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.     

Mainstream vs. sidestream capnometry for prediction of arterial carbon dioxide tension during supine craniotomy

We compared the performance of mainstream capnometry as a measure of arterial carbon dioxide tension (Paco2) with sidestream recordings in adult neurosurgical patients undergoing supine craniotomy. Two hundred and forty patients were randomly assigned so that the end-tidal carbon dioxide tension (PEco2) was measured using either a mainstream or sidestream infrared capnometer. All patients received propofol anaesthesia and ventilation was adjusted according to clinical requirement. Arterial blood gas analyses were performed after induction, prior to dural incision, during surgery and before wound closure. Simultaneous haemodynamic and ventilatory parameters were also recorded. For 1007 paired measurements of PEco2 and Paco2 (mainstream, n = 503; sidestream, n = 504), the mean (SD) mainstream arterial to end-tidal carbon dioxide tension difference, 0.64 (0.16) kPa, was smaller than the corresponding sidestream values, 0.99 (0.40) kPa (p < 0.001). The limits of agreement for the mainstream analyser, 0.32-0.96 kPa, were also narrower than the sidestream recordings, 0.19-1.79 kPa (p < 0.001). In both capnometers, the arterial to end-tidal difference in carbon dioxide tension did not change with time. However, there was greater within-patient variation in the sidestream group. Our study showed that mainstream PEco2 provided a more accurate estimation of Paco2 than sidestream measurement.     

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.     

Single breath end-tidal Pco2, measurement during high frequency jet ventilation in critical care patients

The relationship between arterial and end-tidal carbon dioxide tensions following a single large breath was investigated in seven critically ill patients receiving high frequency jet ventilation. There was a close correlation (r = 0.989) between arterial and end-tidal carbon dioxide tensions over a wide range (3.29-8.95 kPa). Measurement of the end-tidal carbon dioxide tension following a single large breath may be useful in monitoring the efficiency of high frequency jet ventilation in the elimination of carbon dioxide.     

Arterial to end-tidal carbon dioxide tension difference in children under halothane anaesthesia

Using blood gas determinations and capnography, the relationship between arterial and end-tidal PCO₂ was investigated in 20 children under halothane anaesthesia with spontaneous respiration. A median arterial to end-tidal carbon dioxide tension difference of 0.66 kPa (5 mm Hg) was found. There was a close correlation between Pa_co2 and the magnitude of the carbon dioxide difference. Our findings may largely be explained by an increase in Vd/Vt (presumably mainly due to a reduction of Vt) causing admixture of dead space air throughout expiration. It is concluded that though end-tidal carbon dioxide does not exactly reflect Pa_co2 capnography may be of value as a monitor of respiration in paediatric anaesthesia at normal or near-normal values of end-tidal carbon dioxide.     

End-tidal carbon dioxide during thoracotomy. Its relation to blood level in adults and children

The concentration of carbon dioxide in end-tidal gas was compared with the tension in arterial or superior vena caval blood during thoracotomy in twelve patients. In six adults requiring pulmonary resection, one-lung anaesthesia did not change the difference between the two measurements. In six children in whom a systemic to pulmonary arterial anastomosis was being created to improve pulmonary blood flow impaired by cyanotic congenital heart disease, occlusion of the pulmonary artery caused in increase in the blood-end-tidal carbon dioxide gradient. This change was particularly marked in two neonates and was of sufficient magnitude to render end-tidal monitoring unreliable in these circumstances.     

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.     

Accuracy of end-tidal carbon dioxide monitoring using the NBP-75 microstream capnometer. A study in intubated ventilated and spontaneously breathing nonintubated patients

