Porth's Essentials of Pathophysiology, 4e

202

Integrative Body Functions

U N I T 2

hyperventilation syndrome, which is characterized by recurring episodes of overbreathing often associ- ated with anxiety. Persons experiencing panic attacks frequently present in the emergency room with mani- festations of acute respiratory alkalosis. Progesterone increases ventilation in women; during the progesterone phase of the menstrual cycle, normal women decrease their PCO 2 values by 2 to 4 mm Hg and increase their pH by 0.01 to 0.02. 74 Women can also develop sub- stantial decrease in PCO 2 during pregnancy, most nota- bly during the last trimester. 72 Respiratory alkalosis has long been recognized as a common acid–base disorder in critically ill patients, and is a consistent finding in both septic shock and the systemic inflammatory response syndrome (see Chapter 20). Hypoxemia exerts its effect on pH through the peripheral chemoreceptors in the carotid bodies. Stimulation of peripheral chemoreceptors occurs in conditions that cause hypoxemia with rela- tively unimpaired CO 2 transport such as exposure to high altitudes. Mechanical ventilation may produce respiratory alkalosis if the rate and tidal volume are set so that CO 2 elimination exceeds CO 2 production. Because car- bon dioxide crosses the alveolar capillary membrane 20 times more rapidly than oxygen, the increased min- ute ventilation may be necessary to maintain adequate oxygen levels while producing a concomitant decrease in CO 2 levels. In some cases, respiratory alkalosis may be induced through mechanical ventilation as a means of controlling disorders such as severe intracranial hypertension. Manifestations. Respiratory alkalosis manifests with a decrease in PCO 2 and a H 2 CO 3 deficit. The pH is above 7.45, arterial PCO 2 is below 35 mm Hg, and serum HCO 3 – levels usually are below 24 mEq/L (24 mmol/L). The signs and symptoms of respiratory alkalosis are associated with hyperexcitability of the nervous sys- tem and a decrease in cerebral blood flow 75 (see Table 8-11). A decrease in the CO 2 content of the blood causes constriction of cerebral blood vessels. CO 2 crosses the blood–brain barrier rather quickly; thus, the manifesta- tions of acute respiratory alkalosis are usually of sudden onset. The person often experiences light-headedness, dizziness, tingling, and numbness of the fingers and toes. These manifestations may be accompanied by sweating, palpitations, panic, air hunger, and dyspnea. Chvostek and Trousseau signs may be positive, and tetany and convulsions may occur. Because CO 2 provides the stimu- lus for short-term regulation of respiration, short peri- ods of apnea may occur in persons with acute episodes of hyperventilation. Treatment. The treatment of respiratory alkalosis focuses on measures to correct the underlying cause. Hypoxia may be corrected by administration of supplemental oxy- gen. Changing ventilator settings may be used to prevent or treat respiratory alkalosis in persons who are being mechanically ventilated. Persons with hyperventilation

syndrome may benefit from reassurance, rebreathing from a paper bag during symptomatic attacks, and attention to the psychological stress associated with the disorder.

SUMMARY CONCEPTS

■■ Normal body function depends on the precise regulation of acid–base balance. Metabolic processes produce the volatile carbonic acid (H 2 CO 3 ) in equilibrium with dissolved carbon dioxide (PCO 2 ), which is eliminated through the lungs, and nonvolatile acids, which are excreted by the kidneys. ■■ Because of its low concentration in body fluids, the hydrogen (H + ) concentration is expressed as pH, or the negative log of the H + ion concentration. It is the ratio of the bicarbonate (HCO 3 – ) concentration to H 2 CO 3 (PCO 2 ), normally 20:1, that determines body pH. ■■ The ability of the body to maintain pH within the normal range depends on intracellular and extracellular buffers, as well as respiratory and renal compensatory mechanisms. The respiratory regulation of pH, which relies on pulmonary ventilation for release of CO 2 into the environment, is rapid but does not return the pH completely to normal. Renal mechanisms, which rely on the elimination of H + ions and conservation of HCO 3 – ions, take longer but return pH to normal or near-normal levels. ■■ Metabolic acid and base disorders reflect an decrease or increase in HCO 3 – . Metabolic acidosis, which reflects a decrease in pH due to a decrease in HCO 3 – , is caused by conditions that prompt an excessive production and accumulation of metabolic acids or excessive loss of HCO 3 – . Metabolic alkalosis, which reflects an increase in pH due to an increase in HCO 3 – , is caused by conditions that produce a gain in HCO 3 – or a decrease in H + . ■■ Respiratory acid–base disorders reflect an increase or decrease in PCO 2 levels due to altered pulmonary ventilation. Respiratory acidosis, which reflects a decrease in pH due an increase in PCO 2 levels, is caused by conditions that produce hypoventilation. Respiratory alkalosis, which reflects an increase in pH due to a decrease in PCO 2 levels, is caused by conditions that produce hyperventilation.