From the Guidelines
Tachypnea in patients undergoing pressure support ventilation (PSV) is primarily caused by inadequate ventilatory support, patient-ventilator asynchrony, or underlying patient conditions, with the most recent evidence suggesting that excessive ventilatory drive, inadequate pressure support levels, and patient-ventilator asynchrony are key contributing factors 1.
Causes of Tachypnea in PSV
The causes of tachypnea in PSV can be multifactorial, including:
- Inadequate ventilatory support, where the set pressure is too low to meet the patient's respiratory demands, forcing them to breathe more rapidly to maintain adequate minute ventilation
- Patient-ventilator asynchrony, particularly trigger asynchrony or cycle asynchrony, which can lead to tachypnea as patients struggle against the ventilator's timing
- Underlying conditions such as anxiety, pain, fever, metabolic acidosis, hypoxemia, or worsening of the primary respiratory condition
- Excessive dead space in the ventilator circuit or inappropriate sensitivity settings, which can increase work of breathing and trigger rapid breathing
Mechanisms and Contributing Factors
The mechanism of central apnea during PSV is explained by the decrease in PaCO2 below the apneic threshold when the level of support and tidal volume are increased 1. This effect may be worsened in patients with heart failure who have a tendency to develop central apnea via the mechanism of decreased cardiac output, increase in left ventricular filling pressure, prolonged circulation time, and increased chemoreceptor sensitivity. Adjusting the level of assistance in accordance with the patient’s demands, to prevent respiratory events and sleep fragmentation, is essential to improve sleep and to prevent sleep fragmentation 1.
Recommendations for Management
To address tachypnea in PSV, clinicians should first optimize the pressure support level, typically starting at 8-12 cmH2O and titrating based on respiratory rate and tidal volume, adjust trigger and cycle sensitivity settings, and treat any underlying conditions 1. Careful assessment of ventilator waveforms can help identify specific asynchronies contributing to the rapid breathing pattern. Understanding these mechanisms is essential because persistent tachypnea increases work of breathing, causes respiratory muscle fatigue, and may lead to ventilator dependence if not properly addressed.
From the Research
Causes of Tachypnoea in Patients Undergoing Pressure Support Ventilation (PSV)
- Tachypnoea in patients undergoing PSV can be caused by various factors, including incomplete respiratory muscle unloading, high levels of pressure support, and equipment factors such as gas leaks in the ventilator system 2, 3.
- A study published in the journal Chest found that minimizing work fully accounts for relief of tachypnea during PSV, but the level of PSV where work is minimized may not necessarily normalize frequency 2.
- The same study found that frequency was normalized only at PSV levels 131 to 193% of the levels of pressure at the crossover from partial to total respiratory muscle unloading, and that this normalization coincided with increasing tidal volume rather than decreasing work 2.
- Another study published in the journal Anaesthesia reported a case of severe hyperventilation and respiratory alkalosis during PSV due to a gas leak in the ventilator system and autocycling of the ventilator 3.
- Patient factors such as low respiratory impedance and high levels of pressure support can also contribute to tachypnoea during PSV 3.
- The effectiveness of PSV in supplying tidal volumes adequate to improve hypercarbia at lower airway pressures than other modes of ventilation, such as assist control ventilation, can also impact tachypnoea 4.
- Physiological changes occurring with positive pressure ventilation, such as increased airway resistance and intrathoracic pressures, and altered lung mechanics, can also contribute to tachypnoea 5.
- Noninvasive positive-pressure ventilation has been shown to be effective in alleviating distress and avoiding intubation in patients with acute respiratory failure, but may not be suitable for all patients 6.