Arterial carbon dioxide partial pressure measurements using the NBP-75 microstream capnometer were compared with direct PaCO2 values in patients who were (a) not intubated and spontaneously breathing, and (b) patients receiving intermittent positive pressure ventilation of the lungs and endotracheal anaesthesia. Twenty ASA physical status I-III patients, undergoing general anaesthesia for orthopaedic or vascular surgery were included in a prospective crossover study. After a 20-min equilibration period following the induction of general anaesthesia, arterial blood was drawn from an indwelling radial catheter, while the end-tidal carbon dioxide partial pressure was measured at the angle between the tracheal tube and the ventilation circuit using a microstream capnometer (NBP-75, Nellcor Puritan Bennett, Plesanton, CA, USA) with an aspiration flow rate of 30 mL min(-1). Patients were extubated at the end of surgery and transferred to the postanaesthesia care unit, where end-tidal carbon dioxide was sampled through a nasal cannula (Nasal FilterLine, Nellcor, Plesanton, CA, USA) and measured using the same microstream capnometer. In each patient six measurements were performed, three during mechanical ventilation and three during spontaneous breathing. A good correlation between arterial and end-tidal carbon dioxide partial pressure was observed both during mechanical ventilation (r = 0.59; P = 0.0005) and spontaneous breathing (r = 0.41; P = 0.001); while no differences in the arterial to end-tidal carbon dioxide tension difference were observed when patients were intubated and mechanically ventilated (7. 3 +/- 4 mmHg; CI95: 6.3-8.4) compared to values measured during spontaneous breathing in the postanesthesia care unit, after patients had been awakened and extubated (6.5 +/- 4.8 mmHg; CI95: 5. 2-7.8) (P = 0.311). The mean difference between the arterial to end-tidal carbon dioxide tension gradient measured in intubated and non-intubated spontaneously breathing patients was 1 +/- 6 mmHg (CI95: -11-+13). We conclude that measuring the end-tidal carbon dioxide partial pressure through a nasal cannula using the NBP-75 microstream capnometer provides an estimation of arterial carbon dioxide partial pressure similar to that provided when the same patients are intubated and mechanically ventilated.     

COMPARISON OF END-TIDAL AND ARTERIAL CARBON DIOXIDE MEASUREMENTS DURING ANAESTHESIA WITH THE LARYNGEAL MASK AIRWAY

We have confirmed the value of measurement of end-tidal carbondioxide concentration as an indicator of arterial carbon dioxidetension during the use of the laryngeal mask airway in healthypatients breathing spontaneously. The mean difference betweenarterial and end-tidal carbon dioxide tension was 0.52 kPa (range0–1.5 kPa), which is similar to the difference which hasbeen reported when a tracheal tube has been used. (Br. J. Anaesth.1993; 71: 734–735) ^*Present address, for correspondence: Department of Anaesthesia,St George's Hospital, Blackshaw Road, London SW17 OQT.     

Relationship between end-tidal and arterial carbon dioxide partial pressure using a cuffed oropharyngeal airway and a tracheal tube

We have compared the differences between end-tidal PE'CO2 and arterial PaCO2 carbon dioxide partial pressures during general anaesthesia using either a cuffed oropharyngeal airway (COPA) or a tracheal tube (TT) in spontaneously breathing adult patients. After induction of anaesthesia, a COPA was inserted in 20 patients who were allowed to breathe spontaneously. When steady state was reached, PE'CO2 and PaCO2 were recorded. The COPA was removed, the trachea intubated with a TT and spontaneous ventilation allowed to resume. After a stable PE'CO2 was reestablished, PaCO2 was measured again and PE'CO2 recorded. Mean difference between PaCO2 and PE'CO2 with the COPA was 0.72 (SD 0.45) kPa and with the TT 0.64 (0.40) kPa (ns; paired t test). Our results suggest that Pe'CO2 is a clinically acceptable indicator of PaCO2 in adults breathing spontaneously via a COPA.      

Respiratory dead space under anaesthesia in patients with mitral stenosis.

Physiological deadspace (VDphys) and arterial to end-tidal carbon dioxide tension difference [P(a-E)CO2] were calculated under anaesthesia in 27 patients with mitral stenosis planned for close mitral commissurotomy and in 15 healthy individuals for elective non-thoracic surgical procedures. A square wave inspiratory flow pattern and an end-inspiratory pause (25% and 10% of cycle time respectively) were given with a SERVO 900B ventilator used at respiratory rate of approximately 16 per min. An infra-red CO2 analyser was used to measure CO2 production and end-tidal CO2 concentration. Measurements were made prior to the start of the surgery after a minimum of 10 min of stable ventilation to avoid the effect of surgery. Patients with multiple stenosis had significantly higher VDphys (4.28 +/- 1.02 ml kg-1 as compared to 2.10 +/- 0.52 ml kg-1 in controls, P less than 0.001), higher P(a-E)CO2 [0.43 +/- 0.51 kPa as compared to -0.02 +/- 0.23 kPa, P less than 0.01] and lower respiratory system compliance (Crs). Péco2 was positively correlated with PaCO2 in both groups (P less than 0.01). PaO2 was lower in mitral stenosis patients and P(A-a)O2 negatively correlated to Crs (P less than 0.01).     

The influence of changes in end-tidal carbon dioxide upon the Bispectral Index

Carbon dioxide is known to affect consciousness in animals and humans. We surmised that changes in end-tidal carbon dioxide during anaesthesia might affect the Bispectral Index. Twenty-four patients due to undergo surgery were anaesthetised with fentanyl and a propofol infusion. The Bispectral Index, pulse rate and blood pressure were recorded while end-tidal carbon dioxide levels were changed. The patients acted as their own controls as they were subjected to high, normal and low levels of end-tidal carbon dioxide (3-12 kPa) according to a randomised sequence. There were no changes in the Bispectral Index or haemodynamic variables resulting from manipulation of the end-tidal carbon dioxide. At the level of hypnosis involved in this study, changes in end-tidal carbon dioxide, within the range tested, do not result in changes in the Bispectral Index.     

