Correlation between arterial and end tidal carbon dioxide partial pressure during laparoscopic surgeries

Upendra M Kanzarkar^1*, Anjali Savargaonkar²

Abstract               Background: End-tidal carbon dioxide pressure (etCO₂) monitoring is used as an indirect measure of arterial carbon dioxide partial pressure (PaCO₂). To prevent hypercapnia, close intraoperative monitoring of etCO₂ or PaCO₂ and appropriate ventilatory changes are necessary. Aim: To evaluate the correlation between arterial and end-tidal carbon dioxide during laparoscopic surgeries. Material and Methods: A total of 40 non-obese adult patients of either sex, between 18 to 60 years of age and ASA status I, posted for elective laparoscopic surgery were included. Measurement and comparison done between preinsufflation values of etCO₂, a-etCO₂ gradient and their respective predesufflation values. Results: In present study, a-etCO₂ gradient increased with pneumoperitoneum to 7.58±2.11 mmHg at predesufflation period as compared to 5.55±2.07 mmHg at preinsufflation period and this change of 2.03±1.35 mmHg was found to be highly significant (p<0.001). We observed change in a-etCO₂ gradient to be more in Trendelenburg position (2.20±1.51 mmHg) as compared to antitrendelenburg position (1.82±1.15 mmHg); however, this difference is not significant statistically. Change in a-etCO₂ gradient has highly significant, positive correlation with duration of pneumoperitoneum (p< 0.001). Conclusion: Increase in a-etCO₂ gradient with pneumoperitoneum indicating loss of correlation between etCO₂ and PaCO₂ during pneumoperitoneum and thus etCO₂ is less accurate in reflecting PaCO₂.

Key Words: Laparoscopic surgeries, pneumoperitoneum, End-tidal carbon dioxide pressure, arterial carbon dioxide partial pressure.

Laparoscopy is a minimally invasive surgery and promoted as gentle surgery, still the procedure is not risk free. Reduction in functional residual capacity (FRC) occurs with general anaesthesia is compounded by CO₂ induced pneumoperitoneum and Trendelenburg positioning of patients.¹End-tidal carbon dioxide pressure (etCO₂) monitoring is used as an indirect measure of arterial carbon dioxide partial pressure (PaCO₂). To prevent hypercapnia, close intraoperative monitoring of etCO₂ or PaCO₂ and appropriate ventilatory changes are necessary. There is some disagreement whether etCO₂ is reliable in predicting the PaCO₂ in laparoscopic procedures with CO₂ insufflations. Although etCO₂ is an easily accessible monitoring parameter, it often underestimates the true level of PaCO2.² Thus, relying on etCO₂, anaesthesiologist may not be able to execute appropriate ventilatory changes to eliminate increased CO₂ load to prevent systemic acidosis. Keeping in mind all these controversies regarding dependence on etCO₂ for monitoring of PaCO₂ during laparoscopic surgeries even in healthy patients and necessity of their anticipation in compromised patients, the present study was conducted to evaluate the correlation between arterial and end-tidal carbon dioxide during laparoscopic surgeries.

MATERIAL AND METHODS

The present study was carried out in the Department of Anaesthesiology of a Tertiary Care Hospital, after approval by the hospital’s ethics committee.Patients posted for elective laparoscopic surgery, which included laparoscopic appendicectomy, cholecystectomy, adhesiolysis, hydatid cysts removal, or aspiration, abdominoperineal resection, and gonadal vein ligation.

Inclusion Criteria

• Age from 18-60 years
• Patients of either sex.
• Patients posted for elective laparoscopic surgery.
• ASA physical status I/II.

Exclusion Criteria

• Significant cardiovascular morbidity (ASA III and IV).
• Patients with respiratory disorders e.g. asthma, chronic-obstructive-pulmonary disorder, pulmonary tuberculosis, history of pulmonary resection.
• Abnormal modified Allen’s test suggestive of inadequate collateral circulation in the hand.
• Coagulopathy or patients on anti-coagulant therapy.
• Chronic smokers,
• Obese patients with BMI > 25 Kg/m^2.
• Patient’s refusal.

