What should the nurse expect when assessing a patient with respiratory alkalosis

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As with electrolytes, correct balance of acids and bases in the body is essential to proper body functioning. Even a slight variance outside of normal can be life-threatening, so it is important to understand normal acid-base values, as well their causes and how to correct them. The kidneys and lungs work together to correct slight imbalances as they occur. As a result, the kidneys compensate for shortcomings of the lungs, and the lungs compensate for shortcomings of the kidneys.

Arterial Blood Gases

Arterial blood gases [ABG] are measured by collecting blood from an artery, rather than a vein, and are most commonly collected via the radial artery. ABGs measure the pH level of the blood, the partial pressure of arterial oxygen [PaO2], the partial pressure of arterial carbon dioxide [PaCO2], the bicarbonate level [HCO3], and the oxygen saturation level [SaO2].

pH

pH is a scale from 0-14 used to determine the acidity or alkalinity of a substance. A neutral pH is 7, which is the same pH as water. Normally, the blood has a pH between 7.35 and 7.45. A blood pH of less than 7.35 is considered acidic, and a blood pH of more than 7.45 is considered alkaline.

The pH of blood is a measure of hydrogen ion concentration. A low pH, less than 7.35, occurs in acidosis when the blood has a high hydrogen ion concentration. A high pH, greater than 7.45, occurs in alkalosis when the blood has a low hydrogen ion concentration. Hydrogen ions are by-products of the metabolism of substances such as proteins, fats, and carbohydrates. These by-products create extra hydrogen ions [H+] in the blood that need to be balanced and kept within normal range as described earlier.

The body has several mechanisms for maintaining blood pH. The lungs are essential for maintaining pH and the kidneys also play a role. For example, when the pH is too low [i.e., during acidosis], the respiratory rate quickly increases to eliminate acid in the form of carbon dioxide [CO2]. The kidneys excrete additional hydrogen ions [acid] in the urine and retain bicarbonate [base]. Conversely, when the pH is too high [i.e., during alkalosis], the respiratory rate decreases to retain acid in the form of CO2. The kidneys excrete bicarbonate [base] in the urine and retain hydrogen ions [acid].

PaCO2

PaCO2 is the partial pressure of arterial carbon dioxide in the blood. The normal PaCO2 level is 35-45 mmHg. CO2 forms an acid in the blood that is regulated by the lungs by changing the rate or depth of respirations.

As the respiratory rate increases or becomes deeper, additional CO2 is removed causing decreased acid [H+] levels in the blood and increased pH [so the blood becomes more alkaline]. As the respiratory rate decreases or becomes more shallow, less CO2 is removed causing increased acid [H+] levels in the blood and decreased pH [so the blood becomes more acidic].

Generally, the lungs work quickly to regulate the PaCO2 levels and cause a quick change in the pH. Therefore, an acid-base problem caused by hypoventilation can be quickly corrected by increasing ventilation, and a problem caused by hyperventilation can be quickly corrected by decreasing ventilation. For example, if an anxious patient is hyperventilating, they may be asked to breathe into a paper bag to rebreathe some of the CO2 they are blowing off. Conversely, a postoperative patient who is experiencing hypoventilation due to the sedative effects of receiving morphine is asked to cough and deep breathe to blow off more CO2.

HCO3

HCO3 is the bicarbonate level of the blood and the normal range is 22-26. HCO3 is a base managed by the kidneys and helps to make the blood more alkaline. The kidneys take longer than the lungs to adjust the acidity or alkalinity of the blood, and the response is not visible upon assessment. As the kidneys sense an alteration in pH, they begin to retain or excrete HCO3, depending on what is needed. If the pH becomes acidic, the kidneys retain HCO3 to increase the amount of bases present in the blood to increase the pH. Conversely, if the pH becomes alkalotic, the kidneys excrete more HCO3, causing the pH to decrease.

PaO2

PaO2 is the partial pressure of arterial oxygen in the blood. It more accurately measures a patient’s oxygenation status than SaO2 [the measurement of hemoglobin saturation with oxygen]. Therefore, ABG results are also used to manage patients in respiratory distress.

Read more information about interpreting ABG results in the “Oxygen Therapy” chapter in Open RN Nursing Skills.

See Table 15.5a for a review of ABG components, normal values, and key critical values. A critical ABG value means there is a greater risk of serious complications and even death if not corrected rapidly. For example, a pH of 7.10, a shift of only 0.25 below normal, is often fatal because this level of acidosis can cause cardiac or respiratory arrest or significant hyperkalemia.[1] As you can see, failure to recognize ABG abnormalities can have serious consequences for your patients.

Table 15.5a ABG Components, Descriptions, Adult Normal Values, and Critical Values[2]