Changes in central venous to arterial carbon dioxide gap (PCO₂ gap) in response to acute changes in ventilation

Early diagnosis of shock is a predetermining factor for a good prognosis in intensive care. An elevated central venous to arterial PCO₂ difference (∆PCO₂) over 0.8 kPa (6 mm Hg) is indicative of low blood flow states. Disturbances around the time of blood sampling could result in inaccurate calculations of ∆PCO₂, thereby misrepresenting the patient status. This study aimed to determine the influences of acute changes in ventilation on ∆PCO₂ and understand its clinical implications.

To investigate the isolated effects of changes in ventilation on ∆PCO₂, eight pigs were studied in a prospective observational cohort. Arterial and central venous catheters were inserted following anaesthetisation. Baseline ventilator settings were titrated to achieve an EtCO₂ of 5±0.5 kPa (V_T = 8 mL/kg, Freq = 14 ± 2/min). Blood was sampled simultaneously from both catheters at baseline and 30, 60, 90, 120, 180 and 240 s after a change in ventilation. Pigs were subjected to both hyperventilation and hypoventilation, wherein the respiratory frequency was doubled or halved from baseline. ∆PCO₂ changes from baseline were analysed using repeated measures ANOVA with post-hoc analysis using Bonferroni's correction.

∆PCO₂ at baseline for all pigs was 0.76±0.29 kPa (5.7±2.2 mm Hg). Following hyperventilation, there was a rapid increase in the ∆PCO₂, increasing maximally to 1.35±0.29 kPa (10.1±2.2 mm Hg). A corresponding decrease in the ∆PCO₂ was seen following hypoventilation, decreasing maximally to 0.23±0.31 kPa (1.7±2.3 mm Hg). These changes were statistically significant from baseline 30 s after the change in ventilation.

Disturbances around the time of blood sampling can rapidly affect the PCO₂, leading to inaccurate calculations of the ∆PCO₂, resulting in misinterpretation of patient status. Care should be taken when interpreting blood gases, if there is doubt as to the presence of acute and transient changes in ventilation.