False positives in determining the proper location of the esophageal pressure catheter during occlusion test

Diego José Duque, León Darío Jiménez, Luis Santiago Correa, Alejandro Rivera*

* Grupo de Investigación en Ventilación Mecánica, Hospital San Vicente De Paúl, Medellín, Colombia. E-mail: djd@une.net.co, leondario@emco.com.co, soporte@emco.com.co

Recibido: noviembre 3 de 2009. Enviado para modificaciones: marzo 11 de 2010. Aceptado: febrero 16 de 2010

ABSTRACT

Objective: To show that the esophageal pressure (P_es) - airway pressure (P_ao) loop, with airway occlusion at the end of expiration - known as the occlusion test and which is the gold standard to determine the proper placement of the esophageal pressure catheter, could yield false positive results when the catheter is accidentally inserted into the airway.

Methodology: Case report of 5 Adults with cranioencephalic trauma and no pulmonary involvement or any other trauma or associated pathology from a trauma intensive care (ICU) unit of a University Hospital. Placement of an esophageal balloon catheter for monitoring mechanical ventilation at weaning off the invasive mechanical ventilation system, is done.

Results: Once the esophageal catheter is placed, P_es-Time, P_ao-Time and P_es-P_ao loop charts are developed with airway expiratory occlusion. The datalogger software and the Galileo Gold ventilator (Hamilton Medical, Rhäzüns, Suiza) were used, together with an advanced application developed in Excel (Microsoft Office 2003) to organize and plot the variables monitored.

Conclusion: The occlusion test (P_es-P_ao Loop with airway occlusion at the end of the expiration) generates a 45º-slope plot, when the catheter is properly placed inside the Esophagus. This is considered the gold standard test. However, a similar plot can also be obtained if the esophageal catheter is accidentally introduced into the airway and the clinician inadvertedly overlooks the situation.

Key words: Esophagus, pressure, catheterization, pulmonary ventilation, software (Source: MeSH, NLM)

INTRODUCTION

The dynamic measurement of the esophageal pressure (p_es) is not a common practice in patients with mechanical ventilation. However, it is a minimally invasive measurement that requires experienced hands and provides very valuable information to study the respiratory mechanics: the mechanics of the chest wall, the respiratory muscle activity in patients with spontaneous breathing and respiratory work, inter alia. (1,2,3).

Measuring the esophageal wall requires a catheter with a balloon at the tip, connected to a transducer through an air-filled tube.

The catheter advances into the stomach while the patient is spontaneously breathing. Then the catheter is removed up to the distal third of the Esophagus. Its proper placement is confirmed with the occlusion test, having the patient spontaneously breathing in a semi-sitting position, when the esophageal pressure vs. airway pressure loop ( p_es-p_ao Loop) changes during the test to a straight line at a 45°angle.

The reliability of this measurement depends on the excellent placement of the catheter; hence, checking its proper placement is crucial. There are many studies validating this measurement (4-13).

MATERIALS AND METHODS

At the trauma ICU of the San Vicente de Paul University Hospital in Medellin, upon the approval by the ethics committee of the hospital and by the research committee to use and study the esophageal pressure in a patient in whom this measurement was taken, an abnormal behavior of the p_ao-time and p_es-time curves was identified; both had a very similar tracing, despite a positive occlusion test. So the decision was made to compare four patients in whom an esophageal catheter had already been placed and the graphical records were available. The charts compared were p_ao-time, p_es-time and the p_es-p_ao Loop.

All patients included had serious cranioencephalic trauma, with no previous illness, no pulmonary involvement and no other associated trauma. The patients were hemodynamically stable and about to be weaned off their assisted mechanical ventilation.

A Galileo Gold (Hamilton Medical, Rhäzüns, Suiza) ventilator was used, connected to a PC with the Datalogger software to acquire the data; the data captured are the variables: p_ao, p_es, airway flow (F) and tidal volume (Vt), with a sampling frequency of 14,7 m.

An advanced application developed in Excel (Microsoft Office, 2003) enabled the organization and plotting of the p_es-p_ao loop with airway occlusion at the end of expiration and of simultaneous p_es and p_ao charts over time.

The occlusion test is the reference test to determine the proper placement of the catheter in the distal third of the Esophagus and hence it should be administered to every patient.

RESULTS

The occlusion test in the five patients is shown in Figure 1. As can be seen, all are positive because the slope is at a 45° angle.

When looking a the tracings of the charts developed simultaneously for p_ao and p_es vs. time (figure 2), patient #1 shows an extreme coincidence in the tracings for the variables p_es and p_ao. This situation leads us to suspect that the catheter is inside the airway.

In the other four patients, there is a clear difference in the make-up and shape of these two signals, corresponding to a normal esophageal pressure tracing and a normal airway pressure (figure 2) (1).

DISCUSSION

When introducing the esophageal catheter there is a chance that it may be placed inside the airway. The simultaneous analysis of the p_ao-time and p_es-time curves may apparently help to solve this issue because in both cases the measurements are taken in the same compartment (p_es, directly in the airway, p_ao, at the beginning of the endotracheal tube), so they are pretty similar in terms of their shape and configuration. Actually, it should be suggested that in order to determine the proper placement of the esophageal catheter and rule out airway placement, in addition to having a positive occlusion test, the shape of the p_es-time and p_ao-time tracings should also be considered as complementary tools.

Consequently, a stronger emphasis should be made on the detailed analysis of the p_es-time and p_ao-time curves, following a positive occlusion test result, as a complementary tool to rule out any false positives and be absolutely certain that the catheter is inside the esophagus.

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