CHANGES IN DPCO2(V-A) OR PCO2 GAP IN RESPONSE TO ACUTE CHANGES IN VENTILATION

Background Early diagnosis of shock is a pre-determining factor for a good prognosis in intensive care. An elevated central venous to arterial PCO2 difference (ΔPCO2) over 0.8 kPa (6 mmHg) is indicative of low blood flow states. Disturbances around the time of blood sampling could result in inaccurate calculations of ΔPCO2, thereby misrepresenting the patient status. This study aimed to determine the influences of acute changes in ventilation on the ΔPCO2. Methods Eight pigs without cardiovascular or respiratory disease were studied. Arterial and central venous catheters were inserted following anaesthetization. Baseline ventilator settings were titrated to achieve an EtCO2 of 5 ± 0.5 kPa (VT = 8 ml/kg, Freq = 14±2 breaths per minute). Blood was sampled simultaneously from both catheters at baseline and 30, 60, 90, 120, 180 and 240 seconds after a change in ventilation. Pigs were subjected to both hyperventilation and hypoventilation, wherein the respiratory frequency was doubled or halved from baseline. ΔPCO2 changes from baseline were analysed using Repeated Measures ANOVA with post-hoc analysis using Bonferroni’s correction. Results Response of ΔPCO2 to acute changes in ventilation are illustrated in figure 1. ΔPCO2 at baseline was 0.76 ± 0.29 kPa (5.7 ± 2.2 mmHg). Following hyperventilation there was a rapid increase in the ΔPCO2, plateauing at 1.31 ± 0.24 kPa (9.75 ± 1.80 mmHg). There was a corresponding decrease in the ΔPCO2 following hypoventilation, reaching a maximum at 0.23 ± 0.31 (1.73 ± 2.33 mmHg). These changes were statistically significant from baseline 30 seconds after the change in ventilation. Conclusion Disturbances around the time of blood sampling can rapidly affect the PCO2, represented here by the changes in ventilation. This leads to inaccurate calculations of the ΔPCO2 resulting in misinterpretation of patient status, possibly affecting patient management decisions. We, therefore, advocate mindfulness when interpreting blood gases and caution with the use of these parameters while assessing patient status, especially if there is doubt as to the presence of a transient change in the patient’s ventilation status.