pressure, and heart rate. RESULTS: In both groups, PSV showed lower end tidal carbon dioxide (P < 0.001), higher oxygen saturation, (P < 0.001), and higher expired tidal volume (P < 0.001) compared with CPAP. In both groups, PSV had similar leak fraction, respiratory rate, mean arterial pressure, and heart rate compared with CPAP. In group 1, measurements for CPAP were similar before and after PSV. In group 2, measurements for PSV were similar before and after CPAP. CONCLUSION: The authors concluded that PSV provides more effective gas exchange than does unassisted ventilation with CPAP during LMA anesthesia while preserving leak fraction and hemodynamic homeostasis.     

Onset of vecuronium neuromuscular block is more rapid in patients undergoing Caesarean section

This investigation was carried out in ten patients undergoing elective Caesarean section and the results were compared with those of a control group of ten nonpregnant females of the same age group. The study investigated the onset of vecuronium neuromuscular block and the conditions of tracheal intubation when ketamine (1.5 mg.kg-1)-vecuronium 100 micrograms.kg-1) sequence was used for rapid-sequence induction of anaesthesia. The ulnar nerve was stimulated supra-maximally at the wrist with train-of-four stimuli every 20 sec, and the electromyographic response of the adductor pollicis muscle was displayed. The onset of 50% neuromuscular block as monitored by electromyography was shorter in the Caesarean group (80 +/- 30 sec) than in the control group (144 +/- 43 sec). The conditions of intubation at 50% block were adequate in both groups. Also, the onset of 90% block was shorter in the Caesarean group. The time of recovery to T1/control ratio of 25% was longer in the Caesarean group (46 +/- 10 min) than in the control patients (28 +/- 10 min). The results show that administration of vecuronium according to body weight results in a more rapid onset and delayed recovery of neuromuscular block in pregnant women undergoing Caesarean section than in the nonpregnant control patients.     

Transcutaneous carbon dioxide monitoring during prolonged apnoea with high-flow nasal oxygen

Background

The duration of apnoeic oxygenation with high-flow nasal oxygen is limited by hypercapnia and acidosis and monitoring of arterial carbon dioxide level is therefore essential. We have performed a study in patients undergoing prolonged apnoeic oxygenation where we monitored the progressive hypercapnia with transcutaneous carbon dioxide. In this paper, we compared the transcutaneous carbon dioxide level with arterial carbon dioxide tension.

Methods

This is a secondary publication based on data from a study exploring the limits of apnoeic oxygenation. We compared transcutaneous carbon dioxide monitoring with arterial carbon dioxide tension using Bland–Altman analyses in anaesthetised and paralysed patients undergoing prolonged apnoeic oxygenation until a predefined limit of pH 7.15 or PCO₂ of 12 kPa was reached.

Results

We included 35 patients with a median apnoea duration of 25 min. Mean pH was 7.14 and mean arterial carbon dioxide tension was 11.2 kPa at the termination of apnoeic oxygenation. Transcutaneous carbon dioxide monitoring initially slightly underestimated the arterial tension but at carbon dioxide levels above 10 kPa it overestimated the value. Bias ranged from −0.55 to 0.81 kPa with limits of agreement between −1.25 and 2.11 kPa.

Conclusion

Transcutaneous carbon dioxide monitoring provided a clinically acceptable substitute for arterial blood gases but as hypercapnia developed to considerable levels, we observed overestimation at high carbon dioxide tensions in patients undergoing apnoeic oxygenation with high-flow nasal oxygen.     

Supplementary oxygen administration for elective Caesarean section under spinal anaesthesia

We investigated the necessity for administration of supplementary oxygen to mothers undergoing elective Caesarean section under spinal anaesthesia. Sixty-nine women undergoing elective Caesarean section were randomly allocated to one of three groups to be given either oxygen (40%) by facemask, air by facemask or oxygen at 2 l x min(-1) by nasal cannulae. Umbilical arterial and venous blood samples were taken and analysed immediately after delivery. The results showed that there were no significant differences in the umbilical arterial or venous pH, partial pressure of oxygen and partial pressure of carbon dioxide between any of the three groups. We also assessed the patient acceptability of oxygen administered by facemask vs. nasal cannulae should the need for supplementary oxygen arise. It was found that use of the facemask impeded communication.     

Cardiac dysrhythmias during oral surgery. Comparison of hyoscine and droperidol premedication

The incidence of cardiac dysrhythmias during inpatient dental anaesthesia under halothane was studied following either hyoscine or droperidol as a supplement to papaveretum for premedication. Forty-three percent of patients given hyoscine exhibited cardiac dysrhythmias compared to 23 percent of the group given droperidol (p = 0.03). The incidence of dysrhythmias bore no relationship to the age, sex or weight of the patient, nor to the end tidal carbon dioxide tension.     

