Mixed metabolic and respiratory acid-base disturbances: diagnosis and treatment

Clinical approach to patients with acid-base disorders

Simple relationships that hold during compensation for metabolic or respiratory acid-base disturbances have been emphasized. When considered by the physician, they should allow for the interpretation and diagnosis of both simple and mixed disturbances. Since the correct diagnosis of an acid-base disturbance is central to initiation of correct therapy, these considerations are not merely academic exercises but are essential steps to follow in careful management. Table 6 summarizes the stepwise approach to diagnosis of an acid-base disorder. Application of these principles in a routine fashion combined with a careful history and physical examination often results in the successful diagnosis of complicated disturbances. Understanding of the pathophysiologic derangements in each specific disorder is necessary for full appreciation of acid-base balance in disease.

Compensatory hypoventilation in metabolic alkalosis

Although hyperventilation is a well-known compensatory mechanism in metabolic acidosis, compensatory hypoventilation has been inconsistent and controversial in metabolic alkalosis. Six healthy subjects were studied under baseline conditions and during steady-state metabolic acidosis (seven episodes) and alkalosis (14 episodes). Minute ventilation (VE) fell in metabolic alkalosis and rose in metabolic acidosis. These changes in ventilation were entirely due to reduction and elevation of tidal volume (VT) respectively, while respiratory frequency (f) remained unchanged. Alveolar ventilation fell during metabolic alkalosis and resulted in elevation of arterial PCO2 in all subjects. The ventilatory response to arterial PCO2 in all subjects. The ventilatory response to CO2 breathing was also diminished. There was a linear relationship between PaCO2 and plasma [HCO-3] in metabolic acidosis and alkalosis which was defined as PaCO2 (mm Hg = 0.7 [HCO-a] + 20 (+/- SEM), r = 0.95. Although arterial PO2 and plasma [K+] fell during metabolic alkalosis, minute ventilation did not change upon breathing oxygen and there was no correlation between changes in plasma [K+] and plasma H+ regulation.

Mixed acid base disturbances: a clinical approach

The analysis of a mixed acid-base disturbance begins with the history and physical examination from which data can be derived that make the clinician suspect a specific disturbance(s). The electrolytes are then evaluated with emphasis on the meaning of the values for serum bicarbonate, potassium and chloride concentration and on the level of the anion gap. Other laboratory data such as serum creatinine or glucose concentrations, blood cultures, and so forth, should also be reviewed for further clues to a possible disturbance(s). When it is clinically indicated, values for pH and Pco2 are obtained by blood gas determination. If the evidence up to this point indicates the presence of at least one disturbance, the data are examined to see if compensation for this disturbance is appropriate. If not, a mixed disturbance must be present. A normal pH in the setting of an abnormal serum HCO3(-) concentration or Pco2 also suggests a mixed disturbance since compensation rarely corrects the pH back to normal. Of course, a pH deviated in the opposite direction than that expected for a known primary disturbance makes the diagnosis of a mixed disturbance certain. The diagnosis of a mixed acid-base disturbance is therefore based on an analysis of all the clinical data and not just the blood gas measurements. Treatment of the disorders should be directed at maintaining a normal or near normal pH. Some combined acid-base disorders are important to recognize because they can result in a severe deviation in blood pH that demands immediate, specific therapy. Other mixed disturbances result in a pH which is near normal but are important to recognize since they can alert the clinician to the possibility of certain clinical derangements such as septic shock or drug ingestion. Careful analysis of mixed acid-base disturbances in this way is not peutic information to be used in caring for his (her) patients.

Respiratory failure

Patients with respiratory failure should be approached in a systematic way, with emphasis both in diagnosis and treatment on arterial blood gases. The intelligent assessment of oxygenation, ventilation and acid-base balance, based on physiologic principles, can make the management of these patients very rewarding. The physiologic principles outlined here should be well understood by anyone entrusted with the care of these patients. They provide the cornerstone of diagnosis and management, and will remain valid long after current clinical dogma has been revised.

Effects of sodium bicarbonate administration during cardiopulmonary resuscitation

To study whether sodium bicarbonate given in cardiopulmonary resuscitation may produce life-threatening hyperosmolality or hypernatremia, arterial blood was analysed for blood gas, alcohol, blood urea nitrogen, electrolyte and osmolality. The blood was drawn after resuscitation in successful cases, and while effective massage and ventilation were being applied in unsuccessful resuscitations. Seven of the 17 resuscitations were successful. Serum sodium concentrations ranged from 135 to 154 with one exception and did not correlate with the amount of sodium bicarbonate administered. Arterial pH ranged from 6.38 to 7.71; only one patient had metabolic alkalosis. Serum osmolality ranged from 301 to 407. The data suggests a net increase in osmolality of 6 mOsm/50 mEq of sodium bicarbonate.