7.6 Ventilator Dyssynchrony & Troubleshooting

7.6 Ventilator Dyssynchrony & Troubleshooting

Patient-Ventilator Dyssynchrony: Classification & Remediation

Dyssynchrony occurs when the mechanical ventilator fails to match the patient's neural respiratory drive, increasing work of breathing, oxygen consumption, and sedation requirements.

1. Trigger Dyssynchrony

• Ineffective Triggering (Missed Efforts):
  • Visual Sign: A sharp downward negative deflection on the pressure scalar or positive blip on the expiratory flow scalar that is not followed by a ventilator-delivered breath.
  • Cause: Severe auto-PEEP. The patient must inhale with enough muscular force to overcome both intrinsic PEEP and the ventilator trigger threshold before the machine registers an effort: P_effort = PEEP_auto + P_trigger. If muscle strength is insufficient, the effort is wasted.
  • Remedy: Decrease auto-PEEP by extending Te. If auto-PEEP persists, apply extrinsic PEEP to match 80% of auto-PEEP (counter-balancing the threshold load) or optimize flow trigger sensitivity.
• Auto-Triggering:
  • Visual Sign: The ventilator delivers rapid, unprompted breaths without any patient effort.
  • Cause: Circuit condensation sloshing back and forth across the flow sensor, cardiogenic oscillations from a hyperdynamic myocardium, or a trigger threshold set too sensitively (e.g., flow trigger set to 0.05 L/min).
  • Remedy: Drain circuit condensation, increase trigger threshold slightly (e.g., from 0.2 to 0.5 L/min).
• Double Triggering:
  • Visual Sign: The ventilator fires two consecutive mechanical breaths separated by an expiratory time < 0.5 x Ti.
  • Cause: Patient's neural inspiratory time is longer than the set mechanical Ti, or the set tidal volume is too low to meet patient demand.
  • Remedy: Increase inspiratory time (Ti), increase tidal volume, or switch to a proportional mode (e.g., NAVA).

2. Flow Dyssynchrony (Flow Starvation)

• Visual Sign: The pressure-time scalar dips concavely downward during inspiration rather than rising smoothly.
• Cause: The set inspiratory flow rate is lower than the patient's peak inspiratory demand.
• Remedy: Increase peak flow rate, shorten rise time, or transition to Pressure Control.

3. Cycle Dyssynchrony

• Premature Cycling: The ventilator terminates inspiration while the patient is still actively contracting the diaphragm. Triggers double-triggering or high work of breathing. Remedy: Increase Ti or decrease expiratory flow cycling percentage (Esens).
• Delayed Cycling: The ventilator continues inspiration after the patient has relaxed their diaphragm and begun active exhalation. Pressure spikes at end-inspiration. Remedy: Shorten Ti or increase Esens (e.g., from 25% to 40–50% of peak flow).

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Table 7.3.1: Troubleshooting Guide for Ventilator Waveforms & Loops

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Worked Clinical Calculation: Air Leak Percentage & Auto-PEEP

Clinical Scenario

A 10-year-old mechanically ventilated child with severe status asthmaticus is receiving Volume Control ventilation. The respiratory therapist observes the following bedside parameters:

• Set Inspiratory Tidal Volume (Vti) = 350 mL
• Measured Expiratory Tidal Volume (Vte) = 280 mL
• The expiratory flow tracing fails to return to zero baseline before the next breath, continuing at -12 L/min.

Step-by-Step Graphic & Quantitative Analysis

1. Calculate the Air Leak Percentage:

Air Leak % = [(Vti - Vte) / Vti] x 100 = [(350 mL - 280 mL) / 350 mL] x 100 = (70 / 350) x 100 = 20.0%

1. Evaluate the Air Leak: A 20% air leak exceeds the acceptable threshold (<= 10–15%). The therapist checks cuff pressure using a calibrated manometer and discovers the cuff pressure is 10 cmH2O. Inflating the cuff to 22 cmH2O seals the peritubular gap, restoring measured Vte to 345 mL (leak reduced to 1.4%).
2. Resolve the Auto-PEEP: Despite resolving the leak, the flow-time scalar continues to show incomplete exhalation due to severe bronchospasm and a short expiratory time (Te = 1.0 s at rate 24 bpm, Ti = 1.5 s). The therapist decreases the set rate from 24 to 16 bpm, expanding total cycle time from 2.5 s to 3.75 s and lengthening Te to 2.25 seconds. Expiratory flow now successfully returns to baseline zero prior to each breath, abolishing auto-PEEP and hemodynamic depression.

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NPS Exam Traps Callout Box: Graphics & Dyssynchrony

[!WARNING] NPS Exam Trap 1: Auto-PEEP vs. Secretions on Flow Tracings Do not confuse a failure to return to baseline (auto-PEEP) with a sawtooth pattern (secretions). If the line doesn't reach zero, change the ventilatory timing (lengthen Te). If the line is jagged and wavy, suction the patient!

NPS Exam Trap 2: Treating the "Beak" on a P-V Loop A pressure-volume loop that flattens or develops a terminal beak at high volume raises concern for overdistension. Verify scale, effort, leak, chest-wall mechanics, and measured plateau pressure, then reduce tidal volume or pressure when the full picture confirms overdistension. PEEP is titrated from recruitability and oxygenation and is not chosen from loop shape alone.

NPS Exam Trap 3: Missed Triggers and Auto-PEEP A negative pressure deflection without a supported breath is an ineffective effort. Check auto-PEEP and inadequate expiratory time, but also inspect trigger sensitivity, leak compensation, circuit water, sensor function, weakness, sedation, and effort amplitude. When obstructive auto-PEEP is present, lengthen expiration and treat resistance; extrinsic PEEP is individualized and not automatically set to 80%.