Arterial to end-tidal carbon dioxide tension difference in anaesthetized adults mechanically ventilated via a laryngeal mask or a cuffed oropharyngeal airway.

To evaluate arterial (PaCO2), end-tidal (PETCO2) and carbon dioxide tension difference during mechanical ventilation with extratracheal airways, 60 patients ASA physical status I-II, receiving general anaesthesia for minor extra-abdominal procedures were randomly allocated to receive either a cuffed oropharyngeal airway (group COPA, n = 30) or a laryngeal mask (group LMA, n = 30). The lungs were mechanically ventilated by IPPV using a 60% nitrous oxide and 1-1.5% isoflurane in oxygen mixture (VT = 8 mL kg-1; RR = 12 b min-1; l/E = 1/2). After PETCO2 had been stable for at least 10 min after airway placement, haemodynamic variables and PETCO2 were recorded and an arterial blood sample was obtained for measurement of PaCO2. No differences in anthropometric parameters, smoking habit, haemodynamic variables and incidence of untoward events were observed between the two groups. Airway manipulation, to maintain adequate ventilation, was required in only nine patients in the cuffed oropharyngeal airway group (30%) (P < 0.0005); however, in no case was it necessary to remove the designated extratracheal airway due to unsuccessful mechanical ventilation. The mean difference between arterial and end-tidal carbon dioxide partial pressure was 0.4 +/- 0.3 KPa in the laryngeal mask group (95% confidence intervals: 0.3-0.5 KPa) and 0.3 +/- 0.26 KPa in the cuffed oropharyngeal airway group (95% confidence intervals: 0.24-0.4 KPa) (P = NS). We conclude that in healthy adults who are mechanically ventilated via the cuffed oropharyngeal airway, the end-tidal carbon dioxide determination is as accurate an indicator of PaCO2 as that measured via the laryngeal mask, allowing capnometry to be reliably used to evaluate the adequacy of ventilation.     

The alveolar-arterial difference in oxygen tension increases with temperature-corrected determination during moderate hypothermia

Moderate hypothermia (32-33 degrees C) occurs in anesthetic practice. However, intrapulmonary gas exchange and the effect of temperature correction of blood gases on oxygen and carbon dioxide exchange have not been investigated in these patients. We investigated alveolar-arterial difference in oxygen tension (AaDO2) and arterial to end-tidal difference in carbon dioxide (Pa-ETCO2) during rewarming of eight ASA physical status I patients from hypothermia of 32 degrees C. Anesthesia was maintained with fentanyl/propofol. AaDO2 and Pa-ETCO2 were assessed by analyzing arterial blood gases and saturated water vapor pressure, uncorrected or corrected to actual body temperature. The respiratory quotient (RQ) was measured by calorimetry. After temperature correction of blood gases and water vapor pressure, the AaDO2 was significantly higher at 33 and 32 degrees C compared with 36 degrees C (56 +/- 13 and 64 +/- 14 vs 39 +/- 10 mm Hg; P < 0.05 and P < 0.01). The deterioration of pulmonary oxygen exchange was not detected if arterial blood gases and water vapor pressure were not corrected. The RQ did not change during moderate hypothermia compared with 36 +/-C. The temperature-corrected Pa-ETCO2 was not affected by hypothermia. We conclude that AaDO2 is increased during moderate hypothermia. This is only detected when water vapor pressure and arterial blood gases are corrected to actual body temperature. IMPLICATIONS: We investigated intrapulmonary oxygen and carbon dioxide exchange during moderate hypothermia (32 degrees C) in eight patients. If oxygen, carbon dioxide, and water vapor pressure were corrected to actual body temperature, the alveolar-arterial oxygen tension difference was increased during hypothermia. The carbon dioxide tension difference and the respiratory quotient were unaffected by hypothermia.     

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.      

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.     

