This patient's elevated blood pH and decrease in PaCO2 is consistent with acute respiratory alkalosis.
Respiratory acid-base disorders are caused by primary changes in PaCO2, whereas metabolic acid-base disorders are due to primary changes in the concentration of HCO3-. A primary rise in PaCO2 or a fall in plasma HCO3- reduces the pH (acidemia), whereas the opposite increase the pH (alkalemia). Patients suffering from pneumonia can have tachypnea due to hypoxia. Increased minute ventilation reduces arterial CO2, an acid, resulting in alkalosis. A slight decrease in bicarbonate level may be seen due to early renal compensation.
Foster et al. discuss respiratory alkalosis, which may result from pulmonary or extrapulmonary etiology. Hyperventilation syndrome is common in the emergency setting and is a diagnosis of exclusion. There are many cardiac effects of respiratory alkalosis, such as tachycardia, ventricular and atrial arrhythmias, and ischemic and nonischemic chest pain. In the lungs, vasodilation occurs, and in the gastrointestinal system there are changes in perfusion, motility, and electrolyte handling.
Wunderink and Waterer discuss the management of community-acquired pneumonia. The majority of hospitalized patients with community-acquired pneumonia can be treated with either a respiratory fluoroquinolone or a combination of cephalosporin and a macrolide. Patients with three or more minor criteria for severe community-acquired pneumonia (e.g., elevated BUN, confusion, and a high respiratory rate) should receive extensive intervention in the emergency department and be considered for admission to the intensive care unit.
Illustration A depicts a diagram of the various acid-base disorders and the expected compensatory effects.
Illustration B demonstrates a table with the normal compensatory responses to primary acid-base disorders.
Answer 1: Normal pH range is 7.35-7.45. This patient's pH is outside this range, which indicates an acid-base disturbance.
Answer 2: A decreased pH and an decrease in HCO3- would be consistent with metabolic acidosis.
Answer 3: An elevated pH and an increase in HCO3- would be consistent with metabolic alkalosis.
Answer 4: Respiratory acidosis results from decreased alveolar ventilation, which causes increased arterial CO2 levels.
Foster GT, Vaziri ND, Sassoon CS. Respiratory alkalosis. Respir Care. 2001 Apr;46(4):384-91.
PMID: 11262557 (Link to Abstract)
Wunderink RG, Waterer GW. Clinical practice. Community-acquired pneumonia. N Engl J Med. 2014 Feb 6;370(6):543-51.
PMID: 24499212 (Link to Abstract)