Ventilation in ARDS and asthma: the optimal blood gas values.

Artificial ventilation of patients with acute respiratory diseases, i.e. ARDS and severe asthma, may involve the risk of pulmonary oxygen toxicity as well as volutrauma. The relationship between ventilator treatment and volutrauma suggests that only in patients with normal lungs the aim of ventilator treatment should be an arterial carbon dioxide tension and pH within the normal ranges. In patients suffering from a lung disease the clinical target must be based not only upon the arterial blood gases but also upon airway pressure and respiratory tidal volume. Thus during artificial ventilation of a patient with an acute pulmonary disease the following arterial pH and pCO2 optima are proposed: pH 7.35, with a range from 7.1 to 7.4; pCO2 is related to pH but an acceptable range is 5-12 kPa. The lowest acceptable fraction of inspired oxygen and thereby the safe lower level of arterial pO2 for an individual patient depends on many factors. The lower limit may be about 3 kPa, but the arterial pO2 should not be evaluated as an isolated parameter. It is related to the general oxygen transport capability of arterial blood, extractable oxygen, cardiac output and the microcirculation.