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Technology, Computing, and Simulation: Brief Report

Leaking Esophageal Probe May Lead to False Ventilator Settings When Guiding Positive End-Expiratory Pressure by Transpulmonary Pressure

Eichler, Lars MD; Truskowska, Katarzyna; Goetz, Alwin E. MD; Zöllner, Christian MD

Author Information
doi: 10.1213/ANE.0b013e31829ec090
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In critically ill patients, mechanical ventilation guided by transpulmonary pressure or pressure across the lung (PL) has been demonstrated to improve oxygenation and outcome as compared with positive end-expiratory pressure (PEEP) adjustment according to the acute respiratory distress syndrome network table.1 A recently published series of patients suffering from acute respiratory distress syndrome due to influenza A (H1N1) pneumonia used PL as the basis for selecting between further increase of PEEP or initiation of extracorporeal membrane oxygenation therapy.2 Setting PEEP according to the mechanical properties of the respiratory system is one approach to finding the optimal PEEP.3 Goal-directed ventilation using PL is not widely used but might find broader acceptance in the future.4

Esophageal pressure (Pes) can be used as a surrogate for intrapleural pressure, permitting calculation of the PL as PL = Paw − Pes, where Paw is the airway pressure. Nasogastric tubes equipped with a pressure monitoring balloon are commercially available and can be placed in the same way as a conventional nasogastral tube. Advanced into the lower esophagus, heart beat synchronous pressure waves are monitored, and probes are withdrawn slowly until only pressure waves synchronous with respiration are detected. A positive PL during the entire respiratory cycle would result in an outward force that prevents lung collapse and atelectasis formation.

In this brief report, we demonstrate a potential pitfall of this promising technique. This highlights the necessity of thorough interpretation even of supposedly simple monitor values to avoid false therapeutic decisions.

METHODS

Our initial observations were made during a clinical study in patients undergoing bariatric surgery. The local ethics committee approved our study protocol, and all patients granted written informed consent on the day before surgery. The study is registered under DRKS-ID: DRKS00004223 at the German Clinical Trials Register.

The lungs of patients scheduled for bariatric surgery were mechanicaly ventilated with a respirator allowing for simultaneous measurements of intraesophageal pressure via a nasogastric tube (Avea™, CareFusion, Höchberg, Germany). Anesthesia was provided as a target-controlled infusion using remifentanil and propofol (Perfusor® Space, Braun, Melsungen, Germany). PL was derived from the equation PL = Paw − Pes and displayed on the respirator’s monitor. The PEEP level was adjusted to achieve a PL between 0 and 5 cm H2O at the end of expiration.

RESULTS

After setting the PEEP to 20 cm H2O, resulting in a PL of 0 to 5 cm H2O at the end of expiration, we saw a gradual increase in PL over several minutes. This change took place even though Paw, intra-abdominal pressure, and patient position remained constant. In response to these changes, we decreased the PEEP to reach the desired PL level of 0 to 5 cm H2O. A recalibration of the pressure-measuring device restored the initial values for 2 consecutive breaths. After this, the same gradual increase in PL values was seen (Fig. 1).

Figure 1
Figure 1:
Original recording of airway pressure (Paw), esophagus pressure (Pes), and transpulmonary pressure (PL). At an initial positive end-expiratory pressure (PEEP) level of 20 cm H2O, a target PL between 0 and 5 cm H2O was reached. While a stable Paw was set on the respirator, a gradual decline of Pes led to a corresponding increase in PL. After 1 minute, this increase in PL was answered by a reduction of PEEP level bringing PL back to a target level of 0 to 5 cm H2O. This value, however, did not remain stable as the decline in Pes measurement continued. Recalibration of the esophageal probe (involves deflation and refilling of the balloon) after 5 minutes could not restore a stable measurement. Eventually, Pes reached a value of 0 resulting in a PL equal to the set Paw.

The study protocol required a corresponding decrement in PEEP to avoid unnecessarily high PLs. For lack of plausibility in the context of a subject with body mass index of 44 kg/m2, where one would expect to apply PEEP values between 15 and 25 cm H2O to reach a positive PL, this patient was excluded and treated with a standard PEEP of 10 cm H2O.

