Abstract
Background and aims
The pCO2 gap, calculated by the difference between central venous and arterial partial pressure of carbon dioxide, has been evaluated as a global index of tissue perfusion. We aimed to evaluate whether the pCO2 gap could be used to identify patients in the immediate postoperative period following high-risk major abdominal surgery to predict the occurrence of postoperative complications.
Patients and methods
A prospective observational study included all postoperative high-risk adult surgical patients admitted to the intensive care unit (ICU) after major abdominal surgery. The pCO2 gap, central venous oxygen saturation (ScvO2), lactate levels, and postoperative complications were recorded at ICU admission, and after 12–24 hours.
Results
Sixty-three patients undergoing major abdominal surgery were included in the study. The mean age of the study population was 53 ± 20 years. The patients who had persistently high pCO2 gap >6 mm Hg at 24 hours postoperatively had a higher incidence of complications (n = 19.31%) compared to the group of patients with pCO2 gap <6 mm Hg (n = 41.68%). The mean pCO2 gap in the postoperative period at 24 hours was higher in patients with complications than those without complications (7.30 ± 0.60 vs 4.24 ± 1.30 mm Hg). Higher pCO2 gap and higher sequential organ failure assessment (SOFA) score were independently associated with postoperative complications. The pCO2 gap at ICU admission had a higher sensitivity and specificity than ScvO2 and lactate for differentiating between patients who did and who did not develop complications.
Conclusion
High pCO2 gap (>6 mm Hg) can be used as a complementary tool to predict postoperative complications in high-risk patients following major abdominal surgery.
How to cite this article
Kaur A, Paul G, Gautam PL, Sama A, Singh VP. Central Venous-arterial Carbon Dioxide Difference (pCO2 gap) as a Guide to Predict Complications in Postoperative High-risk Surgical Patients: A Prospective Observational Study. Indian J Crit Care Med 2026;30(1):62–67.
Keywords: pCO2 gap, postoperative complications, ScvO2, Serum lactate
Highlights
The majority of studies on pCO2 gap included critically ill patients, and there is limited data on the use of pCO2 gaps in the perioperative setting. While there are other studies in this area, none in the Indian context have studied the association of pCO2 gap trends and postoperative complications. A high pCO2 gap (>6 mm Hg) can be used as a complementary tool to predict postoperative complications following major abdominal surgery.
Introduction
The recognition of the signs of inadequate tissue perfusion during the early resuscitation of patients with sepsis plays a pivotal role in preventing postoperative organ failure and mortality after major surgery.1 The primary goal in circulatory shock is to restore cardiac output (CO), thereby improving the oxygen delivery to the tissues and correcting the oxygen demand-supply mismatch, which is characteristic of shock.2 Various markers of impaired tissue perfusion have been used to help identify patients at an increased risk of complications. Serum lactate levels were earlier described as a surrogate marker of tissue hypoperfusion but can be increased in conditions other than anaerobic metabolism making it a less sensitive marker of hypoperfusion. Central venous oxygen saturation (ScvO2) is often found to be in the normal range in septic shock patients despite anaerobic metabolism, because of the alteration of tissue oxygen extraction, limiting its usefulness.3 The high mortality due to circulatory shock necessitates the need for biomarkers to assess tissue perfusion which could provide important information about prognosis.4 The difference between venous partial pressure of CO2 (PvCO2) and arterial (PaCO2), i.e., pCO2 gap, was proposed to overcome the limitations of previous markers. pCO2 gap is known to be inversely related to CO and hence can be used as a marker for the adequacy of CO and tissue perfusion in patients.5 Raised pCO2 gap has been recognized as a marker of poor outcome during circulatory shock and has also been used as part of clinical evaluation. As CO2 is about 20 times more soluble than O2, it diffuses more rapidly out of tissues into the venous compartment making it a sensitive marker of hypoperfusion than ScvO2.6 The purpose of our study was to evaluate the association of pCO2 gap with postoperative complications in high-risk abdominal surgery patients and its comparison with other markers of impaired tissue perfusion and oxygenation (serum lactate and ScvO2).
Patients and Methods
This was a prospective single-center observational study conducted over a period of 6 months (April–September 2024) in the Surgery intensive care unit (ICU) of a tertiary care hospital in North India after taking approval from the institutional ethics committee (IEC No:2022-714) and registration with Clinical Trial Registry of India (CTRI/2024/04/065711). All consecutive adult (aged >18 years) patients received in the ICU following high-risk abdominal surgery over a period of 6 months were included in this prospective observational study.
