| Critical Care | Case Report: NAVA | Case contributed by Lukas Brander, MD; Daniel Tuchscherer, MD; and Anna Brunello, MD; University Hospital – Inselspital, Bern, Switzerland PATIENT CASE REPORT. CATEGORY: ADULT 59 YEAR OLD WOMAN WITH SEVERE HEMOPTYSIS In a 59 year old woman with severe hemoptysis, resection of the left lower lung lobe was performed after an attempt to control bleeding by angiographic coiling failed. In the histological examination, a ruptured artery within a large lung abscess was identified as the source of bleeding. The early postoperative course was complicated by extensive infiltrations of the left upper lobe (Figure 1 A) and by the development of an acute respiratory distress syndrome (ARDS). While on mechanical ventilation with volume controlled mode (tidal volume 4.9 ml/kg PBW; PEEP 6 cmH 2 O; FiO 2 0.7), oxygenation was severely impaired (PaO 2 /FiO 2 ratio around 90, PaCO 2 45-50 mmHg), and the compliance of the respiratory system was as low as 8-15 ml/cmH 2 O. Several attempts to wean the patient from mechanical ventilation using Pressure Support mode ventilation were unsuccessful because of excessively high respiratory rates, agitation, and distress regardless of the assist and PEEP levels used, and independent of the trigger-on and cycling-off settings (Figure 2). Unusually high amounts of sedative drugs had to be administered to achieve an acceptable level of comfort and to suppress the high respiratory drive of the patient. A catheter to detect the diaphragmatic electrical activity (Edi) was inserted and an Edi amplitude of about 50 μV could be derived. It became obvious that despite using relatively high Pressure Support levels, a sensitive flow-based trigger-on threshold, and a cycling-off at 1% of maximum inspiratory flow (a level chosen arbitrarily based on the low respiratory system compliance), the delivery of pressure was not only somewhat delayed but was also too short compared to the neural inspiratory time (defined as the time between the onset of Edi and about 70% of its peak, where neural expiration starts) (Figure 2). Delivery of the assist was asynchronous and out of proportion to the patient’s respiratory demand, forcing her to increase her respiratory drive and resulting in discomfort and distress. The NAVA preview option was used to adjust the NAVA level such that the mean airway pressure during inspiration (Paw) approximately matched the inspiratory Paw during Pressure Support ventilation. Within the first 24 hours after implementing NAVA, the respiratory rate decreased to about 25 breaths per minute and the tidal volume increased to about 10 ml/kg PBW (Figure 3; Figure 4, panel C). Simultaneously, the respiratory drive, as reflected by the Edi, decrease to approximately 50-60% of its value during Pressure Support ventilation. Early after initiation of NAVA and temporary increase of the PEEP level to 14 cmH 2 O, FiO 2 could be reduced to 0.4. Figure 1 Chest x-ray images. A: early after resection of the left lower lung lobe. B: before discharge from the ICU. A B
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| Critical Care | Case Report: NAVA |
Case contributed by Lukas Brander, MD; Daniel Tuchscherer, MD; and Anna Brunello, MD; University Hospital – Inselspital, Bern, Switzerland
PATIENT CASE REPORT. CATEGORY: ADULT59 YEAR OLD WOMAN WITH SEVERE HEMOPTYSIS
In a 59 year old woman with severe hemoptysis, resection of the left lower lung lobe was performed after an attempt to control bleeding by angiographic coiling failed. In the histological examination, a ruptured artery within a large lung abscess was identified as the source of bleeding. The early postoperative course was complicated by extensive infiltrations of the left upper lobe (Figure 1 A) and by the development of an acute respiratory distress syndrome (ARDS).
While on mechanical ventilation with volume controlled mode (tidal volume 4.9 ml/kg PBW; PEEP 6 cmH2O; FiO2 0.7), oxygenation was severely impaired (PaO2/FiO2 ratio around 90, PaCO2 45-50 mmHg), and the compliance of the respiratory system was as low as 8-15 ml/cmH2O. Several attempts to wean the patient from mechanical ventilation using Pressure Support mode ventilation were unsuccessful because of excessively high respiratory rates, agitation, and distress regardless of the assist and PEEP levels used, and independent of the trigger-on and cycling-off settings (Figure 2). Unusually high amounts of sedative drugs had to be administered to achieve an acceptable level of comfort and to suppress the high respiratory drive of the patient. A catheter to detect the diaphragmatic electrical activity (Edi) was inserted and an Edi amplitude of about 50 μV could be derived. It became obvious
that despite using relatively high Pressure Support levels, a sensitive flow-based trigger-on threshold, and a cycling-off at 1% of maximum inspiratory flow (a level chosen arbitrarily based on the low respiratory system compliance), the delivery of pressure was not only somewhat delayed but was also too short compared to the neural inspiratory time (defined as the time between the onset of Edi and about 70% of its peak, where neural expiration starts) (Figure 2). Delivery of the assist was asynchronous and out of proportion to the patient’s respiratory demand, forcing her to increase her respiratory drive and resulting in discomfort and distress.
The NAVA preview option was used to adjust the NAVA level such that the mean airway pressure during inspiration (Paw) approximately matched the inspiratory Paw during Pressure Support ventilation. Within the first 24 hours after implementing NAVA, the respiratory rate decreased to about 25 breaths per minute and the tidal volume increased to about 10 ml/kg PBW (Figure 3; Figure 4, panel C). Simultaneously, the respiratory drive, as reflected by the Edi, decrease to approximately 50-60% of its value during Pressure Support ventilation.
Early after initiation of NAVA and temporary increase of the PEEP level to 14 cmH2O, FiO2 could be reduced to 0.4.
