Ventilator graphic analysis: Obstructive lung disease
Ventilator graphics provide a rapid display of a patient’s pulmonary mechanics and interaction with the ventilator. Analyzing ventilator waveforms in a patient with acute respiratory failure is as essential as monitoring the telemetry of a patient with suspected cardiac dysrhythmia. Ventilator waveforms can provide the first clue of worsening airway resistance, dynamic hyperinflation and patient-ventilator asynchrony, enabling clinicians to both diagnose and manage ventilator emergencies. Dynamic hyperinflation is commonly seen in patients with obstructive lung disease and occurs when the expiratory time is insufficient to return to resting lung volume (Figure 1).
Figure 1. Intrinsic PEEP as evidenced by active flow at end-expiration.
Dynamic hyperinflation observed on flow waveform analysis during mechanical ventilation is typically caused by collapse of unstable airways early in exhalation or by setting the respiratory rate too high to allow a complete exhalation of the delivered tidal volume. Auto-PEEP (or intrinsic PEEP, PEEPi) refers to end-expiratory alveolar pressure that exceeds the end-expiratory airway pressure set by the clinician (applied PEEP). The degree of dynamic hyperinflation can be quantified by performing an end-expiratory occlusion maneuver to measure the total PEEP, which is the sum of applied PEEP and intrinsic PEEP. In patients with severe asthma, intrinsic PEEP can reach 10 to 15 cm H20 or more.
We know that patients with obstructive lung disease have elevated peak inspiratory pressure (PIP) due to increased airway resistance. However, it is important to also understand that dynamic hyperinflation can increase the plateau pressure (Pplat), as the trapped gas raises average end-inspiratory alveolar pressure. Because asthmatic patients typically have normal lung compliance, elevated plateau pressure should raise concern for significant auto-PEEP, even in the absence of significantly elevated end-expiratory pressure. The end-expiratory hold maneuver is not perfectly sensitive for the detection of auto-PEEP as patients with status asthmaticus can have low values of PEEPi, if there is a large proportion of small airway closure obscuring accurate assessment of end-expiratory alveolar pressure. Can you calculate the PEEPi from the ventilator graphics below (Figure 2)?
Figure 2. Patient with acute respiratory failure due to status asthmaticus.
What is the danger of intrinsic PEEP? Auto-PEEP has several adverse effects, including increased inspiratory workload resulting from suboptimal pulmonary function at a higher resting lung volume, hypotension resulting from increased intrathoracic pressure compromising venous return, ventilation-perfusion mismatch from regional hyperinflation and increased risk of barotrauma. Auto-PEEP imposes a substantial inspiratory muscle load to trigger a breath, as it 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 (Figure 3).
Figure 3. Trigger asynchrony resulting from an increased trigger threshold from auto-PEEP.
Strategies to reduce dynamic hyperinflation include reduction of respiratory rate to 10-12 breaths per minute (lower rates may be required when Pplat > 30 cm H20), reducing inspiratory time to extend expiratory time (avoiding drastic increases in PIP from elevated flow rates), permissive hypercapnia, medical therapies to reduce bronchoconstriction and neuromuscular blockade. If there is no urgent reason to correct acidemia from hypercapnia, like cardiac dysrhythmias or hemodynamic instability, it is best to avoid attempts to lower PaCO2 levels by increasing minute ventilation (rate and tidal volume), which will only worsen hyperinflation and further increase dead space. If severe trigger asynchrony exists, increasing the applied PEEP to lower the trigger threshold can be considered in rare instances, but only if it does not significantly raise the plateau pressure.
Answer: PEEPi = 15 cm H20
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