Memory Tip
For respiratory disorders, pH and PaCO₂ move in opposite directions. In respiratory alkalosis, pH goes up while PaCO₂ goes down.
Respiratory alkalosis occurs when ventilation is excessive and carbon dioxide is blown off faster than the body produces it. For respiratory therapy students, the key ABG pattern is a high pH with a low PaCO₂.
Respiratory alkalosis is an acid-base disorder caused by decreased carbon dioxide in the blood. Since carbon dioxide acts as an acid in the blood, removing too much CO₂ makes the blood more alkaline.
The classic respiratory alkalosis pattern is a pH above 7.45 with a PaCO₂ below 35 mmHg.
| Value | Finding | Meaning |
|---|---|---|
| pH 7.52 | High | Alkalemia is present. |
| PaCO₂ 28 | Low | The respiratory system is causing alkalosis by removing too much CO₂. |
| HCO₃ 24 | Normal | No metabolic compensation is present in this example. |
| PaO₂ 86 | Normal | Oxygenation is normal in this example. |
For respiratory disorders, pH and PaCO₂ move in opposite directions. In respiratory alkalosis, pH goes up while PaCO₂ goes down.
Respiratory alkalosis usually means the patient is breathing faster or deeper than needed for current CO₂ production. The cause may be respiratory, neurologic, metabolic, medication-related, or mechanical.
| Cause | Why PaCO₂ Falls | RT Consideration |
|---|---|---|
| Anxiety or panic | Hyperventilation blows off CO₂. | Assess severity and rule out hypoxemia or other causes before assuming anxiety. |
| Pain | Pain can increase respiratory rate and depth. | Treat the underlying trigger while monitoring ventilation. |
| Hypoxemia | Low oxygen stimulates increased ventilation. | Correct oxygenation and identify the reason for low PaO₂ or SpO₂. |
| Pulmonary embolism | Dead space and hypoxemia may trigger tachypnea. | Sudden dyspnea, chest pain, and low PaO₂ should raise concern. |
| Sepsis or fever | Increased metabolic demand can drive tachypnea. | Look for infection signs and worsening clinical status. |
| Excessive mechanical ventilation | Set minute ventilation may be too high. | Evaluate respiratory rate, tidal volume, minute ventilation, and ABG trend. |
Respiratory alkalosis is not diagnosed by symptoms alone, but clinical presentation helps explain why the patient is hyperventilating.
The kidneys compensate for chronic respiratory alkalosis by lowering bicarbonate. Compensation takes time, so acute respiratory alkalosis may show a normal HCO₃.
| Pattern | ABG Example | Interpretation |
|---|---|---|
| No compensation | pH 7.52 / PaCO₂ 28 / HCO₃ 24 | Uncompensated respiratory alkalosis. |
| Partial compensation | pH 7.48 / PaCO₂ 28 / HCO₃ 20 | Partially compensated respiratory alkalosis. |
| Full compensation | pH 7.41 / PaCO₂ 30 / HCO₃ 19 | Fully compensated respiratory alkalosis. |
If the pH is still alkalotic and HCO₃ has moved downward, compensation is partial. If pH has returned to normal while PaCO₂ and HCO₃ are both abnormal, compensation is complete.
The RT priority is not simply to label the ABG. The priority is to identify why the patient is hyperventilating and whether oxygenation, ventilation, or ventilator settings need intervention.
| Scenario | ABG | Likely Interpretation |
|---|---|---|
| An anxious patient breathing rapidly after a procedure | pH 7.51 / PaCO₂ 29 / HCO₃ 23 / PaO₂ 92 | Uncompensated respiratory alkalosis with normal oxygenation. |
| Patient with suspected pulmonary embolism and sudden dyspnea | pH 7.49 / PaCO₂ 30 / HCO₃ 22 / PaO₂ 58 | Uncompensated respiratory alkalosis with moderate hypoxemia. |
| Ventilated patient with excessive minute ventilation | pH 7.47 / PaCO₂ 27 / HCO₃ 19 / PaO₂ 98 | Partially compensated respiratory alkalosis with normal oxygenation. |