The inclusion criteria for high-risk surgical patients were classified by three parameters:
(1) demographic criteria (age ≥ 70 years, ASA class ≥ 3, respiratory illness, chronic renal failure, chronic liver failure), (2) surgical criteria (major abdominal surgery, prolonged surgery ≥8 hours, emergency surgery, esophagectomy, gastrectomy, small bowel resection, large bowel resection, necrosectomy), (3) intensive care criteria (shock requiring vasopressors to maintain a mean arterial pressure of 65 mm Hg, on mechanical ventilation).7 Patients were considered eligible if they had one of the following criteria: One demographic and one surgical, or three or more demographic, or one or more surgical, and or one intensive care criteria. All our patients were on inotropic support and on mechanical ventilation, which itself fulfilled the high-risk criteria. As this was an observational study, informed consent was waived.
Data Collection
Central venous catheter and arterial catheters were placed intraoperatively in all patients as part of hemodynamic monitoring. After surgery, patients were shifted to the ICU immediately, and standard postoperative monitoring was done. Apart from the routine laboratory investigations, arterial blood gas and simultaneous central venous analysis were done to evaluate pCO2, lactate, pH,pO2, and ScvO2, respectively. These baseline values were noted at ICU admission (0 hours) and repeated at 12 hours and 24 hours after ICU admission. The difference between the central venous partial pressure of carbon dioxide and the arterial partial pressure of carbon dioxide was interpreted as pCO2gap. Fluid resuscitation postoperatively was continued in all patients according to their clinical condition, taking pulse pressure variation (PPV) <10–15% and inferior vena cava collapsibility index <40% till these end points were achieved. Hemodynamic parameters were also noted at ICU admission, 12 hours, and 24 hours. Apart from the above, routine laboratory tests were sent postoperatively on ICU admission. Demographic data, type of comorbidities, type of surgery, inotropic dose and duration, duration of mechanical ventilation, and sequential organ failure assessment (SOFA) score were recorded on a predesigned pro forma.
Postoperative organ failure was assessed with the organ failure criteria as any of:
Circulatory failure: Use of vasopressors to maintain MAP >65 mm Hg after fluid resuscitation; acute respiratory failure: On mechanical ventilation with PaO2/FiO2 <200; acute kidney injury: Increase in creatinine of 1.5 times, increased creatinine >0.3 mg/dL or decrease in urine output <0.5 mL/kg per hour for 6 hours, and neurological: Glasgow Coma Scale Score of <8.8 Primary outcome was the occurrence of postoperative complications which were classified as postoperative sepsis, including pneumonia, surgical site infections, peritonitis, urinary tract infections, blood stream infections; acute kidney injury, cardiac failure, postoperative hemorrhage, or any other ischemic events.9 Detailed definitions for chronic comorbidities and various postoperative complications are in the Supplementary Material. Prolonged mechanical ventilation was defined as failure to extubate within 72 hours postoperatively, and high dose of inotropes was defined as the need for two or more inotropes or vasopressors for more than 48 hours postoperatively. Length of ICU stay and 28-day mortality postoperatively were also recorded.
Statistical Analysis
The patients were divided into two groups based on the occurrence of postoperative complications. Data were described in terms of range, mean ± standard deviation (±SD), median (IQR), frequencies (number of cases), and relative frequencies (percentages) as appropriate. Comparison between the study groups at baseline was analyzed using Mann–Whitney U test for continuous variables. Chi-square (χ2) test was performed for comparing categorical, and Fisher's exact test was used when the expected frequency was less than 5. To test the associations of various variables with postoperative complications, univariate analysis was done. Receiver operator characteristics (ROC) curves were generated, and the value was estimated depending on the specificity and sensitivity. Area under the curve (AUC) was measured. A probability value (p-value) less than 0.05 was considered statistically significant. Further to understand the effect of pCO2 gap on clinical profile, patients were also divided into two groups on the basis of pCO2 gap, as below 6 mm Hg and equal to or above 6 mm Hg. Statistical Package for the Social Science 21.0 version (SPSS Inc., Chicago, USA) statistical program for Microsoft Windows was used for all statistical calculations.
Results
During the 6-month period, 63 patients who fulfilled the inclusion criteria were included in the study. Three patients died within 24 hours and hence were excluded. The mean age of the study population was 53 ± 20 years, and 62% were males. There was no statistically significant difference in the demographic characteristics, comorbidities, and baseline parameters of the study groups, as shown in Table 1. All the patients at ICU admission were on mechanical ventilation and required inotropic infusions. The median length of stay in ICU in the cohort was 8 (7–12) days. The overall 28-day mortality was 21%.
