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Fig. 27. Volumes and capacities of pulmonary ventilation

Total lung capacity (TLC) is a sum of breathing, reserve and residual volumes and makes up 5-6 liters;

Vital capacity of the lungs (VC) is equal to the sum of reserve volumes of inspiration and expiration and a breathing volume (4.5 1);

Respiratory volume (RV) — it is a volume of air inhaled and exhaled in normal respiration equal to 0.5 1;

Inspiratory reserve volume (IRV) is 1.5 1 it is the volume that a man can inhale, if after usual inspiration he will make a maximal inhalation;

Expiratory reserve volume (ERV) is 1.5-2.0 1 it is the volume that a man can exhale, if after a normal expiration he will make a maximal exhalation;

Residual volume (RV) equal to 1.0-1.5 1 it is the air left in the lungs after a maximal exhalation.

Anatomical dead space makes up about 150 ml. Under the influence of different factors (elevation of pressure in the respiratory tracts, vagolytics, adreno- and sympathomimetics) anatomical dead space may increase. Therefore, a notion of physiologic dead space was introduced. It is due to the nonuniformity of blood circulation and ventilation in various parts of the lungs and their division into functional Vest's zones.

In the upper zone (apices of the lungs) an average alveolar pressure during a respiratory cycle prevails over arterial pressure that, in its turn, is higher than a venous one: P_A > P_a > P_v.

Here, in the norm, a pulmonary blood flow is implemented during inhalation when PA becomes lower than an atmospheric pressure.

In the medium zone the arterial pressure becomes higher than the average alveolar pressure, but the latter prevails over a venous one or equal to it: P > P_A > P_v. Here, a pulmonary blood flow is implemented owing to the gradient P_a - P_A.

In the third zone the average alveolar pressure in the course of respiratory cycle is lower than the arterial and venous pressure: P_a > P > P_A. It is here that the pulmonary blood circulation is the most intensive.

In the fourth zone (basal parts of the lungs) exist the same relations:

^Pa > Pv > ^PA-

But perfusion decreases again because of local increase of the interstitial pressure upon the precapillaries. In this zone filtration of fluid occurs the most intensively.

As a result of this an anesthesiologist should remember that in a man lying on his back during carrying out a narcosis, the dorsal sections are worse ventilated, but better perfused, in the process VC decreases by 9%. In the prone position VC decreases by 5-6%, and in Trendelenburg's position — by 15%.

Gas diffusion. Transition of 0₂ from the alveolar air into the blood of pulmonary capillaries and C0₂ in the reverse direction is implemented by means of diffusion by the concentration gradient of gases in the indicated media (Table 10).

Table 10 Indices of gas exchange in the organism

Fig. 28. Dependence of 0₂ blood saturation on P0₂.

Diffusion ability of the lungs (DL) — it is the number of milliliters of gas passing through a pulmonary membrane per 1 min in transmembrane difference of gas partial pressures equal to 1 mm Hg. In the norm DL for oxygen makes up about 15 ml 0₂/min/mm Hg. DL for C0₂ is about 300 ml 0₂/min/mm Hg.

One of the basic conditions ensuring the diffusion is a laminar flow, but this does not occur if a turbulent flow appears (provided that Raynolds number is more than 2000). A turbulent flow arises in case of obstructive changes in the respiratory tracts (spasm, edema of mucous membrane, obstruction of small bronchi, etc.) that leads to the increased functioning of respiratory musculature and elevated load upon respiratory center. As far as a volume rate of gas movement in the turbulent flow is in inverse proportion to its density, the application of gases with a lesser density (helium-oxygen mixture) decreases the resistance of airways facilitating the work of respiratory muscles.

A lengthening of the diffusion path of 0₂ is observed in thickening of the layer of fluid on the alveolar surface, swelling of alveolar membrane, increase of the volume of interstitial fluid and blood plasma fraction (Hamman-Rich syndrome, RDS, pulmonary edema and others).

Ventilation-perfusion ratio — it is a relation of minute respiratory volume to minute pulmonary perfusion — makes up 8 1/10 1 = 0.8, i.e. 0.8 1 of air that passes through the alveoli accounts for each liter of blood that flows through the pulmonary capillaries per minute.

A decrease of ventilation/blood flow ratio is observed in local alveolar hypoventilation (obstructive syndrome, atelectases) and may approximate to zero. As a result of this, a volume of functional dead space increases and, respectively — a "venous admixture" in the blood, flowing out from the lungs. The increase of the given ratio occurs when perfusion decreases (TEPA, spasm and obliteration of pulmonary vessels), and as a consequence of this — the increase of venous admixture in the blood in pulmonary veins.

There are three anatomical pathways in the lungs for a collateral ventilation (ensures a retrograde supply of air into the alveoli) — interalveolar Cohn's pores, bronchoalveolar Lambert's canals and interbronchial Martin's canals. Reduction of collateral ventilation increases the risk of development of air leakage syndromes.

