Elise Anderson is a 6-year-old girl who presents to the emergency department with a history of vomiting for the past 3 days. The nurse notices that her respiratory rate is abnormally low. What is the likely reason for this change in ventilation? Predict what Elise's arterial blood gas values would show.

Verified step by step guidance

1. Understand the physiological context: Vomiting for 3 days can lead to a significant loss of stomach acid (hydrochloric acid, HCl), which contains hydrogen ions (H⁺). This loss can result in a condition called metabolic alkalosis, where the blood becomes more alkaline (higher pH).
2. Relate metabolic alkalosis to respiratory compensation: The body attempts to compensate for metabolic alkalosis by reducing the respiratory rate. This hypoventilation leads to an increase in carbon dioxide (CO₂) levels in the blood, which helps lower the pH back toward normal by forming more carbonic acid (H₂CO₃).
3. Predict arterial blood gas (ABG) values: In metabolic alkalosis, you would expect the following changes: (1) Elevated blood pH (greater than 7.45), (2) Decreased hydrogen ion concentration ([H⁺]), (3) Elevated bicarbonate (HCO₃⁻) levels due to the alkalosis, and (4) Elevated partial pressure of carbon dioxide (PaCO₂) as a result of hypoventilation.
4. Explain the link between low respiratory rate and ABG values: The low respiratory rate observed in Elise is the body's compensatory mechanism to retain CO₂. This retention increases PaCO₂, which helps counteract the alkalosis by shifting the bicarbonate buffer system equilibrium toward producing more H⁺ ions: \( \text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3 \leftrightarrow \text{H}^+ + \text{HCO}_3^- \).
5. Summarize the likely findings: Elise's ABG values would likely show a high pH (alkalosis), high bicarbonate (HCO₃⁻), and high PaCO₂ due to respiratory compensation. These findings are consistent with metabolic alkalosis and the body's attempt to restore acid-base balance.

Key Concepts

Respiratory Rate and Ventilation

Respiratory rate refers to the number of breaths taken per minute and is a critical indicator of respiratory function. In children, a low respiratory rate can indicate hypoventilation, where the body is not expelling enough carbon dioxide (CO2). This can lead to respiratory acidosis, where the blood becomes more acidic due to increased CO2 levels.

Arterial Blood Gas (ABG) Analysis

Arterial blood gas analysis measures the levels of oxygen (O2), carbon dioxide (CO2), and the pH of the blood. It helps assess respiratory and metabolic function. In cases of hypoventilation, ABG values typically show elevated CO2 (hypercapnia) and decreased pH (acidosis), indicating respiratory failure or distress.

Causes of Vomiting and Its Effects

Vomiting can lead to dehydration and electrolyte imbalances, which may affect respiratory function. In children, prolonged vomiting can result in metabolic alkalosis due to loss of gastric acid. This condition can further complicate the respiratory status, potentially leading to changes in ventilation patterns and respiratory rate.