Chest
Volume 131, Issue 3, March 2007, Pages 711-717
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Original Research: Critical Care Medicine
Inspiratory Muscle Unloading by Neurally Adjusted Ventilatory Assist During Maximal Inspiratory Efforts in Healthy Subjects

https://doi.org/10.1378/chest.06-1909Get rights and content

Abstract

Background:Neurally adjusted ventilatory assist (NAVA) is a mode of mechanical ventilation in which the ventilator is controlled by the electrical activity of the diaphragm (EAdi). During maximal inspirations, the pressure delivered can theoretically reach extreme levels that may cause harm to the lungs. The aims of this study were to evaluate whether NAVA could efficiently unload the respiratory muscles during maximal inspiratory efforts, and if a high level of NAVA would suppress EAdi without increasing lung-distending pressures.

Method:In awake healthy subjects (n = 9), NAVA was applied at increasing levels in a stepwise fashion during quiet breathing and maximal inspirations. EAdi and airway pressure (Paw), esophageal pressure (Pes), and gastric pressure, flow, and volume were measured.

Results:During maximal inspirations with a high NAVA level, peak Paw was 37.1 ± 11.0 cm H2O (mean ± SD). This reduced Pes deflections from − 14.2 ± 2.7 to 2.3 ± 2.3 cm H2O (p < 0.001) and EAdi to 43 ± 7% (p < 0.001), compared to maximal inspirations with no assist. At high NAVA levels, inspiratory capacity showed a modest increase of 11 ± 11% (p = 0.024).

Conclusion:In healthy subjects, NAVA can safely and efficiently unload the respiratory muscles during maximal inspiratory maneuvers, without failing to cycle-off ventilatory assist and without causing excessive lung distention. Despite maximal unloading of the diaphragm at high levels of NAVA, EAdi is still present and able to control the ventilator.

Section snippets

Subjects

Nine healthy subjects (one woman) were studied. Their mean (± SD) age, height, and weight were 37 ± 8 years, 172 ± 7 cm, and 71 ± 8 kg, respectively. Two subjects had prior knowledge of mechanical ventilation. The study was approved by the Scientific and Ethical Committees of Sainte-Justine's Hospital, Montreal, Canada, and all subjects gave their informed consent.

Measurements

Electrical signals of the diaphragm were obtained using a multiple-array esophageal electrode (nine electrodes spaced 10-mm apart). Balloons were mounted on the same catheter for measurements of esophageal pressure (Pes), gastric pressure (Pga), and transdiaphragmatic pressure (Pdi). Flow was measured with a pneumotachograph (No. 2; Hewlett Packard; Palo Alto, CA) connected to a pressure transducer (± 3 cm H2O; Ohmega Engineering; Stanford, CT). Airway pressure (Paw) was measured with a

EAdi Signal Processing

Signal processing of EAdi followed American Thoracic Society recommendations.4Filters and algorithms giving the highest possible signal-to-disturbance ratio were applied.5Changes in diaphragm position along the array were accounted for,5, 6yielding a signal not artifactually affected by changes in lung volume or chest wall configuration.7, 8The root-mean-square was used to quantify EAdi every 16 ms.9, 10Signal segments with residual disturbances were replaced by the previously accepted value,

Method for NAVA

The processed EAdi was used to control a Servo 300 ventilator according to Sinderby et al.1NAVA is based on transforming the EAdi amplitude into a voltage every 16 ms and sending it to the Servo 300 ventilator, which responds by adjusting the pressure level according to a linear function. The EAdi can be multiplied by a number, which essentially is a proportionality factor determining the amount of pressure is delivered for a given EAdi. This factor is referred to as theNAVA levelin the present

Experimental Protocol

Subjects were studied in sitting position, breathing at rest through a mouth piece connected to the ventilator. Subjects breathed at rest for 3 to 5 min and performed at least two maximal inspirations toward the end of the period. This was subsequently repeated with increasing NAVA levels, as long as increasing NAVA levels decreased the negative Pes deflection observed on the computer monitor. In the present study, no positive end-expiratory pressure was applied.

