ventilation-perfusion ratio


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.

Gradient vertical pression pleurale

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.

Areas of West

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

E- Research breakdown report- perfusion :

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 :

Regional distribution of pulmonary ventilation

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