Permissive hypercapnia

Source:wikipedia
Permissive hypercapnia is hypercapnia, (i.e. high concentration of carbon dioxide in blood), in respiratory insufficient patients in which oxygenation has become so difficult that the optimal mode of mechanical ventilation (with oxygenation in mind) is not capable of exchanging enough carbon dioxide. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs.

In acute respiratory distress syndrome (ARDS), decreasing the tidal volume on the ventilator (usually 8-12 mL/kg) to 4-6 mL/kg may decrease barotrauma by decreasing ventilatory peak airway pressures and leads to improved respiratory recovery. Hypercapnia (increased pCO2) sometimes needs to be tolerated in order to achieve these lower tidal volumes. The permissive hypercapnia leads to respiratory acidosis which might have negative side effects, but given that the patient is in ARDS, improving ventilatory function is more important.

Since hypoxemia is a major life threatening condition and hypercapnia is not, one might choose to accept the latter. Hence the term, "permissive hypercapnia."
Altogether, the negative side effects of permissive hypercapnia may outweigh the benefits. For that reason, the implementation of extracorporeal CO2 removal (iLA Membrane Ventilator, Novalung) at an early stage of ARDS, has become a well established standard to allow for protective ventilation and avoid respiratory acidosis.

Symptoms

Symptoms of early hypercapnia (i.e. where PaCO2 is elevated but not extremely so) include flushed skin, full pulse, extrasystoles, muscle twitches, hand flaps, and possibly a raised blood pressure. In severe hypercapnia (generally PaCO2 greater than 10 kPa or 75 mmHg), symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death. other discription about permissive hypercapnia in ARDS patient Mechanical ventilation using high tidal volume (VT) and transpulmonary pressure can damage the lung, causing ventilator-induced lung injury. Permissive hypercapnia, a ventilatory strategy for acute respiratory failure in which the lungs are ventilated with a low inspiratory volume and pressure, has been accepted progressively in critical care for adult, pediatric, and neonatal patients requiring mechanical ventilation and is one of the central components of current protective ventilatory strategies.

Read more...

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'.)

Read more...