Leave the sedation alone! Diagnosis and management of patient-ventilator asynchrony
You are called to the bedside of a mechanically ventilated patient who appears uncomfortable. The minute ventilation is high, and the patient is using accessory muscles of respiration. Vital signs are normal. Oxygenation and ventilation are adequate and there are no metabolic derangements driving an elevated minute ventilation. He denies pain. You consider whether there are any solutions other than targeting deep sedation and initiating neuromuscular blockade.
Patient-ventilator asynchrony (PVA) occurs when there is a mismatch between the patient’s respiratory system and the ventilator regarding breath size (volume), duration, flow rate, or applied pressure. Asynchrony is often referred to as a “patient bucking the ventilator,” however, it should be actually be described as the “ventilator bucking the patient” as the onus of synchronization lies on the clinician matching the ventilator with the patient’s efforts, not the other way around. PVA is underrecognized yet associated with increased mortality, ICU length of stay and duration of mechanical ventilation in critical illness. Asynchrony between the patient and the ventilator can occur during each stage of the mechanical breath and can be readily diagnosed by examination of the ventilator waveforms. The respiratory cycle of a mechanical breath can be divided into 4 distinct stages: 1) initiation of inspiration (triggering), 2) gas delivery or inspiratory flow (target), 3) termination of inspiration (cycling), and 4) expiration. The stages of the mechanical breath can be visualized on the pressure, flow and volume scalars (figure A).
Which of the following types of patient-ventilator asynchrony is pictured in figure B?
a) Trigger asynchrony
b) Flow asynchrony
c) Cycle asynchrony
d) Expiratory asynchrony
What mode of mechanical ventilation is depicted in figures A and B?
a) Trigger asynchrony
Figure B demonstrates ineffective triggering, which is a type of trigger asynchrony that occurs when a patient makes an inspiratory effort without delivery of a mechanical breath, as evidenced by a decrease in airway pressure with a simultaneous reversal in expiratory flow. Ineffective triggering is most easily identified on the flow scalar, rather than the pressure scalar, as the patient’s ineffective inspiratory effort results in a greater change in flow than pressure. On the pressure scalar, you may see a subtle negative deflection in the airway pressure tracing representing the patient’s negative inspiratory effort, followed by a slight rise in airway pressure as the patient actively exhales after the ineffective trigger effort. If a patient attempts inspiration during mechanical expiration, there will be a reversal of air flow as the expiratory flow waveform moves toward the zero baseline (or x-axis). This is followed by a small deflection of expiratory flow away from the baseline as the patient exhales again after the ineffective trigger. Clinically, ineffective triggering can be recognized by placing one hand on the patient’s chest, allowing detection of a respiratory effort, while simultaneously observing the lack of an assisted breath on the ventilator graphics.
Ineffective triggering can be caused by an inappropriate sensitivity setting on the ventilator, which can be easily resolved by increasing the sensitivity as high as possible without causing auto-triggering (i.e. when a mechanical breath is artifactually "triggered" without an actual patient effort). You can also change a pressure trigger to a flow trigger, which is inherently more sensitive. Alternatively, the presence of intrinsic PEEP can lead to ineffective triggering by imposing a triggering load on the patient. Any condition that causes incomplete emptying of the lungs leads to an increase in the end-expiratory volume. This air-trapping, also termed dynamic hyperinflation, raises the alveolar pressure above the PEEP level set by the clinician. The presence of intrinsic PEEP creates a large pressure gradient between end-expiratory alveolar pressure and the ventilator circuit pressure that the patient must overcome to trigger an assisted breath (read more about auto-PEEP here: https://emdaily.cooperhealth.org/content/ventilator-graphic-analysis-obstructive-lung-disease). If the ineffective trigger is caused by auto-PEEP, the clinician’s attention should first be directed at minimizing dynamic hyperinflation by lengthening the expiratory time through reduction of respiratory rate and tidal volume as well as reducing airway resistance with bronchodilators. Once intrinsic PEEP is minimized, the applied PEEP can be cautiously increased to reduce the differential between the lung and the circuit pressures, thereby reducing the trigger load.
Ineffective triggering is the most common type of patient-ventilator asynchrony and can significantly increase oxygen consumption and respiratory effort in a critically ill patient. Moreover, it causes severe dyspnea and discomfort for patients that can be alleviated when recognized. The solution to patient-ventilator asynchrony is rarely deep sedation, but rather matching the ventilator’s pressure, flow and volume delivery with the patient’s effort during triggering, targeting and cycling of a mechanical breath.
Figure A: Volume-assist control (VAC)
Figure B: Pressure-assist control (PAC)
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