Sample Size: Based on a previous study,³ a sample size of 40 patients was calculated using values of confidence level = 90%, level of significance=5% and relative precision= 5%.

Methodology: A detailed preanaesthetic evaluation including history of present illness, general examination, systemic examination, airway assessment was carried out in each patient.Appropriate laboratory tests and radiological investigations were carried out and results noted. Patients were premedicated withi.v. Ranitidine 50mg andi.v.Glycopyrrolate 0.004mg/kg. Sedation given with i.v. Midazolam 0.02mg/kg and i.v. Fentanyl 2mg/kg. Baseline vital parameters were noted after premedication and sedation, but before induction. Patients were preoxygenated with 100% O₂ on mask for 3 min.Induction of general anaesthesia was done with i.v.Propofol 2mg/kg and i.v.Vecuronium 0.15mg/kg and the patient was ventilated for 3 min on mask on O₂ + N₂O + Sevoflurane / Isoflurane.Intubation was done, controlled mechanical ventilation was started and anaesthesia was maintained on O₂ and N₂O in ratio of 50:50 along with inhalational anaesthetic agent (2% Sevoflurane or 1.2% Isoflurane) on semiclosed circle system with CO₂ absorber and flow rate of 4 lit/min. Mechanical ventilation was adjusted by changing respiratory rate to maintain etCO₂ between 30 to 40 mmHg while tidal volume was kept constant at 10 ml/kg.Intraoperatively, intermittent doses of I.V. Vecuronium bromide 0.02 mg/kg were used for muscle relaxation according to neuromuscular monitoring. All patients received i.m. Diclofenac sodium 75 mg after endotracheal intubation. Maintenance doses of i.v. Fentanyl 0.5 µg/kg were given intraoperatively at every 60 min. Baseline arterial blood sample was taken anaerobically. etCO₂ value was noted down at the time of arterial blood sampling. After arterial sampling, pneumoperitoneum was created with carbon dioxide.Intra-abdominal pressure [I.A.P.] was maintained between 12-16 mmHg, and was monitored, and recorded. Apart from this, other parameters recorded during pneumoperitoneum were (1) Rate of CO₂ insufflation and (2) patient positioning. Second arterial blood sample was taken just before carbon-dioxide desufflation by the above-mentioned technique with simultaneous recording of etCO₂ at the time of sampling. Any intraoperative complications were managed accordingly. At the end of surgery, reversal of neuromuscular blockade was done with i.v. Neostigmine 0.05 mg/kg and i.v.Glycopyrrolate 10 mgm/kg. Postoperatively, vital parameters like respiratory rate, heart rate, and blood pressure were monitored and any complaints including nausea, vomiting managed accordingly before the patients were shifted to ward. Intra and postoperative complications were noted.

Statistical Analysis: Data was analyzed on statistical software STATA 10.0.Continuous variables were presented as mean ± standard deviation, categorical variables were expressed in percentage (%). All Continuous variables were compared by paired t-test. Mean change in a-etCO₂ gradient as compared to I.A.P. at 3 levels was compared by analysis of variance (ANOVA) with multiple comparison test of Bonferroni; correlation between duration of pneumoperitoneum and a-etCO₂ gradient was assessed by calculating correlation coefficient “r”. Association between number of patients and acid base alterations was assessed by chi-square test.p value < 0.05 was considered as statistically significant.

RESULTS

The study included 40 adult patients of either sex, scheduled to undergo elective laparoscopic surgical procedures. In present study, age group ranged from 18-60 years, mean age of study population was 34.85±12.08 years. Study involved 13 male and 27 female patients. Mean BMI of study population was 19.6±2.65 kg/m² which indicated weight distribution in the patient group. Study involved 45% patients with BMI <18.5 kg/m², while 55% patients were with BMI between 18.6 to 24.99 kg/m².There were no over-weight patients (BMI > 25 kg/m²) in the study. All patients of both sexes belonged to ASA-I category. Laparoscopic appendicectomy19 (47.5%), Laparoscopic cholecystectomy 15 (37.5%) were the major surgeries performed followed by Laparoscopic adhesiolysis and Laparoscopic APR in 2 (5%) patients each, Laparoscopic cyst aspiration and Laparoscopic gonadal vein ligation in one (2.5%) patient each. Average duration of laparoscopic surgery was 81.45 ± 51.38 min with range of 30 to 250 min. Average duration of pneumoperitoneum was 68.62 ± 48.10 min, ranging from 20 to 218 min.