Figure 1 Chest x-ray images. A: early after resection of the left lower lung lobe. B: before discharge from the ICU.
A B
Case contributed by Lukas Brander, MD; Daniel Tuchscherer, MD; and Anna Brunello, MD; University Hospital – Inselspital, Bern, Switzerland
| Critical Care | Case Report: NAVA |
Figure 2 Pressure Support ventilation with an assist level of 20 cmH2O, positive end-expiratory pressure (PEEP) of 12 cmH2O, a sensitive flow-based trigger-on threshold, and cycling-off at 1% of maximum inspiratory flow (a level chosen somewhat arbitrarily based on the low respiratory system compliance) resulted in tidal volumes (TV) of 400-450 ml, and respiratory rates (AF) of 35-40 breaths per minute. The maximum diaphragm electrical activity (Edi) during inspiration was as high as 50 μV. The assist was delivered with some delay and was terminated prematurely (despite the relatively high assist level and the low cycling-off criteria) in some breaths when compared to the patient’s respiratory demand as reflected by her Edi (gray bars indicate neural inspiration). Interestingly, the Edi remained elevated during exhalation (so-called tonic Edi; blue arrows), potentially suggesting that the patient tried to prevent de-recruitment of the lungs at end-expiration by preserving some degree of EAdi. Of note, the ventilatory settings during Pressure Support ventilation were chosen without knowledge about the Edi.
Figure 3 After switching to NAVA, the assist was delivered in synchrony and in proportion to the patient’s Edi. During NAVA the patient reduced her respiratory drive, as reflected by the Edi, to 50-60% of the respiratory drive during Pressure Support ventilation. The elevation of the Edi at the end of exhalation almost completely disappeared, although the PEEP level was not changed. This may indicate that de-recruitment of the lungs was better prevented during NAVA than during Pressure Support ventilation. Note that the number for “respiratory rate (AF)” indicated by the ventilator (red circle) is misleadingly and erroneously high due to multiple false triggering as indicated by the pink arrowheads. (A new algorithm for the respiratory rate of NAVA has since been developed after these early patient evaluations.)
Case contributed by Lukas Brander, MD; Daniel Tuchscherer, MD; and Anna Brunello, MD; University Hospital – Inselspital, Bern, Switzerland
| Critical Care | Case Report: NAVA |
Within four days on NAVA, the PaO2/FiO2 ratio progressively increased from 90 to 200, PaCO2 levels remained at 32-35 mmHg, the respiratory rate decreased to 15-20 breaths per minute, and Vt remained at around 10 ml/kg PBW (Figure 4, panels A - D). PEEP was reduced stepwise to 5 cmH2O after oxygenation improved.
Please note that, due to the pressure waveform, the peak inspiratory Paw is normally higher while the mean inspiratory Paw is often lower during NAVA when compared to conventional, pressure targeted modes of ventilation. As illustrated in our patient (Figure 3), peak inspiratory Paw can reach unusually high levels when respiratory drive is high. Although tracheal and alveolar pressures are much lower than pressures measured in the ventilator circuit, and although the portion of the pressure that ultimately results in distension of the lung depends on the partition of elastance between lung and thoracic wall, the forces applied to lung tissue are normally not available during mechanical ventilation. In the present case, the inspiratory assist, and hence the Vt, was increased during NAVA while the patient’s effort decreased by roughly 50% when compared to Pressure Support ventilation. Estimating the actual change in transpulmonary pressure based on concomitant changes in assist and respiratory drive is difficult. Also, the airway pressure was likely substantially diminished by the resistive load, especially towards the end of inspiration when the airflow is high. Nevertheless, it seems reasonable to choose the upper pressure limit such that excessive high peak airway pressures are prevented during NAVA, especially in patients with high respiratory drive.
The amount of continuously administered sedative and analgesic drugs could be reduced continuously and could finally be stopped within 90 hours after implementing NAVA (Figure 4, panels E and F). The patient became cooperative and communicable. Within 70 hours after starting NAVA, the patient was able to breathe without ventilatory assist for initially short intervals that could progressively be prolonged over the following few days. Ultimately, the patient could be mobilized into a chair for several consecutive hours while still ventilated with NAVA.
Due to the ongoing evaluation of NAVA at that time, pressure support ventilation was resumed after a total of 4 consecutive days on NAVA. The lung condition improved over time (Figure 1 B) and the patient was discharged from the ICU after a total of 52 days. She fully recovered from her pulmonary disease and was discharged home 114 days after admission.
Summary This 59 year old female with ARDS and severely impaired compliance of the respiratory system after resection of the left lower lung lobe was treated with unusually high doses of sedative drugs because of distress and agitation while ventilated with Pressure Support mode ventilation. Delivering the assist in synchrony and proportion to this patient’s high respiratory drive with NAVA better matched her respiratory needs such that the amount of sedative and analgesic drugs administered could rapidly be reduced and the patient became cooperative and communicable.
Figure 4 Time course of respiratory parameters (panels A-D) as well as of sedative and analgesic drugs administered (panels E, F) during 24 hrs before and during 48 hours after NAVA was implemented for a total of four consecutive days. All sedative and analgesic drugs could be reduced during NAVA (cumulative dose during 24 hrs before NAVA to cumulative dose on the fourth day on NAVA): Clonidin from 3.6 mg/24hrs to 0 mg/24hrs; Fentanyl from 5.5 mg/24 hrs to 0.5 mg/24hrs; Haloperidol from 36 mg/24hrs to 16 mg/24 hrs; Remifentanil (not shown) 1.6 mg/24hrs to 0 mg/24hrs. PSV = Pressure Support ventilation.
