A- Introduction :
L & rsquo; pulmonary exchanger is the meeting of two circulations : alveolar and pulmonary capillary, l & rsquo; hematosis pulmonary main function is directly related to the relationship between these two circuit called ventilation-perfusion ratio.
distribution inequalities explain the power of the lungs to adapt their function to the different physiological situations same extreme as the & rsquo; physical effort by recruiting and poorly ventilated areas or poorly perfused at rest
To understand the different values of this report in a lung upright, it is necessary to & rsquo; explain the regional distribution of the two parameters that define namely alveolar ventilation and pulmonary perfusion
B- Regional distribution of alveolar ventilation :
Alveolar ventilation VA is to differentiate the overall ventilation VE, because for the measure is necessary to subtract the volume of & rsquo; dead space VD tidal volume Vt before multiplying all the Fr respiratory rate :
VE = Vt x Fr VA = ( Vt-VD ) x Fr
Upright, under the & rsquo; gravity weight of the viscera of the & rsquo; abdomen pulls the pleura down explaining more negative pleural pressure at the apical regions compared to lower areas so that the most distended alveoli atop receive less d & rsquo; s air & rsquo; inspiration.
C- regional pulmonary perfusion distribution :
The distribution of lung perfusion increases upper to lower parts of the lung.
To understand this distribution the lung is divided vertically into four zones :
Zone 1 : upper part of the lung
PA>Pac>Pv : which explains a crushing of the pulmonary capillary
NB : while the top of the cell can not receive blood perfusion : alveolar dead space
Zone 2 : Pac>PA>Pv
The capillary crashed intermittently
Zone 3 :
Pac>Pv>PA : the capillary is distended, infusion is optimal
Pac : arteriolar-capillary pressure, PA : alveolar pressure, Pv : venular pressure
Zone 4 :
There is a reduction in capillary perfusion related pleural pressure less negative approaching zero positive view explaining that the small vessels are not stretched.
D- Regional distribution of ventilation-perfusion ratio :
Pulmonary perfusion increases more than the & rsquo; observed alveolar ventilation if the lower lung regions compared over more apical areas, when the thorax is obviously upright gravity)
The ventilation-perfusion ratio is therefore not uniform throughout the lung surface
The ventilation lung scan (inhalation of xenon 133) or infusion (injection of the same product) enables a true pulmonary mapping express percentage participation of operation of such area or other relative to the overall pulmonary function
F- Extreme inequality in the distribution of ventilation-perfusion ratio :
If ventilation is void in an area the ratio is equal to 0, This corresponds to an effect schunt opposed the ic true anatom shunt that s & rsquo; observed when veins pours direct part in arteries and contam inant without arterial.
Either normal ventilation is zero infusion, the ratio is equal to & rsquo; infinite, what defines the & rsquo; indeed dead space
Note that the exaggerated inequalities in the distribution of ventilation-perfusion ratio represent the most important cause of & rsquo; hypoxemia
During the PR Aissaoui – Faculty of Constantine