Dyspneic index is the index between breathing reserve and maximum breathing capacity (MBC). Breathing reserve is the balance (difference) between MBC and respiratory minute volume (RMV).

For example, in a normal subject, MBC is 116 L and RMV is 6 L.


Dyspneic index = × 100 MBC

116 – 6 = × 100


= 94.8%.

Dyspnea develops when the dyspneic index decreases below 60%.


Periodic breathing is the abnormal or uneven respiratory rhythm. It is of two types:

1. Cheyne-Stokes breathing

2. Biot breathing.


Features of Cheyne-Stokes Breathing

Cheyne-Stokes breathing is the periodic breathing characterized by rhythmic hyperpnea and apnea. It is

the most common type of periodic breathing. It is marked by two alternate patterns of respiration:

i. Hyperpneic period

ii. Apneic period.

Hyperpneic period – waxing and waning of breathing To begin with, the breathing is shallow. Force of

respiration increases gradually and reaches the maximum (hyperpnea). Then, it decreases gradually

and reaches minimum and is followed by apnea. Gradual increase followed by gradual decrease in force

of respiration is called waxing and waning of breathing.

Apneic period

When, the force of breathing is reduced to minimum, cessation of breathing occurs for a short period. It is

again followed by hyperpneic period and the cycle is repeated. Duration of one cycle is about 1 minute.

Sometimes, waxing and waning of breathing occurs without apnea.

Causes for Waxing and Waning

Initially, during forced breathing, large quantity of carbon dioxide is washed out from blood. When partial

pressure of carbon dioxide decreases, respiratory centers become inactive. It causes apnea. During

apnea, there is accumulation of carbon dioxide (hypercapnea) and reduction in oxygen tension

(hypoxia). Now, the respiratory centers are activated, resulting in gradual increase in the force of breathing. When the force of breathing reaches maximum, the cycle is repeated.

Conditions when Cheyne-Stokes Breathing Occurs

Cheyne-Stokes breathing occurs in both physiological and pathological conditions.

Physiological conditions when Cheyne-Stokes breathing occurs

i. During deep sleep

ii. In high altitude

iii. After prolonged voluntary hyperventilation

iv. During hibernation in animals

v. In newborn babies

vi. After severe muscular exercise.

Pathological conditions when Cheyne-Stokes breathing occurs

i. During increased intracranial pressure

ii. During advanced cardiac diseases, leading to cardiac failure

iii. During advanced renal diseases, leading to uremia

iv. Poisoning by narcotics

v. In premature infants.


Features of Biot Breathing

Biot breathing is another form of periodic breathing characterized by period of apnea and hyperpnea.

Waxing and waning of breathing do not occur. After apneic period, hyperpnea occurs abruptly.

Causes of Abrupt Apnea and Hyperpnea

Due to apnea, carbon dioxide accumulates and it stimulates the respiratory centers, leading to hyperventilation. During hyperventilation, lot of carbon dioxide is washed out. So, the respiratory centers are not stimulated and apnea occurs.

Conditions when Biot Breathing Occurs

Biot breathing does not occur in physiological conditions. It occurs only in pathological conditions. It occurs in conditions involving nervous disorders due to lesions or injuries to brain.

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