Regional cerebrovascular reactivity to carbon dioxide during cardiopulmonary bypass in patients with cerebrovascular disease

In patients with cerebrovascular disease, hypercarbia may cause redistribution of regional cerebral blood flow from marginally perfused to well-perfused regions (intracerebral steal), as evidenced by regional cerebral blood flow studies during carotid endarterectomy. During hypothermic cardiopulmonary bypass, the pH-stat method of acid-base management produces relative hypercarbia. To determine whether pH-stat management produces relative hypercarbia. To determine whether pH-stat management induces intracerebral steals, we investigated nine patients with cerebrovascular disease undergoing coronary artery bypass grafting. During hypothermic cardiopulmonary bypass, arterial carbon dioxide tension was varied in random order between 40 mm Hg and 60 mm Hg (uncorrected for body temperature). Regional cerebral blood flow was measured by clearance of 133 xenon injected into the arterial inflow cannula. Nasopharyngeal temperature (26.8 degrees-28.0 degrees +/- 2.2 degrees-3.0 degrees C), perfusion flow rate (2.14-2.18 +/- 0.70-0.73 L/min/m2), mean arterial pressure (67-68 +/- 6-9 mm Hg), arterial carbon dioxide tension (302-308 +/- 109-113 mm Hg), and hematocrit (23% +/- 4%) were maintained within narrow limits in each patient during arterial carbon dioxide tension manipulation. Global mean cerebral blood flow values were similar to previously reported values in patients free of cerebrovascular disease; patients in this study averaged 15.2 +/- 2.5 ml/100 gm/min at an arterial carbon dioxide tension of 46.1 +/- 8.4 mm Hg and 25.3 +/- 6.1 ml/100 gm/min at an arterial carbon dioxide tension of 71.1 +/- 11.8 mm Hg. Carbon dioxide reactivity, defined as mean global cerebral blood flow (in ml/100 gm/min) divided by arterial carbon dioxide tension (in mm Hg), was similar in the region having the lowest regional cerebral blood flow and in the brain as a whole. No patient developed evidence of an intracerebral steal at the higher arterial carbon dioxide tension. During hypothermic cardiopulmonary bypass, higher levels of arterial carbon dioxide tension, such as those associated with the pH-stat management technique, are apparently not associated with potentially harmful redistribution of cerebral blood flow in patients with cerebrovascular disease.     

Respiration during sleep in kyphoscoliosis.

Eleven subjects with non-paralytic and 10 with paralytic kyphoscoliosis and nine normal control subjects were studied during sleep. The Cobb angle of those with kyphoscoliosis varied from 60 degrees to 140 degrees (median 100 degrees) and the vital capacity varied from 17% to 56% (median 28%) of the value predicted on the basis of span. Recordings made during sleep included expired carbon dioxide tension at the nose, gas flow at the mouth, arterial oxygen saturation, chest wall movement, and the electroencephalogram, electro-oculogram, and electrocardiogram. In three subjects transcutaneous carbon dioxide tension was measured simultaneously. Patients with kyphoscoliosis hypoventilated during sleep, particularly in rapid eye movement sleep, resulting in a rise in end tidal and transcutaneous carbon dioxide tension, and a reduction in oxygen saturation to a degree not observed in normal subjects. Reduced chest wall movement was the major cause of these episodes, which were more frequent and occupied a greater proportion of sleep time in those with kyphoscoliosis than in normal subjects. Serious cardiac arrhythmias were rarely associated. It is concluded that disturbances of respiration during sleep occur in patients with kyphoscoliosis and that these may be important in the pathogenesis of cardiorespiratory failure.     

ADVERSE EFFECTS OF LOW CARBON DIOXIDE TENSIONS DURING MECHANICAL OVER-VENTILATION OF PATIENTS WITH COMBINED HEAD AND CHEST INJURIES

Two patients are described who had suffered injuries both tothe chest and the head. Both patients had evidence of impairmentof ventilation-perfusion relationships, and were ventilatedmechanically. The large minute volumes which were used to inflatethe lungs and to restore normal arterial oxygen tensions severelyreduced the arterial carbon dioxide tension, and produced anarterial alkalaemia. The insertion of a dead-space into theventilation circuit increased the arterial carbon dioxide tension;the arterial oxygen tension was not further impaired, and thelevels of consciousness of both patients improved. A suitabledeadspace allows normal carbon dioxide tension to be maintainedwhen increased ventilation volumes are required for adequatemechanical ventilation.     

The effect of halothani-nitrous oxide anaesthesia and spontaneous ventilation on arterial blood oxygenation and physiological deadspace.

Arterial blood, inspired and expired gas samples were taken from seven patients anaesthetized with halothane (1-2 per cent) and nitrous oxide in oxygen and who breathed spontaneously. Over a two hour period, the average arterial oxygen tension was 75 mm Hg and carbon dioxide tension 49 mm Hg. No significant deterioration of either blood gas value occurred during the two hours. The dead-space/tidal volume ratio and alveolar-arterial oxygen tension difference did not alter significantly during the period of the study.