Mechanism of Tetany in Tachypnea
Tachypnea causes tetany through respiratory alkalosis, which lowers blood CO2 and elevates pH, making nerves and muscles hyperexcitable by reducing ionized calcium availability and directly increasing neuromuscular excitability. 1
Primary Pathophysiological Mechanism
The core mechanism operates through two interconnected pathways:
- Respiratory alkalosis from hyperventilation eliminates more CO2 than is metabolically produced, driving PaCO2 below the normal range of 34-46 mmHg and elevating blood pH 1, 2
- Increased neuromuscular excitability results from this alkalotic state, making nerve cells hyperexcitable and prone to spontaneous firing 1, 3
- Reduced ionized calcium occurs because alkalosis alters the protein-binding of calcium—as pH rises, more calcium binds to albumin, decreasing the physiologically active ionized fraction even when total calcium is normal 4, 5
The Calcium-Alkalosis Relationship
The quantitative relationship between calcium and alkalosis is critical:
- Hypocalcemia and alkalosis are coparticipants in tetany development—either condition alone produces tetany in less than 50% of cases, but their combination always produces tetany 5
- Normocalcemic tetany can occur purely from respiratory alkalosis when hyperventilation is severe enough, as the alkalotic pH shift reduces ionized calcium sufficiently to trigger symptoms 4, 5
- The mechanism is independent of cerebrospinal fluid calcium concentration and operates through peripheral neuromuscular effects 5
Clinical Context and Common Triggers
Understanding the clinical scenarios helps identify the mechanism:
- Anxiety and panic disorders are the most common triggers for hyperventilation syndrome, producing impressive hyperventilation with increased respiratory frequency and respiratory alkalosis 6, 2
- Compensatory hyperventilation in metabolic acidosis can paradoxically cause tetany if ventilation becomes excessive 6
- Iatrogenic causes include excessive mechanical ventilation settings in critical care 6
Manifestations of Neuromuscular Hyperexcitability
The hyperexcitable state manifests as:
- Carpopedal spasm (characteristic hand and foot cramping), laryngospasm, and generalized seizures as typical presentations 3, 7
- Paresthesias (circumoral and distal), accompanied by shortness of breath, palpitations, and dizziness 7
- Chvostek and Trousseau signs become positive as provocative tests for latent tetany 3
Critical Clinical Pitfall
Do not confuse hypocapnia (low CO2 from hyperventilation) with hypercapnia (high CO2 from hypoventilation), as they have opposite causes and require opposite management approaches 1. Tachypnea causing tetany involves hypocapnia and alkalosis, not hypercapnia.
Acute Management Principle
- Calcium infusion is effective as acute therapy for tetany by directly restoring ionized calcium levels and reducing neuromuscular excitability 3
- Addressing the underlying hyperventilation is mandatory—for anxiety-driven hyperventilation, breathing retraining and psychological counseling are successful in over 70% of patients 2
- Magnesium infusion may also be effective, though its benefits can be masked by toxic effects at higher doses 8