Introduction
The Radboud University Nijmegen Medical center is a 1000-bed teaching hospital with 35-bed adult ICU. Our ICU offers a broad spectrum of ICU care including neuro-ICU, medical and surgical ICU, cardiac ICU, extracorporeal CO2 removal, ECMO, expertise center for difficult weaning from mechanical ventilation and an innovative early mobilization service for ICU patients. Research topics include sepsis, mechanical ventilation and respiratory muscle function. All ICU rooms are equipped with a Servo-i ventilator (Maquet Critical Care). Other available ventilators include the BiPAP vision (Philips Respironics) for noninvasive ventilation and the Hamilton Galileo ventilator.
Reasons to start using NAVA in our ICU
During clinical rounds I often noticed that patients on pressure support ventilation (PSV) were often in asynchrony with the ventilator. This could be derived from the flow – volume – pressure curves on the ventilator, but more often when having a closer look at the patient. Some of the patients appeared uncomfortable during PSV. Many had wasted efforts, delayed cycling off or inadequate unloading of the respiratory muscles (use of accessory muscles). In some cases, changing ventilator settings, such as level of PS or cycle off criterion improved comfort. In many other patients, increasing the sedation level was the only way to improve the patient’s comfort. When we first heard about NAVA we were enthusiastic about its principles, in particular the fact that the ventilator directly communicates with the patient’s inspiratory muscles, allowing better synchrony between patient and ventilator.
In addition, the possibility of delivering proportional assist made sense to us. Finally, we were aware that the Edi catheter would be an ideal tool to monitor inspiratory muscle function, even when patients were ventilated in other modes than NAVA. We realized that this was the first real improvement in mechanical ventilation in a long time.
Implementing NAVA
Our ICU consists of 4 different units. We started NAVA implementation in one unit (medical ICU) to assure that the technique worked as it was promised. First, teaching sessions were organized with the help of a Maquet Critical Care representative in our hospital. Separate sessions were organized for nurses and doctors. After these teaching sessions additional training was organized for three nurses and three doctors (“super users”). The super users were available for questions from the other colleagues. In addition, in the first year NAVA was used, one of the super users would visit all patients on NAVA daily. This assured optimal ventilator settings, and moreover time for some bedside teaching. When super users were unable to answer a specific question, a Maquet representative was contacted. Memory cards for the Servo-i were available to save screenshots for later discussion and teaching.
At that time no specific protocols for the use of NAVA were available, so we decided to write our own (see below).
In the first weeks of setting op NAVA, we decided to start with patients that would be a “little forgiving”, such as patients difficult to wean from mechanical ventilation. Now that the team is much more experienced, much more complex patients can be handled effectively with NAVA (acute exacerbation of COPD, ARDS). In addition, after a few months NAVA was implemented in our other ICU units as well.
Hurdles
The practical implementation of NAVA has not been a major problem, but it did take some time to assure that the whole team gained a sufficient level of experience. Training sessions organized by super users were repeated as necessary. Catheter insertion and positioning have not really been an issue, as long as the instructions from the manufacturer were adhered to. As we were among the earliest NAVA users in daily clinical care some hurdles had to be overcome with regards to the settings on the ventilator. The first hurdle was determining how to set the NAVA level. We started using the protocol published by Brander and Colleagues (Chest, 2009). Although the physiological rationale for this NAVA titration protocol is evident, it is time consuming in daily practice. Also, in a fair amount of patients we could not find a real plateau. As we were convinced that a protocol as simple as possible would increase the chance of successful implementation, we used the “overlay window technique”, which involves the viewing of a dedicated window where the predicted NAVA pressure is overlaid on top of the pressure waveform during a conventional mode. Subsequently, the NAVA level is reduced at least twice daily based on patient comfort.
Initially, it seemed more difficult for the nurses and clinicians to decide when the patient was ready to be weaned while on NAVA. We had to develop a ‘clinical feeling” (and understanding !) what low level of support is during NAVA. Again we decided to keep it as simple as possible. As we would do in PSV, when PEEP and Fi,o2 reached certain levels, a rapid shallow breathing index will be determined. If <105, the patient proceeds to a T-tube trial and when successful extubation will follow. When implementing NAVA it is very important to have a weaning protocol (as it is with PSV), otherwise, people might take more time before initiating a weaning trial. Although I am convinced that in the future new parameters / indices (i.e. neuroventilatory efficiency) may help to recognized patients ready to be weaned, for now we chose to keep it as simple as possible in daily clinical care (Rapid shallow breathing index and T-tube trial).
Another hurdle was the fact that breathing (viewed neutrally from the Edi waveform) appeared more chaotic than what we were used to from only observing pressure and flow waveforms. Tidal volume variation was larger and respiratory rate higher. It was important to explain to colleagues that this is a more physiological breathing pattern than the pattern observed with PSV. In addition, it should be kept in mind that the neural respiratory rate was high on PSV as well, but ventilator rate displayed was lower due to wasted efforts.
Finally, it should be recognized that in some patients NAVA is not the optimal ventilator mode. Examples include patients with extremely high respiratory drive (severe respiratory acidosis) and in patients with acute change in diaphragm position, i.e. due to atelectasis or pneumonectomy as this results in continuous diaphragm muscle activation (tonic activity). With regards to high respiratory drive, one should keep in mind that high peak inspiratory pressures may develop. Appropriate setting of alarm levels is for these patients very important.
Benefits
After 2 years NAVA is part of routine clinical care in our unit. All nurses and doctors have been trained and have a sufficient level of understanding to select appropriate patients, initiate and titrate NAVA and solve problems. We are convinced that in specific patient groups optimal synchrony is of clinical benefit (in particular acute exacerbation of COPD, severe diaphragm weakness). But at least as important is that with the implementation of NAVA a valuable tool for respiratory muscle monitoring has become available. With the Edi catheter severe asynchrony can be detected easily in other modes than NAVA such as during PSV or CMV. But the Edi signal can be used help with decisions about setting the cycle off criterion with PSV or inspiration time in controlled ventilation. Over assist is common during PSV, and difficult to detect using only pressure / flow tracings. However, it is very easy to detect with the Edi signal, allowing optimal triggering of the PS level. Finally, the Edi signal can be used to titrate NMBA’s in severe ARDS. Instead of continuous drip, we provide bolus on the base of reappearance of the Edi signal. This prevents overdosing of NMBA.
Conclusion
In our current practice, an Edi catheter is considered in patients difficult to wean from mechanical ventilation, patients with obstructive lung disease or severe ARDS. Having the Edi catheter, a patient can be ventilated in the NAVA mode, but at least as important a valuable tool to monitor activity of the diaphragm has become available in clinical use (see also our recent Critical Care Perspective, Am J Respir Crit Care Med, January 1, 2013). In my opinion this is just as important as mechanical ventilation and critical illness has profound effects on the respiratory muscles. Monitoring its activity may help the clinician to optimize ventilator settings.