Original Contributions
Correlation of arterial Pco2 and Petco2 in prehospital controlled ventilation

This work was presented to the Annual Congress of Emergency Medicine, Paris, April 2002.
https://doi.org/10.1016/j.ajem.2005.04.011Get rights and content

Abstract

Introduction

This study was carried out to estimate the relationship between arterial Pco2 (Paco2) and end-tidal carbon dioxide (Petco2) during prehospital controlled ventilation and also to evaluate variation of the gradient between Pco2 and Petco2 during prehospital transport.

Methods

Measurements of Petco2 from capnography values and Paco2 from arterial blood gases were registered at the beginning (T0) and at the end (Tend) of out-of-hospital management. For all patients requiring invasive ventilation, the gradient between Pco2 and Petco2 was calculated for T0 and Tend, the Paco2 − Petco2 variation between Tend and T0 was also calculated.

Results

One hundred patients were included in this study (mean age, 58.4 ± 16.4 years; 57 were male). There was no variation of the mean gradient (ΔPaco2 − Petco2) during transport (8.64 ± 13.5 mm Hg at T0 and 7.26 ± 12.94 mm Hg at Tend). Thirty-six percent of patients (n = 36) had a gradient above +10 mm Hg, and for 6% of patients (n = 4) the gradient was lower than −10 mm Hg. The Paco2 − Petco2 gradient was not significantly different according to the pathology, but was significantly higher in hypercapnic patients compared with hypocapnic or normocapnic patients. In patients with severe head injury, the capnia was normalized in 80% of patients at the end of the transport according to the last blood gas result. In this subgroup the ΔPaco2 − Petco2 (TendT0) gradient was stable between T0 and Tend except in 20% of the patients for whom the ΔPaco2 − Petco2 was lower than −10 mm Hg. Fifty-four percent of critical care physicians had modified the respiratory setting after the first arterial blood gas results.

Conclusions

The Paco2 cannot be estimated by the Petco2 in the prehospital setting. There is wide variation in the gradient between Pco2 and Petco2 depending on patient condition, and over time, the relationship does not remain constant and thus cannot be useful in prehospital ventilation management.

Introduction

Prehospital critical care teams manage critically ill patients. Endotracheal intubation and controlled ventilation, initiated in the field, are required in these emergency situations. Quality of prehospital ventilation is difficult to monitor. Owing to new sophisticated portable ventilators, progress has been made in assisted prehospital ventilation, which enables a better adaptation to the patient's respiratory requirements. Indeed, the latest achievements of these new ventilators allow for several types of assisted or controlled ventilation; in this way they improve the quality of mechanical prehospital ventilation, which is more appropriate to the patient's ventilation requirements, pathology, and background. Until recently, mechanical prehospital ventilation was monitored on clinical symptoms and capnography monitoring.

Capnography is now an obligation in anesthesia units and in operating rooms [1]. In the same way, monitoring of end-tidal carbon dioxide pressure (Petco2) has become essential in emergency departments and critical care units for many reasons [2], [3]. Capnography confirms the correct placement of the endotracheal tube and monitors the integrity of mechanical ventilation equipment [4], [5], [6]. Petco2 is also very helpful in cardiac resuscitation; indeed, recent studies have shown Petco2 values were a good indication of the adequacy of chest massage and predict survival in cardiac arrest [7], [8], [9], [10].

It is also recommended for prehospital monitoring of severe head trauma patients to avoid hypo- or hypercapnia during transportation [11], [12], [13], [14], [15]. But the ability of Petco2 to predict capnia has not been clearly established in out-of-hospital patients [14]. The aims of this study were to (1) determine the level of Paco2 − Petco2 gradient and (2) analyze the gradient variation during transportation time.

Section snippets

Study setting and population

This prehospital study has been carried out in the French emergency medical services system (SAMU) based on physician-staffed ambulances over a 16-month period. The French emergency medical services system is based on 2 types of ambulances: emergency medical technician (EMT)–staffed ambulances for basic life support and physician-staffed ambulances for advanced life support [16], [17]. The EMTs are members of the fire department or members of the French Red Cross. The telephone number is a

Results

A total of 100 patients were included in this study (mean age, 58.4 ± 16.4 years; 57 were male). Patients' disease data are noted in Table 1. Hemodynamic and ventilation data are shown in Table 2. Arterial blood gases results and calculated Paco2 − Petco2 are recorded in Table 3. The gradient between Paco2 and Petco2 was available for 96 patients at T0 and for 82 patients at Tend. The missing data for 18 patients were due to technical difficulties to obtain blood gas samples at T0 or Tend.

Even

Discussion

The role of capnography in prehospital management is indisputable [25], [26], [27]. Thus, capnometry's role is not any more to show in several applications: ensure correct placement of endotracheal tube [28], [29], verify the integrity of mechanical ventilation equipment [26], [28], and predicting outcome in patient survival. Moreover, because Petco2 seems to be strongly linked to the cardiac output [14], Petco2 is now used also during cardiopulmonary resuscitation to optimize chest massage [8]

Acknowledgment

We thank all the physicians and nurses who took part in this study. The authors gratefully thank Mrs Fiona Richardson for her help with the English revision and Prof Jean Mantz for his help in the response to the reviewers.

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