Table 1.
Baseline characteristics of the two groups
| Variables | Patients with complications (n = 19) | Patients without complications (n = 41) | p-value |
|---|---|---|---|
| Age-mean (years) | 53.05 + 22.09 | 53.34 + 18.26 | 0.956 |
| Gender (M/F) | 10/9 | 27/14 | 0.327 |
| Asthma | 0.0% | 2.4% (1) | 0.492 |
| Diabetes mellitus, n (%) | 5 (26.3%) | 12 (29.3%) | 0.385 |
| Hypertension, n (%) | 2 (10.5%) | 8 (19.5%) | 0.545 |
| Hypothyroidism, n (%) | 0 (0%) | 3 (7.3%) | 0.926 |
A total of 19 (31%) patients developed postoperative complications during their ICU stay of whom 11 (57.9%) patients had sepsis [4 (21%) – peritonitis, 2 (10.5%) – pneumonia, 2 (10.5%) – urinary tract infection, 2 (10.5%) – blood stream infection (BSI), 1 (5.1%) – surgical site infections (SSI)], 5 (26.3%) patients developed acute renal failure, 2 (10.5%) patients had acute myocardial ischemia, and 1 (5.1%) patient had hemorrhage (Fig. 1). Patients with postoperative complications had higher SOFA scores (7.84 ± 2.06 vs 3.83 ± 1.41), longer duration of mechanical ventilation [9 days (47.4%) vs 1-day (2.4%)] and dual inotropic requirement despite fluid resuscitation 6 (31.6%) vs 4 (9.8%) patients. ICU length of stay was higher in the group with postoperative complications (11.63 ± 5.74 vs 7.90 ± 4.24 days), and 28-day mortality was also higher in the group with postoperative complications (21 vs 2.4%), as depicted in Table 2. At the time of ICU admission, the baseline mean lactate, ScvO2, and pCO2 gap in the two groups were 3.93 ± 2.24 vs 3.43 ± 2.64 mmol/L, 67.58 ± 8.73 vs 77.17 ± 10.89 mm Hg, and 7.71 ± 0.35 vs 6.73 ± 0.65 mm Hg, respectively (Table 2B supplementary file).
Fig. 1.
Postoperative complications in high-risk surgical patients
Table 2.
Comparison of variables between the patients with and without complications
| Variables | Patients with complications (n = 19) | Patients without complications (n = 41) | p-value |
|---|---|---|---|
| SOFA score-median (IQR) | 8.00 (6–10.0) | 4.00 (3–4.0) | 0.001 |
| Prolonged ventilation, n (%) | 9 (47.4%) | 1 (2.4%) | 0.001 |
| Dual vasopressor support (nor-epinephrine and vasopressin), n (%) | 6 (31.6%) | 4 (9.8%) | 0.03 |
| LOS, days (mean) | 11.63 ± 5.74 | 7.90 ± 4.24 | 0.001 |
| Mortality, n (%) | 4 (21.1%) | 1 (2.4%) | 0.008 |
IQR, interquartile range; LOS, length of stay; SOFA score, sequential organ failure assessment score
On ICU admission at 0 hours, univariate analysis identified three variables associated with the occurrence of postoperative complications: SOFA score (p = 0.001), pCO2 gap (p = 0.001), and ScvO2 (p = 0.001). Table 2B (supplementary file) depicts the values of lactate, ScvO2, and pCO2 gap that were studied at ICU admission (0 hours), 12 hours, and 24 hours in both study groups. Figure 2 shows the trends that the difference in pH between the two groups was significant at 24 hours; however, the difference in lactate between the two groups was significant at 12 hours and 24 hours, but ScvO2 and pCO2 gap were significant between the two groups at all times of assessment.
Fig. 2.
