End-tidal CO2: physiology in pursuit of clinical applications

Noninvasive Estimation of Arterial CO2 From End-Tidal CO2 in Mechanically Ventilated Children: The GRAeDIENT Pilot Study

OBJECTIVES

The aim of our pilot study was to develop a model to better predict Paco2 in mechanically ventilated children using noninvasive parameters including volumetric capnography.

DESIGN

Prospective clinical pilot study.

SETTING

Level III PICU.

PATIENTS

Sixty-five mechanically ventilated children.

INTERVENTIONS

None.

MATERIALS AND METHODS

We conducted a prospective clinical pilot study that included all children admitted to the PICU (< 18 yr; weight, > 3 kg; mechanically ventilated, > 6 hr; with an arterial line). A predictive model for PaCO2 was developed using linear multivariable regression. Among the data collected in PICU patients, candidate predictors of PaCO2 were defined by a panel of experts and included end-tidal partial pressure of carbon dioxide, ventilation parameters, and data resulting from the analysis of volumetric capnogram recorded 5 minutes before an arterial blood gas. Children with tidal volume less than 30 mL were excluded because of technical limits.

RESULTS

A total of 65 children (43 boys, 65%) (65 [21-150] mo old) were analyzed. By linear multivariable regression, the best model included the mean airway pressure, end-tidal partial pressure of carbon dioxide, FIO2, and the capnographic index with an R equal to 0.90, p value less than 0.001. After correction, 95% (n = 62) of children had an estimated PaCO2 at ± 5 mm Hg.

CONCLUSION

Our model developed provides an accurate estimation of the PaCO2 using end-tidal CO2 and noninvasive variables. Studies are needed to validate the equation in PICUs.

Monitoring CO2 in shock states

The primary end point when treating acute shock is to restore blood circulation, mainly by reaching macrocirculatory parameters. However, even if global haemodynamic goals can be achieved, microcirculatory perfusion may remain impaired, leading to cellular hypoxia and organ damage. Interestingly, few methods are currently available to measure the adequacy of organ blood flow and tissue oxygenation. The rise in tissue partial pressure of carbon dioxide (CO2) has been observed when tissue perfusion is decreased. In this regard, tissue partial pressure of CO2 has been proposed as an early and reliable marker of tissue hypoxia even if the mechanisms of tissue partial pressure in CO2 rise during hypoperfusion remain unclear. Several technologies allow the estimation of CO2 content from different body sites: vascular, tissular (in hollow organs, mucosal or cutaneous), and airway. These tools remain poorly evaluated, and some are used but are not widely used in clinical practice. The present review clarifies the physiology of increasing tissue CO2 during hypoperfusion and underlines the specificities of the different technologies that allow bedside estimation of tissue CO2 content.

Assessment of end-tidal carbon dioxide during pediatric and adult sedation for endoscopic procedures

BACKGROUND

Pulse oximetry has become the standard of care during endoscopic procedures, despite the fact that significant alveolar hypoventilation may be undetected.

OBJECTIVE

To study the value of end-tidal carbon dioxide (EtCO(2)) measurement during pediatric and adult endoscopic procedures with the patient under general anesthesia (GA) and conscious sedation (CS).

DESIGN AND SETTINGS

Oridion Microcap hand-held capnography by using Smart Bite Bloc with oxygen (O(2)) delivery were used for the procedures. Microstream nondispersive infrared (IR) spectroscopy is used to measure the concentration of molecules that absorb IR light in CO(2) exhaled by the subject. For each patient, we defined an "event" based on a combination of a >or=20% change (increase or decrease) in EtCO(2), with at least one of the following: O(2) saturation (SPO(2)) <or=90%, a >or=20% change of pulse rate or respiratory rate.

PATIENTS

We studied 57 patients, with an age range of 4 to 62 years. Nineteen patients (33.3%) had CS and 38 (66.6%) had GA.

RESULTS

Twenty patients had no events, 32 had 1 event, and 5 patients had 2 events. The highest observed frequency of an event was noted during upper endoscopy under GA (0.35), followed by upper endoscopy under CS (0.32). Fitted univariate logistic regression models indicated that higher variability in EtCO(2) is associated with a higher probability for an event (P < .0001) and that an increase in age is associated with a lower probability for an event (P < .0001). Significant differences in the frequencies of SPO(2) events were related to the type of procedure (P = .0002; highest estimated probability for upper endoscopy) and GA (P < .0001). Similar conclusions were obtained based on the fitted multivariate model.

CONCLUSIONS

EtCO(2) contributes significantly to the prediction of events during endoscopy. A lower mean of EtCO(2), higher variability of EtCO(2), younger age, GA, and upper endoscopy increase the probability of an event.

Current status of noninvasive ventilation in stable COPD patients

Noninvasive ventilation (NIV) has been one of the major advances in respiratory medicine in the last decade. NIV improves quality of life, prolongs survival, and improves gas exchange and sleep quality in restrictive patients, but evidence available now does not allow us to establish clear criteria for prescribing NIV in patients with chronic respiratory failure due to COPD. On the basis of the available studies, NIV should not be used as a treatment of choice for all patients with COPD, even when disease is severe. However, there is more evidence that NIV has an important effect in these patients. In fact, a selected group of patients may well benefit from domiciliary mechanical ventilation, and we need to be able to identify who they are. Moreover, NIV can be a new strategy to improve exercise tolerance in COPD patients.

