Canadian Health&Care Mall: Inspiratory Muscle Unloading

Neurally adjusted ventilatory assist (NAVA) is a mode of mechanical ventilation in which positive pressure is applied to the airway opening in proportion to the electrical activation of the diaphragm (EAdi).1 With NAVA, ventilator support is initiated when the neural drive to the diaphragm begins to increase. As the EAdi progressively increases, the assist increases proportionally and, most importantly, the pressure delivered by the ventilator is cycled-off when the EAdi is ended by the respiratory centers. The amount of assist delivered during NAVA depends on a proportionality factor, the so-called “NAVA level,” which defines the magnitude of pressure delivered for a given EAdi. When the NAVA level is changed, the resulting pressure delivered by the ventilator depends on how the respiratory afferents modulate the neural output to the diaphragm. If the response to an increase in NAVA is not a reduction in the EAdi, the delivered pressure increases. However, a reflexive or involuntary reduction in EAdi would mitigate the effects of an increased NAVA level, and the delivered pressure may remain unchanged or be less than anticipated.

Thus, the response in EAdi to an increase in the NAVA level determines the resulting transpulmo-nary pressure (Ptp) and whether volume changes or not. Consequently, during maximal inspirations, when the EAdi is at its highest the pressure delivered could reach extreme levels that may cause harm to the lungs. It is therefore important to determine with increasing NAVA levels whether or not the EAdi is suppressed thereby limiting the pressure delivered during maximal inspiration. Based on the above, the aims of the present study were to determine the following: (1) whether NAVA efficiently unloads respiratory muscles throughout a maximal inspiration in healthy subjects, and (2) if EAdi is suppressed during maximal inspirations with increasing levels of NAVA.

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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 pressure sensor (± 350 cm H2O; Sensym; Milpitas, CA) and was placed with the pneumotach between the mouthpiece and the ventilator (Servo 300; Maquet Critical Care; Solna, Sweden). Respiratory inductance plethysmography (Re-spitrace; Ambulatory Monitoring; Ardsley, NY) was used to evaluate rib cage and abdominal displacements. All signals were acquired simultaneously, displayed on-line to the investigators, and stored for off-line analysis.

EAdi Signal Processing

Signal processing of EAdi followed American Thoracic Society recommendations.4 Filters and algorithms giving the highest possible signal-to-disturbance ratio were applied. Changes in diaphragm position along the array were accounted for,5 6 yielding a signal not artifactually affected by changes in lung volume or chest wall configuration. The root-mean-square was used to quantify EAdi every 16 ms.910 Signal segments with residual disturbances were replaced by the previously accepted value, resulting in a processed EAdi signal.

Method for NAVA

The processed EAdi was used to control a Servo 300 ventilator according to Sinderby et al.1 NAVA 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 the NAVA level in the present work, but has also been referred to as NAVA gain in a previous publication. With an increase in the NAVA level, more pressure is delivered by the ventilator if the EAdi (ie, respiratory drive) does not decrease.

In the current application, NAVA was applied during inspiration, and the assist was cycled-off to zero positive end-expiratory pressure. For triggering on, ventilatory assist was initiated when the EAdi exceeded a threshold increment in EAdi. Given that the variability of the noise level was low, the trigger threshold was set to a fixed level that permitted early detection of increasing diaphragm activation without causing autotriggering when the diaphragm was inactive. For cycling-off, ventilatory assist was terminated when the EAdi fell below a percentage (default 80%) of peak inspiratory activity.

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. Drugs to lower down the arterial pressure are available on Canadian Health&Care Mall.

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 the same conditions for the different subjects, the runs were classified into three NAVA levels. Zero NAVA level refers to the condition in which the subject was breathing on the ventilator circuit without assist. High NAVA was the highest level applied (abolishing or reversing the swing in Pes), and intermediate NAVA was the NAVA level when XPaw was approximately 50% of that observed during the highest NAVA level.

Statistical Analysis

Repeated-measures analysis of variance was used to compare variables between different levels of NAVA. Post hoc comparison 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.