Blood gases transport and tissue respiration. Transportation of 0₂ with blood occurs in the dissolved form and in the form bound with hemoglobin. A saturation of arterial blood with 0₂ and return of 0₂ to the tissues occur because of the difference of oxygen partial pressure (Fig. 28). A dissolution of 0₂ in plasma is implemented by Henry's law. At 37° C and P0₂ 100 mm Hg 0.3 ml 0₂ (0.3 vol. %) are dissolved in 100 ml of plasma. 9 ml 0₂ are dissolved in three liters of plasma. The main function of 0₂ transfer in the organism is fulfilled by hemoglobin. Each gram of hemoglobin (Hb) binds 1.34 ml 0₂ (Guffner's constant).

Oxygen capacity of blood is a maximal amount of 0₂ that can be bound by 100 ml of blood. In the norm this index for arterial blood makes up 19-21vol%, and for venous blood — 14 vol%. HbO, formation and its dissociation (0₂ saturation curve) depend on many factors. Location of Hb0₂ dissociation curve is accepted to estimate by P₅₀ — an oxygen partial tension at which 50% Hb is bound with 0₂ when pH is 7.4 and temperature — 37.0°C. In the norm P₅₀ = 27 mm Hg.

The factors shifting a saturation curve to the right (hemoglobin affinity to oxygen decreases) are hyperthermia, acidosis, hypercapnia, anemia, elevation of atmospheric pressure, heavy physical work, increase of 2.3DPG, ATP, increase of pyruvatekinase in erythrocytes.

The factors shifting a saturation curve to the left (hemoglobin affinity to oxygen increases) are hyperventilation, alkalosis, hypothermia, decrease of atmospheric pressure, HbF, HbAl, decrease of 2.3DPG, CO, MetHb.

Bohr's effect. While pH (in tissues) decreases, a rupture of b,-b₂ bonds in Hb chains, a change of macromolecule configuration and decrease of Hb affinity

to O, occur. A diffusion of oxygen from arterial blood into tissues is based on this effect. A terminal pathway of 0₂ metabolism is a tissue respiration (a complex of coenzymes — NAD⁺, FF⁺, UQ; cytochromes — b, c, a, cytochrome oxidase). While a level of coenzymes decreases and cytochromes are blocked, hypoxemia takes place.

Carbon dioxide is transferred by blood in three forms: 1. Physically dissol­ved — 10%; 2. Carbohemoglobin — 20-30%; 3. In a form of bicarbonates — 60-70%.

Holdein's effect — is the increase of blood affinity to C0₂ in its deo-xygenation.

Anatomicophysiological features of children's respiratory system.

1. Narrow and short nasal passages, trachea, bronchi, hypertrophy of tonsils, a large tongue on account of this grows aerodynamic resistance of respiratory tracts and respiratory function.

2. Larynx is located higher than in adults. A longitudinal axis of larynx is greatly deviated backwards in newborns and forms with the trachea obtuse angle open posteriorly.

3. V-shaped, long, immobile epiglottis is arranged somewhat above the root of the tongue, therefore newborns and infants may be intubated with a spatula or a straight blade.

4. Entrance into the larynx is relatively wide. A true glottis is too high located.

5. A narrow place of the respiratory tracts is a subligamental space (intubation tube is used without an inflated cuff). A great number of mucous glands in the upper respiratory tracts, a more intensive blood supply, looseness of fat in the subligamental space cause a tendency to the edema of respiratory tracts with a rapid reduction of their lumen.

6. A short trachea (4cm) and relatively small crosssection of the trachea leads to a rise of resistance in tachypnea.

7. A less amount of elastic tissue in the lungs and walls of the bronchi, underdevelopment of cartilages and a low stretchability of the lungs increase the risk of development of atelectases, obstructive syndrome and formation of air traps.

8. A collateral ventilation through Cohn's pores and Lambert's canals is absent in the lungs that increases the risk of rise of air leakage syndromes.

9. A thickening of alveolar septa leads to some difficulties of diffusion.

10. A less expression of respiratory musculature and high need in 0₂ (8-10 ml/kg/min) that causes a rapid rise of muscular weakness and rapid development of respiratory failure (RF) in diseases of respiratory organs.

11. Expiratory structure of the chest and high location of the diaphragm restrict the possibilities of increasing a respiratory volume, therefore the increase of pulmonary minute volume occurs at the expense of respiration rate increase.

12. A lower excitability of chemoreceptors to hypoxia, hypercapnea and acidosis causes a later onset of adaptation to disturbances of blood gas composition. Under the influence of changes of the body temperature and environment owing to underdevelopment of the respiratory center in newborns, more rapid disorders of the depth and rate of respiration take place.

Availability of these features of the respiratory system in children may lead to a quick development of RF.

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