Table 1: Comparison of Mean etCO₂during pneumoperitoneum (mmHg) with Preinsufflation value

   NS-Non-significant, S- Significant, HS- Highly significant.

Duration of pneumoperitoneum varied in different patients in our study and affected the baseline value for mean etCO₂ at different time intervals. Mean preinsufflation etCO₂ was 31.18 ± 3.45 mmHg at time of sampling; it showed highly significant increase from baseline at 5, 10, 20, 30, and 40 min. of insufflation (p< 0.001). Again, at 60 min, mean etCO₂ increased highly significantly to 33.55 ± 3.14 mmHg from baseline (p <0.001). At 50, 90, 120 min change was significant (p <0.05). During rest of pneumoperitoneum we may not able to comment on significance of change in mean etCO₂ at time interval of 150, 180 and 210 min as only 3, 2 and 1 patients respectively, had this much period of pneumoperitoneum. etCO₂ showed a tendency to increase with duration of pneumoperitoneum. etCO₂ increased significantly at predesufflation arterial sampling time as compared to preinsufflation arterial sampling time. A total of 18 (45%) patients were in reverse trendelenburg position (head high) during procedure and 22 (55%) patients were in trendelenburg position (head low) during procedure.

     NS-Non-significant, S- Significant, HS- Highly significant

Highly significant (p<0.001) increase in mean PaCO₂ was seen during pneumoperitoneum as compared to preinsufflation value with a mean change of 3.78±4.11mmHg. Mean etCO₂ has shown significant change (p < 0.05) of 1.76 ± 3.61 during pneumoperitoneum from 32.93 ± 3.69 mmHg as compared to preinsufflation value of 31.18 ± 3.45 mmHg. Mean a-etCO₂ gradient changed highly significantly (p < 0.001) by 2.03 ± 1.35 mmHg during pneumoperitoneum as compared to before insufflation value. Mean a-etCO₂ gradient was 5.55 ±2.07 mmHg before pneumoperitoneum, which increased highly significantly to 7.58 ± 2.11 mmHg during pneumoperitoneum. In trendelenburg position, mean a-etCO₂ gradient was on higher side i.e.2.20 ± 1.51 mmHg as compared to 1.82 ± 1.51 mmHg in antitrendelenburg position, but this was not a significant difference statistically (p>0.05). Coefficient of correlation r = +0.6303, and p < 0.001, showed highly significant positive correlation between duration of pneumoperitoneum and a-etCO₂ gradient. Mean a-etCO₂ value showed persistently increasing trend with duration of pneumoperitoneum. The mean a-etCO₂ value was always above the baseline value of a-etCO₂ gradient, during pneumoperitoneum. No significant (p > 0.05) correlation was seen between I.A.P. and mean change in a-etCO₂ gradient with pneumoperitoneum. The change in PaCO₂ has significant positive correlation with change in a-etCO₂ gradient and the change in etCO₂ does not have any significant correlation with change in a-etCO₂ gradient. No major hemodynamic or anaesthesia related complication encountered during operative procedure or pneumoperitoneum.

