Determination of the sensitivity of
the respiratory center to hypoxia
and to hypercapnia
© Department of Physiology, Fac. Of Med., MU, Anna Pekařová, 2016
Control of ventilation
https://sleep.sharepoint.com/siteimages/Chapter%203.png
• 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 – placed bilaterally on the dorsal side of the
medulla oblongata, only inspiratory neurons, sending axons to
motoneurons of inspiratory muscles (diaphragm, external intercostal
muscles; their activation=inspiration, their relaxation=expiration;
participates on inspiration at rest and forced inspiration
• ventral respiratory group - located on the ventrolateral part of the
medulla oblongata, the upper part: neurons whose axons of motor
neurons activate the main and auxiliary inspiratory muscles; the lower
part: expiratory neurons which innervate expiratory muscles (internal
intercostal muscles). Neurons in this group operate only during forced
inspiration and forced expiration.
• Pontine respiratory group - pneumotaxic center - dorsally placed on top of
the pont, contributes to the frequency and depth of breathing; affects the
activity of respiratory neurons in the medulla oblongata.
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.
Mechanism of action: Decreased
ATP production in mitochondria
leads to depolarization of
receptors membrane and to
excitation of chemoreceptor
http://www.medicine.mcgill.ca/physio/resp-web/sect8.htm,
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
Hypercapnia
• 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
Pulse oxymetry
• Is a photometric non-invasive method of measuring the
oxygen saturation of hemoglobin in the arterial circulation.
• The method is based on evaluation of the absorption of light of
two different wavelengths after passage through the tissue
(finger, ear lobe). Reduced hemoglobin absorbs more light at a
wavelength of 660 nm (red spectrum), oxygenated hemoglobin
at 940 nm (infrared spectrum).
• Thus the pulse oxymeter emits beams of two wavelengths to
the tissue and different absorption ratios are the basis for
calculating the factor by which the final value of hemoglobin
saturation is obtained .
• To achieve the targeted measurement of saturation values only
in arterial blood, the measured values between the individual
pulses from the value at the peak of the pulse (pulse wave) is
further used for reading. Calculated component is then equal
to the absorption of the variable component, which is arterial
blood.
Pathology <90% SpO2
https://cs.wikipedia.org/wiki/Pulsn%C3%AD_oxymetrie#/media/File:Absorp%C4%8Dn%C3%AD_spektrum_hemoglobinu.png, http://www.mr-diagnostic.cz/nonin-onyx-vantage-9590
I. Determination of the sensitivity of the
respiratory center to hypoxia
• Objective is to demonstrate changes in ventilation during induction of
hypoxia and to compare the sensitivity of the respiratory center by
different subjects.
• Hypoxia is induced by re-breathing of air expired into Krogh respirometer,
wherein (when closed circuit of respirometer is used) naturally decreases
O2 concentration (to avoid rise of the CO2 soda lime which absorbs CO2 is
inserted into Krogh respirometer).
• The sensitivity of the respiratory center is expressed as Ks parameter that
indicates the slope of the curve
VentilaceV
(l/min)
SpO2(%)
ΔV
ΔSpO2
Ks =
ΔSpO2
ΔV
II. Determination of the sensitivity of the
respiratory center to hypoxia
• Objective is to demonstrate the changes in ventilation
during induced hypercapnia and to compare the sensitivity
of the respiratory center to hypercapnia in several subjects.
• Hypercapnia is induced by re-breathing of air expired into
Krogh respirometer with closed circuit (Krogh respirometer
with oxygen, without the soda lime - naturally decreases
the concentration of O2 and CO2 increases)
• 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).