Respiratory alkalosis is 1 of the 4 basic classifications of blood pH imbalances. Normal human physiological pH is 7.35 to 7.45. A decrease in pH below this range is acidosis, an increase above this range is alkalosis. Respiratory alkalosis is by definition a disease state where the body’s pH is elevated to greater than 7.45 secondary to some respiratory or pulmonary process.
Before going into details about pathology and this disease process, some background information about the physiological pH buffering process is important. The primary pH buffer system in the human body is the HCO3/CO2 chemical equilibrium system. Where:
HCO3 functions as an alkalotic substance. CO2 (carbon dioxide) functions as an acidic substance. Therefore, Increases in HCO3 (bicarbonate) or decreases in CO2 will make blood more alkalotic. The opposite is also true where decreases in HCO3 or an increase in CO2 will make blood more acidic. CO2 levels are physiologically regulated by the pulmonary system through respiration, whereas the HCO3 levels are regulated through the renal system with reabsorption rates. Therefore, respiratory alkalosis is a decrease in serum CO2. While it is theoretically possible to have decreased CO2 production, in every scenario this illness is a result of hyperventilation where CO2 is breathed away.
Respiratory alkalosis is the most common acid-base abnormality with no discrimination between genders. The exact frequency and distribution of disease are dependent upon the etiology. Likewise, the morbidity and mortality rates are dependent on the etiology of the disease.
In almost every scenario, respiratory alkalosis is induced by a process involving hyperventilation. These include central causes, hypoxemic causes, pulmonary causes, and iatrogenic causes. Central sources are a head injury, stroke, hyperthyroidism, anxiety-hyperventilation, pain, fear, stress, drugs, medications such as salicylates, and various toxins. Hypoxic stimulation leads to hyperventilation in an attempt to correct hypoxia at the expense of a CO2 loss. Pulmonary causes include pulmonary embolisms, pneumothorax, pneumonia, and acute asthma or COPD exacerbations. Iatrogenic causes are primarily due to hyperventilation in intubated patients on mechanical ventilation.
Respiratory alkalosis may be an acute process or a chronic process. These are determined based on the level of metabolic compensation for the respiratory disease. Excess HCO3 levels are buffered to reduce levels and maintain a physiological pH through the renal decrease of H secretion and increasing HCO3 secretion; however, this metabolic process occurs over the course of days whereas respiratory disease can adjust CO2 levels in minutes to hours. Therefore, acute respiratory alkalosis is associated with high bicarbonate levels since there has not been sufficient time to lower the HCO3 levels and chronic respiratory alkalosis is associated with low to normal HCO3 levels.
Since the primary cause of all respiratory alkalosis etiologies is hyperventilation, many patients present with complain to shortness of breath. The exact history and physical exam findings are highly variable as there are many pathologies that induce the pH disturbance. These may include acute onset dyspnea, fever, chills, peripheral edema, orthopnea, weakness, confusion, light-headedness, dizziness, anxiety, chest pain, wheezing, hemoptysis, trauma, history of central line catheter, recent surgery, history of thromboembolic disease, history of asthma, history of COPD, acute focal neurological signs, numbness, paresthesia, abdominal pain, nausea, vomiting, tinnitus, or weight loss.
Physical exam findings may be just as varied depending on etiology to include fever, tachycardia, tachypnea, diaphoresis, hyper or hypotension, altered mental status, productive or non-productive cough, wheezing, rales, crackles, cardiac murmur or arrhythmia, jugular venous distension, meningeal signs, focal neurological loss, Trousseau sign, Chvostek sign, jaundice, melena, hematochezia, hepatosplenomegaly, or there may be no definitive signs at all.
With a wide preliminary differential diagnosis list, evaluation should always begin with a thorough history and physical exam to focus diagnostic considerations. In all cases, arterial blood gas is necessary to diagnose the pH imbalance. Serum electrolytes should be measured with particular attention to sodium, potassium, and calcium levels as aberrations in these may lead to further complication. Magnesium and phosphate are also essential to measure. In hypoxic patients, it is important to calculate the A-a gradient to determine the etiology and further diagnosis. If the A-a gradient is wide, be suspicious of pulmonary embolism and appropriately work up the patient. A chest x-ray is important in all patients as it helps discern an anatomical or infectious cause and may rule in/out pulmonary edema. If there is a clinical reason for it, chest CT can play a vital role in achieving a diagnosis. If there is appropriate clinical suspicion for neurological insult, CT or MRI of the head may be appropriate along with lumbar puncture for WBC, glucose, and protein analysis.
Treatment of metabolic alkalosis is targeted at treating the underlying pathology. In anxious patients, anxiolytics may be necessary. In infectious disease, antibiotics targeting sputum or blood cultures are appropriate. In embolic disease, anticoagulation is necessary. Ventilator support may be necessary for patients with acute respiratory failure, acute asthma, or acute, chronic obstructive pulmonary disease (COPD) exacerbation if they show signs of respiratory fatigue. In ventilator controlled patients, it may be necessary to reevaluate their ventilator settings to reduce respiratory rate. If hyperventilation is intentional, monitor the arterial or venous blood gas values closely. In severe cases, pH may be directly reduced using acidic agents. However, this is not routinely done.
Other possibilities include:
Respiratory alkalosis in itself is not a life-threatening diagnosis. However, the prognosis is variable depending on etiology.
Respiratory alkalosis is a pathology that is secondary to hyperventilation.
Hyperventilation typically occurs in response to an insult such as hypoxia, metabolic acidosis, pain, anxiety, or increased metabolic demand.
Respiratory alkalosis in itself is not life-threatening; however, the underlying etiology may be. Always look for and treat the source of the illness. Interventions to reduce pH directly are typically not necessary as there is no mortality benefit to this therapy.