On subsequent analysis of pressure tracings, the observed increases in PLs were recognized as reflecting gradually decreasing intraesophageal pressure measurements. Since PL is calculated by subtracting Pes from Paw, this led to a constantly increasing value for PL eventually approaching the level of Paw. A comparable course was seen in 4 subsequent patients and reported to the company that manufactured the ventilator. Eventually, a leak in the connection between the esophageal balloon and pressure-measurement tubing was detected. The manufacturer solved the problem by introducing an additional silicon seal that prevented air from escaping out of the balloon (Fig. 2).

Figure 2
Figure 2:
Connectors for attaching esophageal probes to the respirator were improved by introduction of a silicon seal (A). The former plastic connector (B) did allow air to escape from the esophageal balloon resulting in gradually decreasing values for esophageal pressure.

DISCUSSION

When guiding mechanical ventilation by PL, it is important to understand how this value is calculated. Our ventilator displayed a tracing of PL in addition to the usual Paw, flow, and CO2 curves. However, the Pes measurements from which PL was derived were not shown, so we were not aware that the gradual increase in PL was the result of decreasing Pes values. The consequence was inappropriate reduction in PEEP, possibly increasing the risk of atelectasis and impaired gas exchange.

DISCLOSURES

Name: Lars Eichler, MD.

Contribution: This author helped design and conduct the study and prepare the manuscript.

Attestation: Lars Eichler approved the final manuscript and attests to the integrity of the original data and the analysis reported in this manuscript. Lars Eichler is the archival author.

Conflicts of Interest: The reported observation was made during the conduct of a clinical study on bariatric surgery patients. The authors received support from CareFusion who provided the respirator (Avea) and esophageal pressure probes.

Name: Katarzyna Truskowska.

Contribution: The author helped design and conduct the study.

Attestation: Katarzyna Truskowska approved the final manuscript and attests to the integrity of the original data and the analysis reported in this manuscript.

Conflicts of Interest: The reported observation was made during the conduct of a clinical study on bariatric surgery patients. The authors received support from CareFusion who provided the respirator (Avea) and esophageal pressure probes.

Name: Alwin E. Goetz, MD.

Contribution: This author helped design the study and prepare the manuscript.

Attestation: Alwin E. Goetz approved the final manuscript and attests to the integrity of the original data and the analysis reported in this manuscript.

Conflicts of Interest: The reported observation was made during the conduct of a clinical study on bariatric surgery patients. The authors received support from CareFusion who provided the respirator (Avea) and esophageal pressure probes.

Name: Christian Zöllner, MD.

Contribution: This author helped design and conduct the study and prepare the manuscript.

Attestation: Christian Zöllner approved the final manuscript and attests to the integrity of the original data and the analysis reported in this manuscript.

Conflicts of Interest: The reported observation was made during the conduct of a clinical study on bariatric surgery patients. The authors received support from CareFusion who provided the respirator (Avea) and esophageal pressure probes.

This manuscript was handled by: Steven L. Shafer, MD.

REFERENCES

1. Talmor D, Sarge T, Malhotra A, O’Donnell CR, Ritz R, Lisbon A, Novack V, Loring SH. Mechanical ventilation guided by esophageal pressure in acute lung injury. N Engl J Med. 2008;359:2095–104
2. Grasso S, Terragni P, Birocco A, Urbino R, Del Sorbo L, Filippini C, Mascia L, Pesenti A, Zangrillo A, Gattinoni L, Ranieri VM. ECMO criteria for influenza A (H1N1)-associated ARDS: role of transpulmonary pressure. Intensive Care Med. 2012;38:395–403
3. Gattinoni L, Carlesso E, Brazzi L, Caironi P. Positive end-expiratory pressure. Curr Opin Crit Care. 2010;16:39–44
4. Soroksky A, Esquinas A. Goal-directed mechanical ventilation: are we aiming at the right goals? A proposal for an alternative approach aiming at optimal lung compliance, guided by esophageal pressure in acute respiratory failure. Crit Care Res Pract. 2012;2012:597932
© 2013 International Anesthesia Research Society