Determination of the sensitivity of the respiratory center to hypercapnia
© Department of Physiology, Fac. Of Med., MU, Anna Pekařová, 2016


Control of ventilation
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•Breathing is an automatic process that takes place unconsciously. Automaticity of breathing comes
from regular (rhythmic) activity of groups of neurons anatomically localized in the medulla and its
vicinity. They can be divided into three main groups:
•
•
•dorsal respiratory group
•ventral respiratory group
•Pontine respiratory group - pneumotaxic center

Chemical factors affecting the respiratory center
Central chemoreceptors
-on the front side of the medulla
-sensitive only to increase of arterial pCO2 (by increasing H+ )
-
-central chemoreceptor are stimulated by other types of acidosis (lactate acidosis, ketoacidosis)
-pCO2 sudden change does not take effect immediately, changes in ventilation through central
chemoreceptors occur after 20-30 s

Peripheral chemoreceptors
– located in the aortic and carotid bodies
-primarily sensitive to decrease in arterial pO2, particularly to decrease of O2 under 10-13 kPa in
the arterial blood.
They convey their sensory information to the medulla via the  vagus nerve and glossopharyngeal
nerve.
http://www.medicine.mcgill.ca/physio/resp-web/sect8.htm,
Chemical factors affecting the respiratory center

Hypoxia, hypoxemia
•Hypoxia is a general name for a lack of oxygen in the body or individual tissues.
•Hypoxemia is lack of oxygen in arterial blood.
•Complete lack of oxygen is known as anoxia.
The most common types of hypoxia:
1.Hypoxic - physiological: stay at higher altitudes, pathological: hypoventilation during lung or
neuromuscular diseases
2.Transport (anemic) - reduced transport capacity of blood for oxygen (anemia, blood loss, CO
poisoning)
3.Ischemic (stagnation) - restricted blood flow to tissue (heart failure, shock states, obstruction
of an artery)
4.Histotoxic - cells are unable to utilize oxygen (cyanide poisoning - damage to the respiratory
chain)
11-16 kPa
vs. ischemie

Hypercapnia
•Increase of concentration of carbon dioxide in the blood or in tissues that is caused by retention
of CO2 in the body
•possible causes: total alveolar hypoventilation - decreased respiration or extension of dead space
•
•mild hypercapnia (5 -7 kPa) causes stimulation of the respiratory center (therapeutic use:
pneumoxid = mixture of oxygen + 2-5% CO2)
•hypercapnia around 10 kPa - CO2 narcosis - respiratory depression (preceded by headache,
confusion, disorientation, a feeling of breathlessness)
•hypercapnia over 12 kPa - significant respiratory depression - coma and death.
5.3-6.65 kPa

Determination of the sensitivity of the respiratory center to hypercapnia
• Objective: to demonstrate the changes in ventilation during induced hypercapnia and to compare
the sensitivity of the respiratory center to hypercapnia in several subjects
•Method: Hypercapnia is induced by re-breathing of air expired into Krogh respirometer with closed
circuit (Krogh respirometer with oxygen, without the soda lime - naturally increases the
concentration CO2)
•Interpretation: The slope of the curve shows the sensitivity of the respiratory center to
hypercapnia (see hypoxia)
•
•Clinical note: the sensitivity of the respiratory center to change of partial pressure of CO2 is
reduced in patients with chronic end-stage lung disease, in patients with heart failure as well as
in subjects training breath holding (e.g. divers without oxygen tanks)
Ks =
ΔCO2exp
CO2exp(%)
ΔCO2 exp