Mechanical ventilation respiratory alkalosis

It is imperative to identify serious causes of respiratory alkalosis and institute effective treatment. In spontaneously breathing patients, respiratory alkalosis is typically only mild or moderate in severity and no specific therapy is indicated. Severe alkalosis generally represents respiratory acidosis imposed on metabolic alkalosis and may improve with sedation. Patients receiving mechanical ventilation are treated with reduced minute ventilation achieved by decreasing the respiratory rate and/or tidal volume. If the alkalosis persists in the ventilated patient, high-level sedation or paralysis is effective. 

Causes of respiratory alkalosis include increases in neurochemical stimulation via central or peripheral mechanisms, or physical increases in ventilation via voluntary or artificial means (e.g., mechanical ventilation). 

Respiratory alkalosis associated with mechanical ventilation can often be corrected by decreasing the number of mechanical breaths per minute, using a capnograph and spirometer to adjust ventilator settings more precisely, or increasing dead space in the ventilator circuit. 

The most common mixed acid-base disorder is respiratory and metabolic alkalosis, which occurs in critically ill surgical patients with respiratory alkalosis caused by mechanical ventilation, hypoxia, sepsis, hypotension,... 

A decrease in PaC02 may occur in patients with cardiogenic, hypovolemic, or septic shock because oxygen delivery to the carotid and aortic chemoreceptors is reduced. This relative deficit in Pa02 stimulates an increase in ventilation. The hyperventilation in sepsis is also mediated via a central mechanism. Hyperventilation-induced respiratory alkalosis with an elevation in cardiac index and hypotension without peripheral vasoconstriction may therefore be an early sign of sepsis. 

The combination of respiratory and metabolic alkalosis is the most common mixed acid-base disorder. This mixed disorder occurs frequently in critically ill surgical patients with respiratory alkalosis caused by mechanical ventilation, hypoxia, sepsis, hypotension, neurologic damage, pain, or drugs, and with metabolic alkalosis caused by vomiting or nasogastric suctioning and massive blood transfusions. It may also occur in patients with hepatic cirrhosis who hyperventilate, receive diuretics, or vomit, as well as in patients with chronic respiratory acidosis and an elevated plasma bicarbonate concentration... 

Respiratory alkalosis is associated with many illnesses. Hyperventilation has several causes. The CNS respiratory centre is stimulated by many factors including anxiety, psychosis, pain and fever. Overdosage of salicylates can initially stimulate ventilation causing respiratory alkalosis which may be followed by metabolic acidosis. Stimulation of the chest receptors by conditions such as pneumothorax, pulmonary embolism and pulmonary oedema can cause hyperventilation and hypocapnia. Other causes include mechanical ventilation, hepatic failure and sepsis... 

This is due to overbreathing. It occurs in the hyperventilation syndrome susceptible subjects hyperventilate, perhaps in response to stress, for minutes, days and even weeks. It also occurs in healthy people at high altitude, where ventilation is stimulated by hypoxia. In surgical practice, hyperventilation occurs in patients being overventilated by a mechanical ventilator. In this situation, CO2 is washed out of the body and respiratory alkalosis supervenes. Because of the dangers of this situation, the arterial blood gas tensions should be frequently checked in such patients and the PCO2 should not be allowed to fall below 30mmHg. 

High muscle pH (above 7.45) is seen in children with a respiratory or metabolic alkalosis. As in acidotic states, respiratory and metabolic causes can be distinguished by the blood PCO2. Most of the children who have developed muscle alkalosis were alkalotic because of hyperventilation while on a mechanical ventilator or because of a metabolic alkalosis caused by the transfusion of large quantities of citrated blood. Muscle pH which is often higher than blood pH in these children has been observed as high as 7.70. 

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