Analysis

The start and end of each maximal inspiration were determined using the flow signal. For each tidal or maximal inspiration, EAdi signal strength was calculated as the mean inspiratory EAdi with a baseline EAdi subtracted (mean electrical activity of the diaphragm [XEAdi]). The mean pressure swings for Paw (mean Paw [XPaw]), Pes (mean Pes [XPes]), Pga (mean Pga [XPga]), Pdi, and Ptp (mean Ptp [XPtp]) were also calculated. Volume was obtained by integration of the flow signal.

In order to compare

Statistical Analysis

Repeated-measures analysis of variance was used to compare variables between different levels of NAVA.Post hoccomparison was performed with a Tukey test. Correlation between the mean and peak pressures was performed with Pearson product moment correlation. The statistical analyses were performed using statistical software (Sigmastat, version 2.0; Jandel Scientific; San Rafael, CA). The level of significance for all statistical tests was p < 0.05. Data are presented as mean ± SD.

Results

During both breathing at rest and maximal inspirations, NAVA was well tolerated by the subjects at all NAVA levels. In all subjects, it was possible to increase the NAVA to a level where the negative Pes deflection generated during the inspiratory capacity (IC) maneuver was abolished or reversed to positive. The group mean data obtained during the quiet breathing periods (zero and high NAVA level) are shown inTable 1.

Figure 1 shows representative tracings for one subject performing maximal

Discussion

The present study demonstrates that NAVA can unload the respiratory muscles of a healthy subject to a level at which it substitutes the inspiratory muscle contribution to distend the lungs at all lung volumes. At such a high NAVA level, the subject still maintains the breathing pattern and full voluntary control of the ventilatory assist. This study also demonstrates that maximal inspirations performed with a high level of NAVA suppress the diaphragm electrical activity, thereby limiting

Termination of Maximal Inspiration

Despite 55 years having passed since Mills14published the first article describing what limits the effort and depth of a voluntary maximal inspiration in a human, there is no consensus on this topic. Although this topic may be of academic relevance only in situations when maximal inspirations are performed without a ventilator, the introduction of a new generation of ventilators that deliver assist in proportion to the patient's effort revives the question of whether diaphragm activity is

Critique of the Study

The present study could be criticized for several reasons. First, the present study was performed in healthy, awake subjects, which today is not conventional for studies on control of breathing. However, awake subjects were a requirement since the main intervention was maximal voluntary inspirations.

Second, while breathing at rest, the tidal volume and minute ventilation were greater than anticipated. This was likely due to the increased dead space of the tubing, connectors, pneumotachograph,

Conclusion

NAVA can efficiently unload the respiratory muscles at all lung volumes. Diaphragm electrical activity is downregulated with increasing NAVA level, limiting lung distension during maximal inspirations in healthy subjects. The findings of the present study suggest that NAVA is well integrated with respiratory control systems and provides assist in response to central respiratory output. Despite maximal unloading of the diaphragm at high levels of NAVA, EAdi is still present and able to control

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The work was performed at Hôpital Sainte-Justine in Montreal, Quebec, Canada.

This work was supported in part by the Canadian Intensive Care Foundation. Dr. Beck and Dr. Sinderby were supported by the Fonds de la Recherche en Santé du Québec.

This disclosure statement has been approved by St-Michael's Hospital, Sunnybrook Health Sciences Centre, and University of Toronto. Dr. Beck and Dr. Sinderby have made inventions related to neural control of mechanical ventilation that are patented. The license for these patents belongs to Maquet Critical Care. Future commercial uses of this technology may provide financial benefit to Dr. Sinderby and Dr. Beck through royalties. Dr. Sinderby and Dr. Beck each own 50% of Neurovent Research Inc. Neurovent Research is a research and development company that builds the equipment and catheters for research studies. Neurovent Research has a consulting agreement with Maquet Critical Care. Dr. Slutsky consults for companies that make ventilators, specifically Maquet Critical Care and Hamilton Medical and is compensated for these consultations. Drs. Spahija, de Marchie, Lacroix, and Navalesi have no conflicts of interest to disclose.

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