Breath stacking, also known as intrinsic positive end-expiratory pressure (auto-PEEP) or dynamic hyperinflation, occurs when a patient on a mechanical ventilator cannot fully exhale before the next breath is delivered. This incomplete exhalation traps air within the alveoli, leading to a progressive build-up of pressure and volume. High intrathoracic pressure can compromise circulation by decreasing blood return to the heart, potentially causing low blood pressure, and can lead to lung damage (barotrauma). Addressing this condition quickly is necessary to prevent these serious complications.
Causes of Air Trapping
The fundamental cause of air trapping is insufficient time for the patient to complete the expiratory phase. When a new breath begins before the lung has fully deflated, residual volume is retained and accumulates with each cycle. This problem stems from a mismatch between the patient’s respiratory needs and the ventilator settings, or an underlying disease process that physically impedes airflow.
Ventilator settings contribute when the respiratory rate is set too high or the inspiratory time is too long, both of which reduce the time available for exhalation. A high minute ventilation, achieved through large tidal volumes or a fast rate, can overwhelm the patient’s ability to empty their lungs completely. This shortens the total breathing cycle time, leaving inadequate time for passive exhalation to finish.
Patient factors that increase airway resistance are the second major contributor. Conditions such as Chronic Obstructive Pulmonary Disease (COPD) or acute asthma cause the airways to narrow, making it harder and slower for air to be pushed out. The natural resistance of the diseased lung tissue requires a significantly longer duration for the expiratory flow to cease. This increased resistance, combined with a short cycle time, guarantees air trapping.
Immediate Management Strategies
When breath stacking is identified, rapid intervention is necessary to immediately decompress the lungs. The most immediate action is to temporarily disconnect the patient from the ventilator circuit. This allows a full, passive exhalation of all trapped air, quickly relieving high intrathoracic pressures and often resulting in an immediate improvement in blood pressure.
Clinicians must also address factors increasing the patient’s respiratory drive, which often causes the stacking event. A patient who is anxious, in pain, or has a high metabolic demand may attempt to breathe faster than the ventilator allows, leading to double-triggering. Increasing sedation or providing analgesia can calm the patient’s breathing, reducing rapid efforts and improving synchrony with the machine.
Underlying physical causes of airflow limitation must also be managed quickly. If thick pulmonary secretions or mucus plugs obstruct the airways, immediate suctioning of the endotracheal tube can remove the blockage and reduce resistance to expiratory flow. If the patient has underlying bronchospasm, as seen in asthma or COPD, administering bronchodilator medication can open the narrowed airways, facilitating a faster and more complete exhalation.
Adjusting Ventilator Settings for Resolution
The definitive long-term solution involves manipulating ventilator settings to maximize the time available for exhalation, known as the expiratory time (\(T_e\)). Since the total duration of a breath cycle is fixed by the respiratory rate, the primary goal is to lengthen \(T_e\) relative to the inspiratory time (\(T_i\)). This adjustment requires a sequential approach to ensure patient safety.
The core principle is to create a longer \(T_e\) to allow the expiratory flow to fully return to zero. Several adjustments can achieve this:
- Lower the set respiratory rate (\(f\)). Reducing the rate directly increases the total time for each breath cycle, providing a proportionally longer period for exhalation. For example, dropping the rate from 20 to 15 breaths per minute significantly increases the available time for passive lung emptying.
- Adjust the Inspiratory-to-Expiratory (I:E) ratio. This ratio dictates the proportion of the breath cycle spent in inhalation versus exhalation. To fix air trapping, the ratio must be widened from a common 1:2 setting to 1:3 or even 1:4, prioritizing the expiratory phase.
- Decrease the tidal volume (\(V_t\)). A smaller delivered breath requires less time to push into the lungs, shortening \(T_i\). This makes more time available for the passive \(T_e\) within the same total cycle time and reduces the amount of air that needs to be exhaled.
- Increase the inspiratory flow rate. Delivering the set tidal volume faster reduces the duration of the inspiratory phase, which leaves a greater amount of the fixed breath cycle time for exhalation.
Verifying and Maintaining Stability
The success of any intervention must be quickly confirmed using objective data from the ventilator. The most immediate visual confirmation comes from observing the flow-time waveform on the ventilator screen. In a healthy breath cycle, the expiratory flow tracing should drop all the way down and touch the zero-flow line before the next breath begins. If breath stacking is resolved, this tracing will return to zero, indicating that all the delivered air has been exhaled.
Quantifying the remaining trapped pressure is done by performing an expiratory hold maneuver. This test temporarily occludes the exhalation port at the end of expiration, allowing the ventilator to measure the true pressure remaining in the alveoli, known as intrinsic PEEP (auto-PEEP). A reduction in this measured pressure confirms that the adjustments have successfully reduced air trapping.
Clinical status must also be monitored to ensure the patient tolerates the new settings and remains stable. Improved hemodynamics, such as an increase in previously low blood pressure, suggests that the high intrathoracic pressure has been relieved. The patient’s oxygen saturation should improve, and signs of patient-ventilator asynchrony, such as double-triggering, should disappear.