Dr. Barton: In the PICU, we are primarily controlling their minute ventilation with their respiratory rate, so asynchrony between the patient and ventilator is a critical issue. With NAVA, the patient-ventilator interaction is greatly improved when compared to other SIMV modes with pressure support or solitary pressure support/volume support modes. With NAVA, the patient’s tidal breathing is simply more natural and at ease.
Rowley: NAVA mode of ventilation has increased my awareness about how underappreciated breath-cycle asynchrony and over assisted ventilation is during conventional ventilation. During NAVA-preview assessments, when patients are in either volume or pressure modes of ventilation, I have consistently identified inspiratory and expiratory trigger asynchronies by comparing real-time superimposed airway pressure waveforms during patient initiated breaths.
Breath-cycle asynchronies that are not detectible by standard airway pressure and inspiratory flow waveform assessments become easily detectible during NAVA preview. When patients are transitioned from conventional modes of ventilation to NAVA mode, breath-cycle synchrony immediately improves because the ventilator breath-cycle responds to the coupling effect of diaphragm myofiber contractility and repolarization, respectively. Over assisted ventilation with pressure delivery to the airways is avoided because airway pressure is delivered in direct proportion to the magnitude of diaphragm contractility. This is measured by the Edi signal.
Dr. Lowson: NAVA has made me aware of just how poor the patient-ventilator interactions really are using existing triggering methods. NAVA has clearly shown that the frequency and degree of asynchrony is far greater than earlier studies suggested. In the majority of cases, it is also almost impossible to diagnose asynchrony without the information that NAVA provides. Events such as missed patient efforts, delayed triggering, or even bilateral diaphragmatic paralysis can go undiagnosed without NAVA.
Another extraordinary finding is how relatively modest levels can completely unload the respiratory muscles to the point of complete diaphragmatic shutdown. This phenomenon is undetectable without placing a NAVA catheter, and it affects approximately one-quarter of patients we have evaluated. The patient triggers the ventilator using either pressure or flow triggering but has no detectable Edi. The Edi only begins to appear when the ventilator mode is switched to CPAP and disappears again when pressure/volume modes are reinstituted. Given the known concerns for diaphragmatic dysfunction/atrophy, it is worrisome that we are discovering this at low levels of ventilator support.
Dr. Barton: Knowledge of Edi objectively assists the clinician to determine the patient’s true respiratory demands. The Edi signal in the critically ill pediatric patient is highly variable from breath to breath. This variability in peak pressures and respiratory rates allows the NAVA mode to interact “real time” with the patient, maintaining minute ventilation with the lowest peak airway pressures possible from breath to breath.
Rowley: NAVA offers a real-time physiologic monitoring tool that quantifies workload of the diaphragm during partially assisted breaths or during trach collar trials. Once a baseline Edi value is established, clinicians may use the value to trend how the diaphragm responds to increased and decreased support. With Edi monitoring, I have learned that pneumatically triggered breaths can occur without diaphragm contractility. Other respiratory muscles are capable of generating negative intrathoracic pressure necessary to cycle the ventilator on before the diaphragm contracts.
Dr. Lowson: NAVA offers the ability to see what a patient’s diaphragm is actually doing and what messages the patient’s ventilatory center is sending in response to different pulmonary pathologies and ventilator settings.
Dr. Barton: NAVA has been a tremendous asset in our ever-going process of improving respiratory care delivery to our patients. In particular, we have seen dramatic improvements in ventilator days, length of sedation requirements, days requiring inotropes, and first time extubation success in our postoperative cardiovascular patients. We have now seen similar improvements in our patients with acute hypoxemic respiratory failure.