Effects of propofol vs isoflurane on respiratory gas exchange during laparoscopic cholecystectomy

Background : Respiratory function and pulmonary gas exchange are affected in laparoscopic procedures where a pneumoperitoneum is introduced using CO₂. Previous studies have shown differing results concerning pulmonary gas exchange during laparoscopic procedures: Whereas in patients undergoing isoflurane anaesthesia decreases in PaO₂ are demonstrated, this factor remains unchanged in patients undergoing propofol anaesthesia. In the present study, the effects of propofol on pulmonary gas exchange were compared with those of isoflurane in patients undergoing elective laparoscopic cholecystectomy in a prospective randomised manner. Methods : Twenty ASA patients with physical status I and II were divided randomly between isoflurane (IG) and propofol groups (PG). After induction of anaesthesia patients were moderately hyperventilated. Respirator settings remained unchanged during pneumoperitoneum (PP) until 10 min after deflation of the peritoneal cavity. Blood gas analyses were performed at 5 time points: 15 min after induction of anaesthesia (giving pre-PP values), immediately before carbon dioxide insufflation (0 min PP), after both 30 and 60 min of PP and 10 min post PP. Inspiration plateau pressure (P_plat), compliance of the respiratory system, and both ins- and expiratory gas concentrations were continuously recorded by an Ultima V® monitor (Datex Corp., Helsinki, Finland). The difference between arterial and end-tidal CO₂ partial pressure (P(a-et)CO₂) was calculated so as to allow assessment of physiological dead space by the modified Bohr equation. Results : Pulmonary gas exchange differed significantly after 30 min of PP between the IG and the PG. At this time, PaO₂ was 19.5 ± 2.9 kPa (mean ± SD) in the IG and 23.1 ± 1.8 kPa in the PG (P<0.01), whereas PaCO₂ was 5.5 ± 0.37 kPa in the IG and 4.9 ± 0.27 kPa in the PG (P<0.01). These discrepancies remained until after carbon dioxide desufflation. At 10 min post PP, PaO₂ was 18.3 ± 2.6 kPa in the isoflurane group and 21.9 ± 2.2 kPa in the propofol group (P<0.01), whereas PaCO₂ was 5.4 ± 0.46 kPa in the IG and 4.8 ± 0.22 kPa in the PG (P<0.01). During carbon dioxide insufflation the P(a-et)CO₂ increased significantly in the IG from 0.47 ± 0.13 kPa to 0.76 ± 0.37 kPa (P<0.05), while the values in the PG remained constant. Conclusion : This study demonstrates that pulmonary gas exchange in patients with laparoscopic cholecystectomy is affected by the choice of anaesthetic procedure. During and after laparoscopic cholecystectomy using isoflurane as the anaesthetic, the PaCO₂ is significantly higher and the PaO₂ significantly lower than they are with propofol.     

FRESH GAS REQUIREMENTS OF AN ENCLOSED AFFERENT RESERVOIR BREATHING SYSTEM DURING CONTROLLED VENTILATION IN CHILDREN

An enclosed afferent reservoir breathing system (EAR) designedby Ohmeda was evaluated during anaesthesia with controlled ventilationin 104 healthy children. Carbon dioxide production and arterialcarbon dioxide tension were measured in 12 children in orderto determine the proportion of fresh gas (VF) involved in gasexchange. When the ratio of minute volume ventilation to freshgas flow (VE:VF) exceeded 1.5, fractional utilization of freshgas with the EAR was 0.92. This value and values of carbon dioxideproduction obtained from 43 children were used to derive a simpleformula relating fresh gas flow requirements to body weight.The formula, VF =0.6 x weight ^0.5, was assessed in 49 childrenweighing 10–70 kg. The mean end-tidal partial pressureof carbon dioxide in these patients was 4.5 kPa (range 3.8–5.2kPa). We conclude that the EAR has an efficiency of 92% in theuse of fresh gas during controlled ventilation in healthy children,provided the VE:VF ratio is greater than 1.5. Under these conditions,normocapnia to mild hypocapnia was produced accurately usingthe formula VF=0.6 x weight ^0.5.     

The effects of the exaggerated lithotomy position for radical perineal prostatectomy on respiratory mechanics

The exaggerated lithotomy position is used during radical perineal prostatectomy to increase perineal exposure. The aim of this study was to evaluate the effects of the exaggerated lithotomy position on respiratory mechanics and arterial blood gases. In the exaggerated lithotomy position, dynamic compliance and static compliance were found to be significantly decreased by 27.4% and 34.8%, respectively, whilst peak, plateau, and mean airway pressures increased significantly by 34.0%, 45.8% and 31.7%, respectively. The physiological dead space/tidal volume ratio and total inspiratory work of breathing increased significantly by 11.1% and 33.7%, respectively. Arterial oxygen tension was significantly decreased by 26.9%; however, no significant differences were seen in end-tidal or arterial carbon dioxide tension. These results indicate that the exaggerated lithotomy position under general anaesthesia can cause significant effects on respiratory system mechanics and arterial oxygenation and highlights the need for careful monitoring of patients placed in this position for surgery.     

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.