Arterial minus end-tidal CO2 as a prognostic factor of hospital survival in patients resuscitated from cardiac arrest

AIMS

The purpose of this study was to determine the clinical value of arterial minus end-tidal CO(2) [P(a-et)CO(2)] and alveolar dead space ventilation ratio (V(dA)/V(t)) as indicators of hospital mortality in patients that have been resuscitated from cardiac arrest at emergency department.

MATERIALS AND METHODS

Forty-four patients with a return of spontaneous circulation (ROSC) after cardiac arrest were studied in the emergency department of a university teaching hospital from March 2004 to February 2006. Mean arterial pressure (MAP), serum lactate, arterial blood gas studies, end-tidal CO(2) (EtCO(2)), P(a-et)CO(2), and V(dA)/V(t) were evaluated at 1 h after ROSC. We compared these variables between hospital survivors and non-survivors.

RESULTS

The rates of ventricular fibrillation and pulseless ventricular tachycardia in hospital survivors were higher than those of non-survivors (53.0 and 9.7%, respectively, p=0.002). Hospital survivors had significantly higher MAP, lower serum lactate, lower P(a-et)CO(2), and lower V(dA)/V(t) value than non-survivors. Receiver operator characteristic (ROC) curves of serum lactate, P(a-et)CO(2), and V(dA)/V(t) showed significant sensitivity and specificity for hospital mortality. Specifically, lactate > or = 10.0 mmol/L, P(a-et)CO(2) > or = 12.5 mmHg, and V(dA)/V(t) > or = 0.348 were all associated with high hospital mortality (p=0.000, 0.001 and 0.000, respectively).

CONCLUSIONS

This study showed that high serum lactate, high P(a-et)CO(2) and high V(dA)/V(t) during early ROSC in cardiac arrest patients suggest high hospital mortality. If future studies validate this model, the P(a-et)CO(2) and V(dA)/V(t) may provide useful guidelines for the early post-resuscitation care of cardiac arrest patients in emergency departments.

Difference in end-tidal CO2 between asphyxia cardiac arrest and ventricular fibrillation/pulseless ventricular tachycardia cardiac arrest in the prehospital setting

INTRODUCTION

There has been increased interest in the use of capnometry in recent years. During cardiopulmonary resuscitation (CPR), the partial pressure of end-tidal carbon dioxide (PetCO2) correlates with cardiac output and, consequently, it has a prognostic value in CPR. This study was undertaken to compare the initial PetCO2 and the PetCO2 after 1 min during CPR in asphyxial cardiac arrest versus primary cardiac arrest.

METHODS

The prospective observational study included two groups of patients: cardiac arrest due to asphyxia with initial rhythm asystole or pulseless electrical activity, and cardiac arrest due to acute myocardial infarction or malignant arrhythmias with initial rhythm ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT). The PetCO2 was measured for both groups immediately after intubation and then repeatedly every minute, both for patients with and without return of spontaneous circulation (ROSC).

RESULTS

We analyzed 44 patients with asphyxial cardiac arrest and 141 patients with primary cardiac arrest. The first group showed no significant difference in the initial value of the PetCO2, even when we compared those with and without ROSC. There was a significant difference in the PetCO2 after 1 min of CPR between those patients with ROSC and those without ROSC. The mean value for all patients was significantly higher in the group with asphyxial arrest. In the group with VF/VT arrest there was a significant difference in the initial PetCO2 between patients without and with ROSC. In all patients with ROSC the initial PetCO2 was higher than 10 mmHg.

CONCLUSIONS

The initial PetCO2 is significantly higher in asphyxial arrest than in VT/VF cardiac arrest. Regarding asphyxial arrest there is also no difference in values of initial PetCO2 between patients with and without ROSC. On the contrary, there is a significant difference in values of the initial PetCO2 in the VF/VT cardiac arrest between patients with and without ROSC. This difference could prove to be useful as one of the methods in prehospital diagnostic procedures and attendance of cardiac arrest. For this reason we should always include other clinical and laboratory tests.

Survey of use of end-tidal carbon dioxide for confirming tracheal tube placement in intensive care units in the UK

The use of end-tidal carbon dioxide monitoring to confirm the correct placement of a tracheal tube immediately after intubation is mandatory in the operating theatre. Tracheal intubation in critically ill patients can be challenging. Quick and accurate confirmation of tracheal tube placement is essential to minimise complications. This survey explored the use of end-tidal carbon dioxide monitoring to confirm tracheal tube placement in intensive care units in the UK. Questionnaires were sent to either the lead clinician or clinical director of randomly selected general adult intensive care units. One hundred and twenty-seven replies were received from the 215 questionnaires sent (response rate 59%). Twenty per cent of the units did not have an end-tidal carbon dioxide monitor, 20% had one end-tidal carbon dioxide monitor per bed and 60% had one end-tidal carbon dioxide monitor between several beds. Only 50% of the units having an end-tidal carbon dioxide monitor use it to confirm correct tracheal tube placement. Of these 50%, only about a third use it for every intubation. Seventy-two per cent of respondents felt that end-tidal carbon dioxide is well suited to confirm correct placement of tracheal tube in critically ill patients, but 50% did not think that confirmation using end-tidal carbon dioxide should be mandatory for intubations outside the operating theatre. Half of the units not having end-tidal a carbon dioxide monitor cited lack of resources as a reason. In summary, although four in every five intensive care units surveyed have end-tidal carbon dioxide monitors, only a small proportion use them to confirm correct placement of tracheal tube after intubation.