Acidosis frequently accompanies many pathological states, such as uremia, severe trauma and diabetes, which are often associated with protein wasting. ICU patients are a group in whom disturbances of pH homeostasis are a common occurrence. Acidosis frequently develops, but although acidosis has been shown experimentally to inhibit muscle protein and albumin synthesis, its role in the profound muscle wasting that occurs in traumatized patients needs to be evaluated. The acidosis can be reversed by a combination of treatment with alkalis and by increasing the ventilation rate, but whether these can reverse the protein loss is not known. However, if the protein loss continues, it will lead to respiratory muscle loss, which will then inhibit the weaning of the patient from the ventilator. It is therefore important that studies of the role of pH in controlling tissue protein metabolism be studied in this group of patients, who themselves are at risk from protein wasting and acidosis. Moreover, the proposed studies would take advantage of the change in pH that occurs at the time that hyperventilation is terminated, without the need to manipulate the treatment of the patient. The purpose of the present proposal is therefore to demonstrate the association between changes in arterial blood pH and rates of protein synthesis in muscle and liver of head trauma patients in the ICU, during the day following cessation of hyperventilation. If the study is successful in demonstrating that more alkalosis is beneficial, this might help us to develop potential interventions to reverse the protein loss after head trauma and in other ventilated ICU patients.