What is breath stacking in mechanical ventilation, particularly for patients with severe respiratory failure, such as acute respiratory distress syndrome (ARDS)?

Breath Stacking in Mechanical Ventilation

Breath stacking is a dangerous form of patient-ventilator asynchrony where a patient initiates a new breath before fully exhaling the previous one, resulting in excessive tidal volumes (often 1.5-2 times the set volume) that can reach 10 mL/kg or higher, substantially increasing the risk of ventilator-induced lung injury even during lung-protective ventilation strategies. 1

Definition and Mechanism

Breath stacking occurs when:

• A patient triggers a new mechanical breath before complete exhalation of the previous breath 2, 1
• The residual volume from the incomplete exhalation adds to the new tidal volume 1
• This results in cumulative volumes that can be 1.44-1.82 times the set tidal volume 1
• The phenomenon persists even with deep sedation (Richmond Agitation Sedation Scale -4 to -5) 1

Clinical Significance in ARDS

The primary danger is that breath stacking undermines lung-protective ventilation by generating excessive transpulmonary pressures and tidal volumes that promote ventilator-induced lung injury. 3, 1

Key concerns include:

• Stacked breaths occur at a mean frequency of 2.3 ± 3.5 per minute during low tidal volume ventilation 1
• Median stacked breath volumes reach 10.1 mL/kg predicted body weight, far exceeding the 6 mL/kg target for ARDS 1
• This defeats the mortality benefit of lung-protective ventilation strategies 3
• Neuromuscular blocking agents may be required to prevent dyssynchrony and breath stacking in severe ARDS, particularly during the first 48 hours 3

Risk Factors

Lower set tidal volumes paradoxically increase the risk of breath stacking (relative risk 0.4 for each 1 mL/kg increase in set tidal volume). 1 This creates a clinical dilemma where the very strategy intended to protect the lung may promote asynchrony.

Additional factors:

• Inadequate sedation-analgesia management 2
• Assist-control mode ventilation 2
• Strong patient respiratory drive 3
• Insufficient inspiratory time relative to patient demand 2

Management Algorithm

First-Line: Ventilator Adjustment (Most Effective)

Adapting the ventilator to patient breathing effort reduces breath-stacking asynchrony significantly more effectively than increasing sedation (asynchrony index reduction: -99% with ventilator changes vs -41% with sedation increases, p<0.001). 2

Specific ventilator adjustments:

• Switch to pressure-support ventilation (independently associated with asynchrony reduction) 2
• Increase inspiratory time to better match patient demand (independently associated with asynchrony reduction) 2
• Consider slightly higher tidal volumes within lung-protective limits (4-8 mL/kg) if breath stacking persists 3, 1
• Adjust trigger sensitivity to improve patient-ventilator synchrony 2

Second-Line: Sedation-Analgesia Optimization

If ventilator adjustment is insufficient:

• Increase sedation-analgesia, though this is less effective than ventilator changes 2
• Titrate according to protocols with regular assessment 3

Third-Line: Neuromuscular Blockade (Severe ARDS Only)

Reserve neuromuscular blocking agents for patients with the most severe ARDS, mainly in the acute phase and during the first 48 hours of mechanical ventilation. 3

Indications:

• Severe ARDS with persistent dyssynchrony despite ventilator optimization 3
• Generation of excessive transpulmonary pressure by inspiratory muscles 3
• Prevention of expiratory efforts causing derecruitment 3

Cautions:

• Requires sustained deep sedation 3
• Risk of myopathy, diaphragm dysfunction, and ICU-acquired weakness, especially with concurrent corticosteroids 3

Monitoring and Detection

Continuous waveform analysis is essential:

• Monitor flow-time and pressure-time waveforms for incomplete exhalation 1
• Calculate asynchrony index (percentage of total breaths that are asynchronous) 2
• Severe breath stacking is defined as asynchrony index ≥10% 2
• Inter-rater reliability for identifying stacked breaths is excellent (kappa 0.99) 1

Integration with Lung-Protective Ventilation

Maintain tidal volumes of 4-8 mL/kg predicted body weight and plateau pressures <30 cm H₂O while actively managing breath stacking through ventilator adjustment rather than accepting higher volumes. 3

The goal is to:

• Prevent alveolar overdistension and recruitment-derecruitment 4
• Minimize mechanical power delivered to the lungs 5
• Achieve patient-ventilator synchrony without abandoning lung protection 6

Common Pitfalls

• Reflexively increasing sedation first rather than adjusting ventilator settings (sedation alone reduces asynchrony by only 41% vs 99% with ventilator changes) 2
• Tolerating the asynchrony without intervention (results in virtually no improvement) 2
• Failing to recognize that lower tidal volumes increase stacking risk, creating a false sense of safety 1
• Not monitoring waveforms continuously to detect the problem early 1