What is the cause of tetany in a patient with hyperventilation (respiratory alkalosis)?

From the Guidelines

Tetany in a hyperventilating patient is primarily caused by respiratory alkalosis, which leads to decreased ionized calcium levels due to increased binding of calcium to albumin as blood pH rises 1. When a patient hyperventilates, they blow off excess carbon dioxide, leading to an increase in blood pH. This increase in pH causes calcium to bind more strongly to albumin, reducing the amount of free calcium available for neuromuscular function. The symptoms of tetany include carpopedal spasms, perioral numbness, tingling in extremities, and in severe cases, muscle cramps or laryngospasm.

To manage this condition, the primary intervention is to correct the hyperventilation.

• The patient should be instructed to breathe slowly and deeply, preferably through pursed lips, to help increase carbon dioxide levels and normalize pH.
• It is crucial to address the underlying cause of hyperventilation, whether it's anxiety, pain, or other medical conditions, to prevent recurrence of tetany symptoms.
• Anxiolytic medications may be considered if anxiety is driving the hyperventilation, as suggested by good practice points regarding patients with suspected hyperventilation 1.
• Notably, rebreathing from a paper bag is not advised as a treatment for hyperventilation due to potential dangers, as highlighted in the BTS guideline for oxygen use in adults in healthcare and emergency settings 1.
• In severe cases with prolonged symptoms, calcium supplementation might be necessary, though this is rarely required as symptoms typically resolve once breathing normalizes.

Key considerations in managing tetany due to hyperventilation include:

• Monitoring oxygen saturation in patients with a definite diagnosis of hyperventilation, as those with normal or high SpO2 do not require oxygen therapy 1.
• Ensuring organic illness is excluded before making a diagnosis of hyperventilation, to rule out other potential causes of symptoms 1.

From the Research

Causes of Tetany in Hyperventilation

The cause of tetany in a patient with hyperventilation (respiratory alkalosis) can be attributed to several factors, including:

• Electrolyte disturbances, such as hypocalcaemia, hypomagnesemia, and hypokalaemia 2
• Alkalosis resulting from hyperventilation, which can lead to a temporary reduction in ionized calcium fraction 3
• Enhanced neuromuscular excitability due to hypocalcemia, hypomagnesemia, or alkalosis 4

Mechanism of Tetany in Hyperventilation

The mechanism of tetany in hyperventilation involves:

• A decrease in PCO2 during hyperventilation, leading to respiratory alkalosis 5
• A reduction in ionized calcium fraction, which can cause neuromuscular hyperactivity and lead to symptoms of tetany 3
• Hyperventilatory alkalosis, which can cause numbness and stiffness in the hands and feet, primarily at peripheral neuromuscular junctions 6

Associated Conditions

Tetany in hyperventilation can be associated with various conditions, including:

• Endocrine disorders, such as hypoparathyroidism 4
• Chronic renal failure, which can lead to electrolyte imbalances and exacerbate heart failure 6
• Hyperventilation syndrome, which can cause multiple somatic symptoms induced by inappropriate hyperventilation 5