Changes in central venous to arterial carbon dioxide gap (PCO2 gap) in response to acute changes in ventilation. Issue 1 (18th March 2021)

• Record Type: Journal Article
• Authors: Shastri, Lisha; Kjærgaard, Benedict; Rees, Stephen Edward; Thomsen, Lars Pilegaard
• Is Part Of: BMJ open respiratory research. Volume 8:Issue 1(2021)
• Publication Date: 2021-03-18
• Subjects: not applicable; Respiratory organs -- Diseases -- Periodicals; Respiratory organs -- Diseases -- Treatment -- Periodicals; Respiratory therapy -- Periodicals; 616.2005
• DOI: 10.1136/bmjresp-2021-000886
• Languages: English
• ISSNs: 2052-4439

Abstract

Abstract : Background: 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. Methods: 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 (VT = 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. Results: ∆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. Conclusion: 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.