Question

How do you assess an eldely patient with acid-base imbalances

Answer 1

Like temperature, blood pressure, osmolality and many other physiological parameters, the human body strives to keep its acid-base balance within tightly controlled limits. The accurate interpretation of laboratory tests in patients with acid-base disorders is critical for understanding pathophysiology, making a diagnosis, planning effective treatment and monitoring progress. This is an important topic particularly for junior medical staff who may encounter acid-base problems outside normal working hours when patients become acutely unwell. These clinical situations may be a source of confusion particularly because of the variety of terms used to describe and classify acid-base disorders.

When it comes to acids and bases, the difference between life and death is balance. The body’s acid-base balance depends on some delicately balanced chemical reactions. The hydrogen ion (H+) affects pH, and pH regulation influences the speed of cellular reactions, cell function, cell permeability, and the very integrity of cell structure.

When an imbalance develops, you can detect it quickly by knowing how to assess your patient and interpret arterial blood gas (ABG) values. And you can restore the balance by targeting your interventions to the specific acid-base disorder you find.

Basics of acid-base balance

Before assessing a patient’s acid-base balance, you need to understand how the H+ affects acids, bases, and pH.

•     An acid is a substance that can donate H+ to a base. Examples include hydrochloric acid, nitric acid, ammonium ion, lactic acid, acetic acid, and carbonic acid (H2CO3).
•     A base is a substance that can accept or bind H+. Examples include ammonia, lactate, acetate, and bicarbonate (HCO3-).
•     pH reflects the overall H+ concentration in body fluids. The higher the number of H+ in the blood, the lower the pH; and the lower the number of H+, the higher the pH.

A solution containing more base than acid has fewer H+ and a higher pH. A solution containing more acid than base has more H+ and a lower pH. The pH of water (H2O), 7.4, is considered neutral.

The pH of blood is slightly alkaline and has a normal range of 7.35 to 7.45. For normal enzyme and cell function and normal metabolism, the blood’s pH must remain in this narrow range. If the blood is acidic, the force of cardiac contractions diminishes. If the blood is alkaline, neuromuscular function becomes impaired. A blood pH below 6.8 or above 7.8 is usually fatal.

pH also reflects the balance between the percentage of H+ and the percentage of HCO3-. Generally, pH is maintained at a ratio of 20 parts HCO3– to 1 part H2CO3.

Regulating acid-base balance

Three regulating systems maintain the body’s pH: chemical buffers, the respiratory system, and the renal system.

Chemical buffers, substances that combine with excess acids or bases, act immediately to maintain pH and are the body’s most efficient pH-balancing force. These buffers appear in blood, intracellular fluid, and extracellular fluid. The main chemical buffers are bicarbonate, phosphate, and protein.

The second line of defense against acid-base imbalances is the respiratory system. The lungs regulate carbon dioxide (CO2) in the blood, which combines with H2O to form H2CO3. Chemoreceptors in the brain sense pH changes and vary the rate and depth of respirations to regulate CO2 levels. Faster, deeper breathing eliminates CO2 from the lungs, and less H2CO3 is formed, so pH rises. Alternatively, slower, shallower breathing reduces CO2 excretion, so pH falls.

The partial pressure of arterial CO2 (Paco2) level reflects the level of CO2 in the blood. Normal Paco2 is 35 to 45 mm Hg. A higher CO2 level indicates hypoventilation from shallow breathing. A lower Paco2 level indicates hyperventilation. The respiratory system, which can handle twice as many acids and bases as the buffer systems, responds in minutes, but compensation is temporary. Long-term adjustments require the renal system.

The renal system maintains acid-base balance by absorbing or excreting acids and bases. Also, the kidneys can produce HCO3– to replenish lost supplies. The normal HCO3– level is 22 to 26 mEq/L. When blood is acidic, the kidneys reabsorb HCO3– and excrete H+. When blood is alkaline, the kidneys excrete HCO3– and retain H+. Unlike the lungs, the kidneys may take 24 hours before starting to restore normal pH.

Compensating for imbalances

The two disorders of acid-base balance are acidosis and alkalosis. In acidosis, the blood has too much acid (or too little base). In alkalosis, the blood has too much base (or too little acid). The cause of these acid-base disorders is either respiratory or metabolic. If the respiratory system is responsible, you’ll detect it by reviewing Paco2 or serum CO2 levels. If the metabolic system is responsible, you’ll detect it by reviewing serum HCO3– levels.

To regain acid-base balance, the lungs may respond to a metabolic disorder, and the kidneys may respond to a respiratory disorder. If pH remains abnormal, the respiratory or metabolic response is called partial compensation. If the pH returns to normal, the response is called complete compensation. Keep in mind that the respiratory or renal system will never overcompensate. A compensatory mechanism won’t make an acidotic patient alkalotic or an alkalotic patient acidotic.

Understanding acidosis and alkalosis

Caused by hypoventilation, respiratory acidosis develops when the lungs don’t adequately eliminate CO2. The hypoventilation may result from diseases that severely affect the lungs, diseases of the nerves and muscles of the chest that impair the mechanics of breathing, or drugs that slow a patient’s respirations. Respiratory acidosis causes a pH below 7.35 and a Paco2 above 45 mm Hg. HCO3– is normal. (See Causes of acid-base imbalances at a glance by clicking the PDF icon above.)

Caused by hyperventilation, respiratory alkalosis develops when the lungs eliminate too much CO2. The most common cause of hyperventilation is anxiety. Respiratory alkalosis causes a pH above 7.45 and a Paco2 below 35 mm Hg. HCO3– is normal.

Metabolic acidosis may result from:

•     ingestion of an acidic substance or a substance that can be metabolized to an acid
•     production of excess acid
•     an inability of the kidneys to excrete normal amounts of acid
•     a loss of base.

Metabolic acidosis causes a HCO3– below 22 mEq/L and a pH below 7.35. Paco2 is normal.

Metabolic alkalosis may result from:

•     loss of stomach acid
•     an excess loss of sodium or potassium
•     a renal loss of H+
•     a gain of base.

Metabolic alkalosis causes a HCO3– above 26 mEq/L and a pH above 7.45. Paco2 is normal.

Treating acid-base imbalances

Treatment for metabolic acidosis focuses on correcting the underlying cause. For a diabetic patient, treatment consists of controlling blood glucose and insulin levels. In a case of poisoning, treatment focuses on eliminating the toxin from the blood. Correcting the underlying cause of sepsis may include antibiotic therapy, fluid administration, and surgery. You may also treat the acidosis directly. If it’s mild, administering I.V. fluid may correct the problem. If acidosis is severe, you may give bicarbonate I.V., as prescribed.

Treatment for metabolic alkalosis also focuses on the underlying cause. Frequently, an electrolyte imbalance causes this disorder, so treatment consists of replacing fluid, sodium, and potassium.

The treatment goal for respiratory acidosis is to improve ventilation. Expect to administer drugs such as bronchodilators to improve breathing and, in severe cases, to use mechanical ventilation. Maintain good pulmonary hygiene.

Usually, the only treatment goal for respiratory alkalosis is to slow the breathing rate. If anxiety is the cause, encourage the patient to slow his or her breathing. Some patients may need an anxiolytic. If pain is causing rapid, shallow breathing, provide pain relief. Breathing into a paper bag allows a patient to rebreathe CO2, raising the level of CO2 in the blood.