DISCUSSION

End-tidal carbon dioxide pressure(etCO₂) monitoring is increasingly used during anaesthesia as an indirect measure of PaCO₂. Routinely anaesthesiologists rely on etCO₂ monitoring for detection of hypercarbia. Value of a-etCO₂ depends on many factors including ventilation – perfusion distribution within lung (V/Q), changes in FRC and changes in CO₂ production (V_CO2). In many countries, etCO₂ monitoring has been made mandatory for laparoscopic surgeries. During laparoscopic surgery, Trendelenburg position together with peritoneal insufflation of CO₂ reduces FRC and increases V_CO2, and leads to changes in V/Q distribution due to basal lung compression and redistribution of hydrostatic forces. Thus, a-etCO₂ is expected to change further during laparoscopy. Many studies concluded that, a-etCO₂ gradient increased with pneumoperitoneum and etCO₂ measurements grossly underestimate PaCO₂ levels and are unreliable for ventilatory adjustments because of increased dead space ventilation secondary to V-Q mismatching.^4-7 In present study, we have analyzed a-etCO₂ gradient before and during CO₂ pneumoperitoneum and evaluated changes in it with pneumoperitoneum in laparoscopic surgeries. We also assessed correlation of a-etCO₂ gradient with duration of pneumoperitoneum, I.A.P., and patients positioning. To study a-etCO₂ gradient in present study, we analyzed arterial blood sample taken before CO₂ insufflation (preinsufflation sample) and just before CO₂ desufflation (predesufflation sample) for PaCO₂ and these values were compared with etCO₂ value at the time of arterial blood sampling. Arterial to end tidal carbon dioxide gradient calculated from this data. In present study, etCO₂ was monitored and maintained constantly between 30 to 40 mmHg during procedure, i.e. before and during CO₂ pneumoperitoneum by increasing respiratory rate and minute ventilation. It is well known fact that, CO₂ absorption and finally its elimination through lung during CO₂ pneumoperitoneum had persistently shown increase in CO₂ concentration in expired tidal volume. Studies have shown persistent increasing trend of etCO₂ value with duration of pneumoperitoneum^4,8,9 Other studies^10,11found an increase in etCO₂ concentration maximum up to 40 to 60 min of pneumoperitoneum, and thereafter it becomes stable. Duration of pneumoperitoneum varied in different patients in our study. Mean etCO₂ values at any period represents only specific number of patients and not all the patients. In our study mean etCO₂ measurement showed increasing trend with duration of pneumoperitoneum but was not significant statistically. Some authors found increase in gradient^4-7 while others found no significant change in it^1,12-14 Preinsufflation values of a-etCO₂ gradients in our patients were similar to other studies^1,15-17and increase in gradient with laparoscopy was only approximately 2.03 mmHg, but still highly significant (p<0.001) change from preinsufflation value. This increase in gradient may have occurred secondary to increase in dead space ventilation following CO₂ pneumoperitoneum and cephalad shift of diaphragm leading to decreased pulmonary compliance or decrease in cardiac output and resultant altered V-Q perfusion relationship. There were no negative values of gradient during laparoscopy among our patients. Several reports^18,19have indicated that a-etCO₂ gradient has tendency to increase or to become unpredictable with time in prolonged laparoscopies and especially in patients with pulmonary disorders, where authors recommended radial artery cannulation and frequent blood gas analysis. In our study, 18 patients are in anti-Trendelenburg position and 22 patients in Trendelenburg position. Mean a-etCO₂ gradient was on higher side i.e. 2.20±1.51 mmHg with Trendelenburg position, as compared to 1.82±1.51 mmHg in anti-Trendelenburg position, which may have occurred because of more decrease in F.R.C. in Trendelenburg positioning, but this was not a significant difference statistically (p>0.05). Analysis of data indicates positioning of patient has minor but non-significant effects on a-etCO₂ gradient. Some other studies, where author studied effect of position on a-etCO2 gradient found that, different patient positioning during pneumoperitoneum does not affect a-etCO₂ gradient significantly.⁴ Increase in mean a-etCO₂ gradient in our patients was 2.03 ± 1.35 mmHg (35%), this much increase was seen in healthy patients. Even higher increase in gradient is expected in patients with cardiac or pulmonary diseases; undergoing laparoscopy⁶. This makes ventilatory adjustments of these patients, based on etCO₂ measurements, difficult and at times dangerous retention of CO₂ with hazardous complications may occur.

CONCLUSION

Increase in a-etCO₂ gradient with pneumoperitoneum indicates loss of correlation between etCO₂ and PaCO₂ during pneumoperitoneum and thus etCO₂ is less accurate in reflecting PaCO₂. The loss of correlation between etCO₂ and PaCO₂ during laparoscopy may even be more exaggerated in patients with cardiovascular and pulmonary diseases with significant anaesthetic implications.

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