Permissive hypercapnic ventilation (PHV)

Source:http://www.uptodate.com/contents/permissive-hypercapnic-ventilation

INTRODUCTION

Permissive hypercapnic ventilation (PHV) is a strategy of mechanical ventilation that accepts deliberate alveolar hypoventilation [1]. Its primary purpose is to allow ventilator changes that reduce alveolar pressure and its associated risks, such as pulmonary barotrauma, ventilator-associated lung injury, and hypotension. Hypercapnic acidosis is a consequence of this strategy and not a goal.

Patient selection, efficacy, and potential harms of PHV are discussed in this topic. In addition, the strategy itself is described. Use of PHV in patients with acute respiratory distress syndrome and asthma is reviewed separately. (See "Mechanical ventilation in acute respiratory distress syndrome" and "Mechanical ventilation in adults with acute exacerbations of asthma".)

PATIENT SELECTION

Selecting patients for PHV requires careful consideration of the indications and contraindications, which are discussed in this section.

Indications — PHV is indicated in situations where decreasing the minute ventilation (ie, lowering the tidal volume or respiratory rate) may be beneficial. It is most commonly utilized in patients with acute lung injury or acute respiratory distress syndrome (collectively referred to as ARDS in this review), an acute exacerbation of asthma, or an acute exacerbation of chronic obstructive pulmonary disease (COPD).

• ARDS — Low tidal volume ventilation improves important clinical outcomes in patients with ARDS. The respiratory rate is routinely increased during low tidal volume ventilation in an effort to maintain an adequate minute ventilation. However, the increased respiratory rate may be insufficient to compensate for the low tidal volumes and hypercapnic acidosis may develop. PHV is indicated in this situation, rather than abandoning low tidal volume ventilation. (See "Mechanical ventilation in acute respiratory distress syndrome", section on 'Low tidal volume ventilation'.)
• Asthma — Widespread bronchoconstriction reduces expiratory airflow during an asthma exacerbation. Auto-PEEP will develop if the expiratory airflow is reduced to such a degree that the next breath is initiated prior to the completion of expiration. The expiratory time should be prolonged in this situation, which can be accomplished by increasing the inspiratory flow rate, decreasing the tidal volume, or decreasing the respiratory rate. Lowering the tidal volume or respiratory rate will decrease the minute ventilation and may result in hypercapnic acidosis. PHV is indicated in this situation, rather than accepting significant auto-PEEP. (See "Mechanical ventilation in adults with acute exacerbations of asthma".)
• COPD — Patients with COPD are similarly at risk for auto-PEEP during mechanical ventilation. PHV is indicated if efforts to reduce auto-PEEP result in hypercapnic acidosis, as described above for patients with an asthma exacerbation. (See "Positive end-expiratory pressure (PEEP)", section on 'Auto (intrinsic) PEEP'.)