Chest

Chest

Volume 113, Issue 2, February 1998, Pages 443-451
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Clinical Investigations in Critical Care
Evaluation of Respiratory Inductive Plethysmography in Controlled Ventilation: Measurement of Tidal Volume and PEEP-Induced Changes of End-Expiratory Lung Volume

https://doi.org/10.1378/chest.113.2.443Get rights and content

Study objective

To determine the accuracy of respiratory inductive plethysmography (RIP) with a respiratory monitor (Respitrace Plus; NIMS Inc., Miami) operating in the DC-mode for the measurement of tidal volumes (Vt) and positive end-expiratory pressure (PEEP)-induced changes of end-expiratory lung volume (ΔEELV) in patients with normal pulmonary function, acute lung injury (ALI), and COPD during volume-controlled ventilation.

Design

Prospective comparison of RIP with pneumotachography (PT) for assessment of Vt and with multibreath nitrogen washout procedure (N2WO) for determination of ΔEELV as reference methods.

Setting

Mixed ICU at a university hospital.

Patients

Thirty-one sedated and paralyzed patients: 12 patients with normal pulmonary function mechanically ventilated after major surgery, 10 patients with respiratory failure due to ALI, and 9 patients with a known history of COPD ventilated after surgery or because of respiratory failure due to bronchopulmonary infection.

Interventions

Stepwise increase of PEEP from 0 to 5 to 10 cm H2O and reduction to 0 cm H2O again. On each PEEP level, N2WO was performed.

Measurements and main results

The baseline drift of RIP averaged 25.4±29.1 mL/min but changed over a wide range even in single patient measurements. Determination of Vt for single minutes revealed that 66.5% and 90.0% of all values were accurate within a range of±10% and±20%, respectively. The deviation for Vt measurements between RIP and PT in patients with COPD was significantly (p<0.05) higher compared to patients with ALI or normal pulmonary function. The difference of ΔEELV between RIP and N2WO was 11.6±174.1 mL with correlation coefficients of 0.77 (postoperative and COPD patients) and 0.86 (ALI patients). However, just 25.8% and 46.2% were precise within±10% and±20%, respectively. ΔEELV determination in COPD patients differed more between RIP and N2WO than in the other groups, but this was not significant.

Conclusion

In a mixed group of patients undergoing controlled ventilation, RIP using the Respitrace Plus monitor was not consistently precise enough for quantitative evaluation of Vt and EELV when compared to our reference methods. This was most evident in patients with COPD. For long-term volume measurements, a better control of the baseline drift of RIP should be achieved.

Section snippets

Materials and Methods

After approval of the protocol by the ethical committee of the local medical faculty, 31 patients were included into the study. Patients undergoing elective surgery gave informed written consent to participate prior to the study; for all other patients, permission was obtained from the next of kin. Exclusion criteria were age younger than 18 years, fraction of inspired oxygen (FIo2) >0.6 to obtain an oxygen saturation of 90%, bronchopulmonary leakage, or circulatory instability that would not

Results

Analysis of the baseline drift of the RIP signal (Fig 2) revealed an average increase of +25.4±29.1 mL/min with 28.2±48.9, 24.5±16.6, and 23.5±20.7 mL/min for the first, second, and third 5-min period, respectively. In 51% of all minutes analyzed, drift was in the range of 0 to +20 mL/min and in 98%, it was in the range of −20 to +80 mL/min. However, in individual patients, the drift was neither stable nor steadily increasing or decreasing with time, but changed over a wide range as indicated

Discussion

Our data show that RIP is not consistently precise enough (deviation <10%) for quantitative measurements of Vt in mechanically ventilated patients. The difference between N2WO and RIP for ΔEELV was even greater than between PT and RIP for Vt measurements. However, this might also partially reflect problems of N2WO to measure the complete intrathoracic gas volume.

Using the Respitrace Plus monitor, we found a high and unstable baseline drift of the RIP signal that certainly hindered a higher

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    Copyright © 1998 The American College of Chest Physicians. Published by Elsevier Inc. All rights reserved.