Noninvasive Carbon Dioxide Monitoring in a Porcine Model of Acute Lung Injury Due to Smoke Inhalation and Burns

Abstract

In critically ill intubated patients, assessment of adequacy of ventilation relies on measuring partial pressure of arterial carbon dioxide (PaCO2), which requires invasive arterial blood gas analysis. Alternative noninvasive technologies include transcutaneous CO2 (tPCO2) and end-tidal CO2 (EtCO2) monitoring. We evaluated accuracy of tPCO2 and EtCO2 monitoring in a porcine model of acute lung injury (ALI) due to smoke inhalation and burns. Eight anesthetized Yorkshire pigs underwent mechanical ventilation, wood-bark smoke inhalation injury, and 40% total body surface area thermal injury. tPCO2 was measured with a SenTec system (SenTec AG, Therwil, Switzerland) and EtCO2 with a Capnostream-20 (Oridion Medical, Jerusalem, Israel). These values were compared with PaCO2 measurements from an arterial blood gas analyzer. Paired measurements of EtCO2-PaCO2 (n = 276) and tPCO2-PaCO2 (n = 250) were recorded in the PaCO2 range of 25 to 85 mmHg. Overlapping data sets were analyzed based on respiratory and hemodynamic status of animals. Acute lung injury was defined as PaO2/FIO2 ≤ 300 mmHg; hemodynamic instability was defined as mean arterial pressure ≤ 60 mmHg. Before ALI, EtCO2 demonstrated moderate correlation with PaCO2 (R = 0.45; P< 0.0001), which deteriorated after onset of ALI (R = 0.12; P < 0.0001). Before ALI, tPCO2 demonstrated moderate correlation (R = 0.51, P < 0.0001), which was sustained after onset of ALI (R = 0.78; P < 0.0001). During hemodynamic stability, EtCO2 demonstrated moderate correlation with PaCO2 (R = 0.44; P < 0.0001). During hemodynamic instability, EtCO2 did not correlate with PaCO2 (R = 0.03; P = 0.29). tPCO2 monitoring demonstrated strong correlation with PaCO2 during hemodynamic stability (R = 0.80, P < 0.0001), which deteriorated under hemodynamically unstable conditions (R = 0.39; P < 0.0001). Noninvasive carbon dioxide monitors are acceptable for monitoring trends in PaCO2 under conditions of hemodynamic and pulmonary stability. Under unstable conditions, reevaluation of patient status and increased caution in the interpretation of results are required.

The relationship between minute ventilation and end tidal CO2 in intubated and spontaneously breathing patients undergoing procedural sedation

“…Other factors which decrease the accuracy and utility of capnography in non-intubated patients include mouth versus nose breathing, changes in flow of oxygen, procedures requiring oral intervention or lack of access to the head of the bed to ensure proper cannula placement [25]. Furthermore, EtCO 2 does not consistently reflect PaCO 2 , particularly in patients with cardiac and respiratory failure and in patients with a high ventilation-perfusion ratio [6–8,26–29]. Even when reliable EtCO 2 measurements are obtained, they provide a lagging indicator of respiratory performance rather than direct measure of changes in respiratory volumes [30].…”

“…End tidal CO 2 (EtCO 2 ) monitoring with capnography has become the standard of care in intubated patients for both confirming endotracheal tube placement and monitoring adequacy of ventilation [1,2]. Capnography with an endotracheal tube in place is considered a reliable method to non-invasively reflect arterial PaCO 2 [3,4], however, measuring EtCO 2 in spontaneously breathing patients can be inaccurate in certain settings, particularly during procedural sedation [5] and post-operatively in the post-anesthesia care unit [6–8]. Variables such as sensor positioning, changes in respiratory patterns, and changes in oxygen supplementation often distort EtCO 2 measurements in non-intubated patients, rendering them unreliable.…”

Methods, Measurements, and Interpretation of Animal Lung Function in Health and Disease

“…Compensating mechanisms, both cardiovascular and respiratory, can mask diffusional deficiencies as the body systematically seeks homeostasis. That is not to say, however, that blood gas balance does not fluctuate in response to challenges where local circulation or rapid shifts in ventilation distribution in the lung might have immediate effects on blood gas levels (Belenkiy et al, 2013;Krishnamoorthy et al, 2012). Steady state diffusion assessment in chronic disease or in health is more frequently of interest.…”

Progress of clinical practice on the management of burn-associated pain: Lessons from animal models

“…Another study examined mobilization of mononuclear cells after a 30%TBSA scald burn [166]. Many previous models that involve extensive burn injuries in large animals also incorporate an associated co-morbidity in the form of LPS injection [167][168][169] or smoke inhalation [170,171]. The addition of these clinically-relevant co-morbidities can be performed to augment/alter the systemic inflammation.…”