Roles of neurally adjusted ventilatory assist in improving gas exchange in a severe acute respiratory distress syndrome patient after weaning from extracorporeal membrane oxygenation: a case report
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Patient-ventilator asynchrony is a major cause of difficult weaning from mechanical ventilation. Neurally adjusted ventilatory assist (NAVA) is reported useful to improve the synchrony in patients with sustained low lung compliance. However, the role of NAVA has not been fully investigated.
The patient was a 63-year-old Japanese man with acute respiratory distress syndrome secondary to respiratory infection. He was treated with extracorporeal membrane oxygenation for 7 days and survived. Dynamic compliance at withdrawal of extracorporeal membrane oxygenation decreased to 20 ml/cmH2O or less, but gas exchange was maintained by full support with assist/control mode. However, weaning from mechanical ventilation using a flow trigger failed repeatedly because of patient-ventilator asynchrony with hypercapnic acidosis during partial ventilator support despite using different types of ventilators and different trigger levels. Weaning using NAVA restored the regular respiration and stable and normal acid-base balance. Electromyographic analysis of the diaphragm clearly showed improved triggering of both the start and the end of spontaneous inspiration. Regional ventilation monitoring using electrical impedance tomography showed an increase in tidal volume and a ventilation shift to the dorsal regions during NAVA, indicating that NAVA could deliver gas flow to the dorsal regions to adjust for the magnitude of diaphragmatic excursion. NAVA was applied for 31 days, followed by partial ventilatory support with a conventional flow trigger. The patient was discharged from the intensive care unit on day 110 and has recovered enough to be able to live without a ventilatory support for 5 h per day.
Our experience showed that NAVA improved not only patient-ventilator synchrony but also regional ventilation distribution in an acute respiratory distress patient with sustained low lung compliance.
KeywordsNeurally adjusted ventilatory assist Compliance Patient-ventilator synchrony Electromyography Electrical impedance tomography Ventilation distribution
electromyographic activity of the diaphragm
extracorporeal membrane oxygenation
electrical impedance tomography
intensive care unit
neurally adjusted ventilator assist
positive end-expiratory pressure
pressure support ventilation
synchronized intermittent mandatory ventilation
Initiation and termination of inspiratory support during conventional partial ventilatory support is triggered by changes in flow (flow trigger, FT) or pressure (pressure trigger) in the airway. Trigger delay can cause late initiation and early termination of mechanical support . This is known as patient-ventilator asynchrony, leading to not only insufficient inspiratory assist but also irregular, disturbed respiration. Asynchrony is more obvious and often a problem when the patient is converted to partial ventilatory support for the weaning from mechanical ventilation and gas exchange is insufficiently maintained, although it is observed during full ventilatory support (e.g., assist/control ventilation). Asynchrony exaggerates pre-existing acute respiratory failure . Neurally adjusted ventilatory assist (NAVA) uses the electromyographic activity of the diaphragm (EAdi) detected through an electrode placed in the stomach. NAVA triggers the initiation and termination of ventilatory support and improves patient-ventilator synchrony [3, 4, 5].
We experienced a case of severe acute respiratory distress syndrome (ARDS) in which initiating NAVA markedly improved the patient’s respiratory status. In this case report, we investigated the roles of NAVA in improving respiratory status.
Peak value of EAdi gradually decreased to 10–20 μV at which level NAVA was successfully converted to conventional PSV of 24 cmH2O 31 days later. The patient was discharged from our ICU on day 110 with a home ventilator to enhance his mobility program. As of day 325, the patient has recovered enough to be able to live without a ventilator support 5 h per day.
While the number of survivors following ECMO increases, we have also seen more cases of difficult weaning from mechanical ventilation in patients whose oxygenation is restored and relatively stable; some appear to have sustained low lung compliance. Patient-ventilator asynchrony is a major reason for difficult weaning from mechanical ventilation , and low lung compliance can cause asynchrony.
