Monitoring Asynchrony During Invasive Mechanical Ventilation

Flow starvation with double-triggering seen on tracings of air flow, airway pressure (P_aw), and tidal volume (V_T) in a mechanically ventilated patient on volume-control continuous mandatory ventilation. Flow starvation (ie, inspiratory flow mismatching) occurs when the ventilator fails to meet the patient’s flow demand. Inspiratory effort continues during mechanical insufflation (green arrow in the P_aw waveform) in each breath; sometimes it is strong enough to trigger a second mechanical insufflation (red arrow) without expiration (ie, double-triggering ) and consequent breath-staking (blue arrows). Notice that the V_T in breath-stacking is almost double with the associated elevated P_aw.

Flow starvation seen on tracings of air flow, airway pressure (P_aw), and tidal volume (V_T) in a mechanically ventilated patient on volume-control continuous mandatory ventilation with decelerating flow. Flow starvation (ie, inspiratory flow mismatching) occurs when the ventilator fails to meet the patient’s flow demand. Inspiratory effort continues during mechanical insufflation in each breath. Notice a negative deflection in the P_aw waveform (arrow) with a different magnitude in every breath.

Short or premature cycling seen on tracings of air flow, airway pressure (P_aw), and esophageal pressure (P_es) in a mechanically ventilated patient on pressure-control continuous mandatory ventilation. Every breath delivered is patient-triggered (ie, assist ventilation) with a decrease in flow, P_aw, and P_es. Short or premature cycling develops when the neural inspiratory time is greater than the mechanical inspiration time. In this example, the inspiratory effort (solid line) continues after the mechanical insufflation ended (dashed line), with the negative deflection of the P_es persisting after the mechanical inflation terminates. This pattern develops in the 4 illustrative breaths. Image courtesy of Tai Pham and Laurent Brochard, Toronto, Canada.

Relationship between tidal volume (V_T) and peak inspiratory pressure (PIP) in a normal breath and in a double-tirggered breath, with or without reverse-triggering. Descriptive notched boxplots for V_T (top) and PIP (bottom) in each ventilatory mode. (A, C) Breaths without double-triggering (DT) versus breaths with DT breaths. (B, D) Breaths with reverse-trigger–induced DT breaths (DT RT) versus patient-triggered DT breaths (DT PT). Dots represent means, and box plots indicate medians and 25th–75th percentiles. PCV = pressure-control continuous mandatory ventilation; VCV = volume-control continuous mandatory ventilation with constant flow; VCVDF = volume-control continuous mandatory ventilation with decelerated flow. Notice the higher V_T, in both patient-triggered and reverse-triggered ventilation, delivered in double-triggered breaths without meaningful changes in PIP. From Reference 44, with permission.

Relationship between comfort and level of assistance. Patient comfort was assessed with 2 different methods: (A) Borg scale; (B) visual analog scale. Notice that the extreme levels of assistance during pressure support ventilation generated the least comfort. From Reference 51, with permission.

Flow, airway pressure (P_aw), and inspiratory and expiratory muscle activity in a patient with COPD who received pressure support ventilation with delayed or prolonged cycling. The electromyograms in the lower portion of the figure show inspiratory muscle activity in the patient’s diaphragm and expiratory muscle activity in the transversus abdominis. The patient’s increased inspiratory effort caused P_aw to fall below the set trigger threshold, and inadequate delivery of flow by the ventilator resulted in a scooped contour on the P_aw curve during inspiration. Although the ventilator was still pumping gas into the patient, his/her expiratory muscles were recruited, causing a bump in the P_aw curve. Notice that neural inspiratory time is not coupled with machine inspiratory time; after the neural inspiration finished, the ventilator still provided air flow while the patient’s expiration begins. The dotted line shows P_aw in another patient who generated just enough effort to trigger the ventilator and in whom there was adequate delivery of gas by the ventilator. From Reference 3, with permission.

Reverse-triggering and elimination of reverse-triggered breaths during an end-expiratory hold: airway pressure (P_aw), flow, esophageal pressure (P_es), and transpulmonary pressure during 6 breaths and an expiratory hold. The first 4 breaths show signs of respiratory entrainment; there seems to be a 2:1 pattern of mandatory breaths to reverse-triggered breaths that start as a passive insufflation but are rapidly followed by patient effort, as can be concluded from the drop in P_es (ie, the mandatory insufflations elicit a neural response that leads to patient effort). Larger drops in P_es than in P_aw trigger the ventilator before exhalation of the previous breath, leading to breath-stacking with high tidal volumes. These breaths with patient effort are likely reverse-triggered. An expiratory hold was administered at the first asterisk and continued until the next asterisk, which abolished respiratory muscle activity for > 12 s. The arrow on the transpulmonary pressure (P_L) curve marks the moment that inspiratory effort was expected if patient effort was generated spontaneously. After the expiratory hold, another mandatory insufflation was administered by the ventilator; patient effort immediately follows. This is suggestive of reverse-triggering because the patient effort seems to depend on the external stimulus provided by mandatory ventilator insufflation. From Reference 71, with permission.

Distribution of asynchronies during mechanical ventilation. The asynchrony index, noted as percentage (%) per hour, was recorded continuously over several days in 4 representative patients. Recordings show that periods of very low asynchrony alternated with periods of high levels of asynchrony. From Reference 4, with permission.

Relationship between asynchrony and dose of sedatives (left) and opioids (right). On opioids-only days (white circles in B, D, F), the opioid dose was inversely associated with AI, IEE, and DC. On sedatives + opioids days (black circles in A, C, E), opioid dose was still inversely associated with AI, IEE, and DT. Sedative + opioid dose was directly associated with AI and IEE (black circles in A, C), but not with DT (black circles in E). On sedatives-only days (white cirlces in A, C, E), the sedative dose was only inversely related to DT. AI = asychrony index; IEE = ineffective expiratory effort; DT = double-triggering. From Reference 113, with permission.