What condition is indicated by a low partial pressure of carbon dioxide (PCO2), low blood pH (acidosis), and a relatively normal partial pressure of oxygen (PO2) on room air?

Metabolic Acidosis with Respiratory Compensation (Hyperventilation)

The arterial blood gas values presented—PO2 90 torr, PCO2 28 torr, and pH 7.16—are most consistent with metabolic acidosis with compensatory hyperventilation (answer d). The severely low pH (7.16) with a low PCO2 (28 torr) indicates the body is attempting to compensate for a metabolic acidosis by hyperventilating to blow off CO2 and raise the pH. 1

Blood Gas Interpretation

pH Analysis

• The pH of 7.16 represents severe acidosis (normal range 7.35-7.45), indicating significant acid-base disturbance requiring urgent attention 1
• Acidosis is defined as pH <7.35, and this patient's value is markedly below this threshold 1

PCO2 Analysis

• The PCO2 of 28 torr is significantly below the normal range of 34-46 mmHg (4.6-6.1 kPa) 1
• This low PCO2 represents hyperventilation, which is the body's compensatory mechanism to eliminate CO2 and raise pH 2
• In respiratory acidosis (alveolar hypoventilation), PCO2 would be elevated, not decreased—ruling out option (b) 1

PO2 Analysis

• The PO2 of 90 torr is at the lower end of normal but not severely hypoxemic 1
• This relatively preserved oxygenation on room air argues against primary respiratory failure and supports a metabolic process 1

Why This Represents Metabolic Acidosis

Metabolic acidosis is characterized by low pH, low serum bicarbonate, and compensatory hyperventilation causing decreased PCO2 2. The pattern here shows:

• Severe acidosis (pH 7.16) that cannot be explained by the low PCO2 alone
• If this were primary respiratory alkalosis from hyperventilation, the pH would be elevated (>7.45), not profoundly decreased 1, 3
• The hyperventilation (low PCO2) is compensatory, attempting to correct the metabolic acidosis 2

Differential Diagnosis Considerations

Why Not Hypovolemic Shock (a)?

• While hypovolemic shock can cause lactic acidosis with this blood gas pattern, the question asks what the blood gases are "indicative of," and the direct physiological finding is hyperventilation with metabolic acidosis 2
• Hypovolemic shock would be a potential underlying cause, but the blood gas pattern itself indicates metabolic acidosis with respiratory compensation

Why Not Alveolar Hypoventilation (b)?

• Alveolar hypoventilation causes respiratory acidosis with elevated PCO2 (typically >45 mmHg), not decreased PCO2 1
• This patient has the opposite: low PCO2 from hyperventilation 1

Why Not Prolonged Nasogastric Suctioning (c)?

• Prolonged NG suctioning causes metabolic alkalosis (elevated pH) from loss of gastric acid, not acidosis 2
• The pH would be >7.45, not 7.16 1

Why Hyperventilation (d)?

• The PCO2 of 28 torr definitively indicates hyperventilation 1, 3
• Hyperventilation in this context is compensatory for metabolic acidosis 2
• The severely low pH despite hyperventilation indicates the primary problem is metabolic, with hyperventilation as the compensatory response 2

Clinical Implications

The combination of severe acidosis (pH 7.16) with compensatory hyperventilation requires immediate investigation for the underlying metabolic cause, which could include:

• Lactic acidosis from tissue hypoxia (shock, sepsis) 1
• Diabetic ketoacidosis 1
• Renal failure with acid retention 2
• Toxic ingestions 2

The relatively preserved PO2 on room air suggests adequate gas exchange at the alveolar level, making primary pulmonary pathology less likely as the cause 1.