During conventional PSV, patients can both trigger and cycle the breath. However, mechanical ventilation using FT is not initiated until airway flow generated by diaphragmatic contraction reaches a pre-determined level. Trigger delay is often observed in patients with low lung compliance. Because of the physical properties of the respiratory system, negative pressure generated by inspiratory muscle contraction is difficult to transmit to the airway flow change. Longer delays between respiratory muscle contraction and actual gas flow generation cause patient-ventilator asynchrony and patient discomfort. Also, with low lung compliance, the inspiratory flow decreases rapidly after the peak and reaches the termination threshold of mechanical support very early. The result is that flow support using FT is initiated with a delay and terminated earlier despite the fact that diaphragmatic contraction continues. These effects result in serious shortening of the duty cycle, increased patient work of breathing, patient-ventilator asynchrony, and the inability to adjust inspiratory effort [8, 9, 10, 11, 12].
Theoretically, NAVA has potential in these patients because it is immediately triggered by the electrical signal from spontaneous diaphragmatic contraction and decreases asynchrony. However, there are few reports of its use. In our case, the clinical symptom showed that FT caused asynchrony; the patient had excessive inspiratory effort and tachypnea because of low lung compliance. Asynchrony exaggerated tachypnea, caused cough, and increased work of breathing. These exhausted the patient and finally led to a cycle of respiratory failure. Another reason for asynchrony was the difference of supporting pressure. Assist/control mode maintained gas exchange, and the patient’s inspiratory effort was overridden. However, conversion from assist/control to SIMV caused vigorous inspiration effort due to insufficient support. Supporting pressure of NAVA was set automatically corresponding to the patient’s inspiratory effort (electrical activity of diaphragm) and was higher than that of PSV. The flow waveform of PSV is a decelerating, triangular pattern (Fig. 3a), while that of NAVA is sinusoidal shape (Fig. 3b) that is identical to the shape of the muscle tension generated by the diaphragm, suggesting the better synchrony during NAVA. Patient-ventilator synchrony improved considerably after the introduction of NAVA, as shown in Fig. 2. More importantly, the improved synchrony decreased the work of breathing and supported the weaning process.
Patients with low lung compliance are considered candidate for NAVA when conventional partial ventilatory support fails to synchronize and worsens respiratory failure. The cost and reliability of long-term use of an EAdi catheter are limitation in the clinical application of NAVA.
A previous study reported the effect of NAVA in difficult weaning cases. Mauri and colleagues evaluated and compared asynchronies between PSV and NAVA in ARDS patients with low static compliance (18 ± 8 mL/cmH2O) undergoing ECMO. The authors found that the incidence of premature cycling, ineffective triggering, double triggering, and auto-triggering decreased . In our case, auto triggering and premature cycling disappeared after initiating NAVA. NAVA broke the cycle of respiratory failure and immediately decreased the respiratory rate.
EAdi did not decrease after NAVA and this finding is compatible with previous studies [17, 18], although another study reported a decrease . Previous studies reported that control mechanical ventilation is potentially sufficient to decrease diaphragm efficiency [20, 21]. PSV often overassists the movement of the diaphragm, whereas NAVA automatically adjusts the level of pressure support to the magnitude of inspiration effort, and reduces the risk of overassistance due to downregulation of the EAdi signal and improved diaphragm efficiency . In our case, the strong inspiratory effort sustained for days after the conversion to NAVA probably due to the low lung compliance. NAVA, however, could maintain the support level not to be excessive and adjust for the magnitude of diaphragmatic excursion, resulted in the ventilation distribution predominant to the dependent lung regions.
The findings in our case suggests that NAVA is worth trying for weaning to improve not only patient-ventilator synchrony but also regional distribution in an ARDS patient with sustained low lung compliance after weaning from ECMO.
The study was approved by our institutional ethics committee (reference number 3013).
Written informed consent was obtained from the patient's kin for publication of this case report. A copy of the written consent is available for re- view by the Editor-in-Chief of this journal.
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