How do you correct dyssynchrony in a patient requiring mechanical ventilation (MV) through ventilator waveform interpretation?

Correcting Ventilator Dyssynchrony Through Waveform Interpretation

To correct patient-ventilator dyssynchrony, systematically analyze pressure-time and flow-time waveforms to identify the specific type of asynchrony, then apply targeted ventilator adjustments: optimize trigger sensitivity and apply external PEEP for ineffective triggering, increase inspiratory flow for flow starvation, adjust cycling criteria to match neural inspiratory time, and ensure adequate minute ventilation to reduce respiratory drive. 1, 2, 3

Systematic Approach to Waveform Analysis

Identify the Type of Dyssynchrony

The first step requires careful observation of ventilator waveforms to classify the specific pattern of dyssynchrony 3, 4:

• Ineffective triggering: Look for negative pressure deflections on the pressure waveform or flow changes that fail to trigger a breath—this indicates the patient's inspiratory effort is insufficient to overcome the trigger threshold 5, 1, 3
• Auto-triggering: Observe ventilator-delivered breaths without preceding patient effort, appearing as breaths triggered by cardiac oscillations, circuit leaks, or condensation 3, 4
• Flow starvation: Identify a scooped-out or concave appearance in the pressure waveform during inspiration, indicating patient demand exceeds delivered flow 2, 3
• Premature cycling: The ventilator terminates inspiration before the patient's neural inspiratory time ends, visible as continued negative pressure deflection after breath termination 4, 6
• Delayed cycling: Inspiration continues beyond the patient's neural inspiratory time, seen as positive pressure deflection at end-inspiration as the patient begins active exhalation 4, 6
• Double triggering: Two ventilator breaths occur within one patient inspiratory effort, appearing as two consecutive breaths without an expiratory pause 4, 6

Correcting Specific Dyssynchronies

Ineffective Triggering (Most Common with Auto-PEEP)

Auto-PEEP creates an inspiratory threshold load that must be overcome before triggering can occur, and setting external PEEP to offset intrinsic PEEP (but not exceed it) reduces triggering effort and improves patient comfort 5, 1:

• Measure auto-PEEP using end-expiratory occlusion technique by performing manual airway occlusion during the last 0.5 seconds of expiration 5
• Apply external PEEP at 80-85% of measured auto-PEEP level to counterbalance the threshold load without causing additional hyperinflation 5, 1
• Reduce auto-PEEP formation by decreasing respiratory rate, reducing tidal volume, or increasing expiratory time (adjust I:E ratio to 1:4 or 1:5 in obstructive disease) 5
• Optimize trigger sensitivity to -1 to -2 cm H₂O, observing pressure deflections before breath delivery to minimize triggering effort 1, 2
• Continue bronchodilator therapy as successful treatment shows decreased peak-to-plateau pressure gradient and improved expiratory flow patterns 1

Flow Starvation (Inadequate Inspiratory Flow)

When the pressure waveform shows a concave or scooped appearance during inspiration 2, 3:

• Increase peak inspiratory flow rate to 80-100 L/min in adults to meet patient demand 5, 2
• Switch from constant (square wave) to decelerating flow pattern, as decelerating flow reduces work of breathing compared to constant flow 1, 2
• In pressure support ventilation, increase the pressure support level to deliver higher initial flow 2, 4
• Ensure delivered minute ventilation is adequate to reduce respiratory drive and ventilatory requirements 2

Premature Cycling

When the ventilator terminates inspiration before the patient's neural inspiratory time ends 4, 6:

• In pressure support ventilation, decrease the expiratory trigger sensitivity (cycle-off criterion) from the typical 25% to 10-15% of peak flow 4
• Increase inspiratory time in volume-cycled modes to better match the patient's neural inspiratory time 2, 4
• Increase pressure support level, which prolongs inspiratory time by maintaining higher flow for longer duration 4

Delayed Cycling

When inspiration continues beyond the patient's neural inspiratory time 4, 6:

• In pressure support ventilation, increase the expiratory trigger sensitivity (cycle-off criterion) from 25% to 40-50% of peak flow 4
• Decrease inspiratory time in volume-cycled modes 2, 4
• Reduce pressure support level if excessive 4
• Consider switching to proportional assist ventilation or neurally adjusted ventilatory assist, which automatically adjust to patient demand 6

Double Triggering

When two ventilator breaths occur within one patient inspiratory effort 4, 6:

• Increase tidal volume if the set volume is insufficient for the patient's demand (common cause when VT <6 mL/kg in non-ARDS patients) 4, 6
• Prolong inspiratory time to match the patient's neural inspiratory time 4, 6
• In pressure support, increase the pressure level to deliver adequate volume in one breath 4

Addressing Patient Factors

Beyond ventilator adjustments, reverse underlying patient factors that contribute to dyssynchrony 2, 4:

• Reduce respiratory drive: Treat fever, pain, anxiety, and metabolic acidosis; ensure adequate sedation without oversedation 2, 4
• Optimize respiratory mechanics: Aggressive bronchodilator therapy for airflow obstruction; diuresis for pulmonary edema; treat pneumothorax if present 1, 2
• Minimize imposed work of breathing: Use the largest endotracheal tube possible (8-9 mm) to decrease airway resistance; ensure circuit is free of condensation and kinks 5, 2

Monitoring Response to Interventions

After making adjustments, reassess waveforms to confirm improvement 3, 4:

• Pressure waveforms should show smooth contours without scooping or abrupt deflections 3
• Flow waveforms should demonstrate complete return to baseline before the next breath 3
• Expiratory flow should reach zero before the next inspiration (absence of breath stacking) 5, 3
• Patient should appear comfortable with reduced accessory muscle use and decreased respiratory rate 2, 4

Critical Pitfalls to Avoid

• Never ignore persistent dyssynchrony, as it prolongs mechanical ventilation duration, increases ICU length of stay, and may worsen outcomes 6
• Avoid excessive sedation or paralysis as the first-line response; optimize ventilator settings first 5, 2
• Do not apply external PEEP exceeding auto-PEEP levels, as this causes additional hyperinflation and hemodynamic compromise 5
• Recognize that low tidal volumes (6-8 mL/kg) appropriate for ARDS may cause double triggering in non-ARDS patients with normal respiratory mechanics 4, 6
• In severe asthma or COPD with auto-PEEP causing hypotension, immediately disconnect from the ventilator circuit to allow passive exhalation and press on the chest wall to assist exhalation 5