Comparison of the trends of variables: pH, ScvO2, lactate, and central venous-to-arterial carbon dioxide difference (pCO2 gap) at ICU admission (0 hours), 12 hours, and 24 hours in patients with and without postoperative complications
Patients were also divided into groups on the basis of who had persistently high pCO2 gap >6 mm Hg (>6 mm Hg) and a pCO2 gap <6 mm Hg at 24 hours. Patients with high pCO2 gap were associated with more organ failure (p = 0.008), an increase in duration of mechanical ventilation (p = 0.001), increased length of hospital stay (p = 0.001), and a higher 28-day mortality rate (0.008)(Table 3). Figure 3 depicts the area under the ROC curve was 0.898 (95% CI: 0.82–0.98) for discrimination of patients with pCO2 gap >7.25 mm Hg at zero hours (89% sensitivity and 87% specificity) and area under the ROC curve was 0.98 (95% CI: 0.00–1.00) for discrimination of patients with pCO2 gap >6.25 mm Hg at 24 hours (94% sensitivity and 95% specificity) to predict the development of postop complications (Fig. 3A). The area under the ROC curve was 0.594 (95% CI: 0.44–0.748) for discrimination of patients with lactate >2.5 at zero hours (68% sensitivity and 52% specificity), as shown in Figure 3B, and the area under the ROC curve was 0.780 (95% CI: 0.653–0.907) for discrimination of patients with ScvO2 >70% at zero hours (sensitivity and specificity of 78%) to predict the development of postoperative complications as depicted in Figure 3C.
Table 3.
Patients divided into groups who had persistently high pCO2 gap (>6 mm Hg) and pCO2 gap <6 mm Hg at 24 hours
| Variables | pCO2 gap <6 mm Hg | pCO2 gap >6 mm Hg | p-value |
|---|---|---|---|
| Prolonged MV, n (%) | 0.0% (0) | 41.7% (10) | 0.001 |
| LOS, days (mean) | 7.25 + 2.45 | 11.83 + 6.53 | 0.001 |
| Organ Failure, n (%) | 0.0% (0) | 20.8% (5) | 0.008 |
| Mortality, n (%) | 0.0% (0) | 20.8% (5) | 0.008 |
LOS, length of stay; MV, mechanical ventilation; pCO2 gap, central venous-to-arterial carbon dioxide difference
Figs 3A to C.
(A) Area under the ROC curve was 0.898 with central venous-arterial difference in carbon dioxide (pCO2 gap) >7.25 mm Hg at ICU admission (89% sensitivity and 87% specificity), and area under the ROC curve was 0.98 with pCO2 gap >6.25 mm Hg at 24 hours (94% sensitivity and 95% specificity) to predict the development of postop complications; (B) At ICU admission, the area under the ROC curve was 0.594 with lactate >2.5 mmol/L (68% sensitivity and 52% specificity); (C) and the area under the ROC curve was 0.780 with ScvO2 >70% at 0 hours (sensitivity and specificity of 78%) to predict the development of postoperative complications
Discussion
The main finding of our study is that a pCO2 gap >7.25 mm Hg at ICU admission in high-risk surgical patients following major abdominal surgery is predictive of postoperative complications with moderate sensitivity and specificity. The patients who had a persistent increase in pCO2 gap >6.25 mm Hg 24 hours postoperatively have higher sensitivity and specificity for predicting the occurrence of postoperative complications. The patients with persistently high pCO2 gap (>6 mm Hg) were also significantly associated with high organ failure, prolonged duration of mechanical ventilation, increased length of hospital stay, and 28-day mortality.
To the best of our knowledge, this is the first Indian study on the relation of pCO2 gap with postoperative complications after major abdominal surgery in high-risk patients. Following trends in pCO2 gap, it was maximum immediately after surgery and remained high even after 24 hours, despite fluid resuscitation in patients who developed complications, compared to those who had no postoperative complications. The diagnostic performance of pCO2 gap was superior to that of ScvO2 and lactate. It also has the advantages of being easily implementable at bedside and can even be measured at ICU admission.
Our results are in coherence with those of a previous study where an elevated pCO2 gap was associated with an increased rate of postoperative complications in patients who were treated with an individualized goal-directed fluid therapy.10 Also, literature has evidence that sepsis or septic shock patients with high pCO2 gap are associated with higher incidence of organ failure and mortality, as shown in our study as well.11,12 However, the thresholds for pCO2 gap in these studies were about 5–6 mm Hg, whereas it was a little higher in our study, showing that there was a margin for hemodynamic optimization when the patient was received in the ICU.13 However, the ARISE trial showed that goal-directed therapy (GDT) in critically ill patients with septic shock did not lead to a statistically significant decrease in mortality, duration of organ support, and length of stay.14 The persistence of a pCO2 gap >6 even after fluid resuscitation in the ICU for 24 hours in patients who later developed complications is consistent with the relationship between increased pCO2 gap and low flow state. Low blood-flow induced state or tissue hypoxia ultimately was likely responsible for complications postoperatively, which were sepsis in most of our cases. Bakker et al.15 in their study of patients with septic shock reported that the pCO2 gap was lower in survivors than nonsurvivors, despite similar values of CO, O2 delivery (DO2), and O2 consumption (VO2).15
Following the trends in ScvO2 in our cohort, they improved with fluid resuscitation to reach the target value of 70% in patients with complications 24 hours after ICU admission, while ScvO2 was well above this target at ICU admission in patients who did not develop complications later on. ScvO2 as a part of early GDT has established role in improving the prognosis of patients.16,17 There is evidence in the literature that also shows that low ScvO2 during and early after high-risk abdominal surgery is associated with an increased risk of postoperative complications, as in our cohort.18 However, a recently published trial reported that there was no significant difference in ScvO2 levels on ICU admission between patients with and without postoperative complications. This can be explained by the difference in the methodology as they hemodynamically optimized patients with a value of ScvO2 above 70% during the surgery before inclusion in the study.
Also, sepsis was the main postoperative complication in our study results, a situation of microcirculation failure. The pCO2 gap is linearly related to CO2 production (VCO2) and inversely related to CO, according to the modified Fick equation.19 In situations where the oxygen demand-supply relationship is adequate, the flow is sufficient to wash out even if there is additional CO2 production. On the other hand, when blood flow is low, the pCO2 gap may increase even without excess CO2 production.20 Together, these factors explain that even when ScvO2 was normal, there was an increase in pCO2 gap in some patients, and this allowed for the prediction of postoperative complications. pCO2 improved in patients where low blood-flow induced tissue hypoxia was optimized following fluid resuscitation but persisted in conditions where hypoxemia was caused by microcirculation dysfunction (sepsis, heart failure). ScvO2 allowed discrimination between patients with or without postoperative complications but with less statistical significance as compared to pCO2 gap.
Considering lactate levels as a tool for predicting postoperative complications showed that the strength of association was lower (0.59) and showed statistical significance at later time intervals than ScvO2 and pCO2 gap. This may be explained by the slower clearance of lactate compared to the dynamic and rapid change in pCO2 levels. Also, lactate is affected by many variables as duration of surgery, intraoperative blood loss, and inotrope use in the immediate postoperative period.21
To conclude, our results highlight in comparison with other markers of tissue hypoperfusion, the pCO2 gap had greater sensitivity and specificity at ICU admission to predict postoperative complications in high-risk patients following major abdominal surgery in comparison with ScvO2, and the prediction was possible earlier than by lactate alone. Hence, a high pCO2 gap (>6 mm Hg) can be used as a complementary tool to identify patients at higher risk of postoperative complications after major abdominal surgery.
Our study has several limitations. Firstly, this was a single-center study and secondly included patients who underwent only high-risk major abdominal surgery. Therefore, these findings may certainly not be extrapolated to other patients undergoing non-abdominal surgeries. This could limit the external validity of the study; hence, the results need to be confirmed by future studies in a larger group of patients. Thirdly, the use of central venous-to-arterial pCO2 difference as a surrogate for mixed venous pCO2 might be a further limitation. But the correlation with CO can still be considered. Lastly, patients in our cohort had myocardial infarction and cardiac failure as postoperative complications, which could also be the reason for the increased pCO2 gap.
Contribution Details (to be ticked marked as applicable)
| Amandeep Kaur | Gunchan Paul | Parshotam L Gautam | Ayushi Sama | Vikram P Singh | |
|---|---|---|---|---|---|
| Concepts | ✓ | ✓ | ✓ | ✓ | ✓ |
| Design | ✓ | ✓ | ✓ | ✓ | ✓ |
| Definition of intellectual content | ✓ | ✓ | ✓ | ✓ | |
| Literature search | ✓ | ✓ | ✓ | ✓ | |
| Data acquisition | ✓ | ✓ | ✓ | ||
| Data analysis | ✓ | ✓ | ✓ | ✓ | |
| Statistical analysis | ✓ | ✓ | |||
| Manuscript preparation | ✓ | ✓ | ✓ | ✓ | |
| Manuscript editing | ✓ | ✓ | ✓ | ✓ | |
| Manuscript review | ✓ | ✓ | ✓ | ✓ | |
| Guarantor | ✓ | ✓ |
Supplementary Materials
The supplementary tables are available online on the website of www.IJCCM.org
Orcid
Amandeep Kaur https://orcid.org/0000-0002-9093-3132
Gunchan Paul https://orcid.org/0000-0002-3834-9852
Parshotam L Gautam https://orcid.org/0000-0002-7615-4781
Ayushi Sama https://orcid.org/0009-0008-8635-9810
Vikram P Singh https://orcid.org/0000-0001-8215-6508
Footnotes
Source of support: Nil
Conflict of interest: None
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