Manual of Equine Anesthesia and Analgesia. Группа авторов

       The lung receives blood from two circulations, the pulmonary artery and the bronchial artery.

       The pulmonary artery receives the total output of the right ventricle, and perfuses the alveolar capillaries.

       The bronchial artery is a branch of the aorta and perfuses the parenchymal structures of the lung (e.g. airways).

       The pulmonary arterial systolic, diastolic, and mean pressures in the horse average 42, 18, and 22 mmHg, respectively. This indicates a low vascular resistance compared to the systemic circulation.

       For gas exchange to occur across an alveolar membrane, the alveoli must be perfused. Optimal gas exchange occurs when the alveolar ventilation and blood flow are equally distributed in the lung.

       The distribution of pulmonary blood flow within the lung of the horse was previously thought to be primarily influenced by gravity; however, there is not a consistent vertical gradient to blood flow in the lungs of horses.

       This implies that gravity does not play a major role in blood flow distribution.

       Endogenous vasoactive mediators (e.g. nitric oxide and endothelin‐1) are now thought to play a major role in the distribution of perfusion within the lung.

       Vasoconstriction and shunting of blood away from alveoli with low oxygen content, is a result of vasoactive mediators acting on pulmonary vasculature.

       In the normal horse, the V/Q ratio is close to 1.0.

       This normal V/Q relationship may be altered by the distribution of ventilation, perfusion and/or a change in their relative distribution.When a lung unit has low or no ventilation relative to perfusion, blood leaving the unit will have lower O2 content than units with optimal V/Q relationships.Figure 4.4 Blood entering the pulmonary capillaries associated with non‐ventilated alveoli is termed “shunt,” and represents a V/Q ratio of 0. Alveoli that are ventilated but not perfused are termed “deadspace” and represents a V/Q ratio of infinity. Alveoli that are equally perfused and ventilated represent a V/Q ratio of 1.

        If the V/Q relationship is 0, the blood leaving this unit will have O2 content similar to pulmonary artery blood.In this situation, the blood leaving this unit is referred to as an intrapulmonary shunt and is most commonly a result of atelectasis, partial or complete airway obstruction.

       The other extreme, a V/Q ratio of infinity, is dead space ventilation.

       The composition of gases in a mixture can be described by their fractional composition or their partial pressures.

       The composition of gas within the alveoli is determined by the movement of gas into and out of the alveoli via the airways or across the alveolar capillary membrane and into or out of the pulmonary capillaries.

       Bulk transfer describes the movement of gases during inspiration and expiration within the proximal large airways.

       Diffusion is the passive movement of gases down the concentration gradient in the distal small airways to the alveolus. It is the process by which gases move (i) in and out of the alveoli into the terminal airways, (ii) across the alveolar capillary membrane, and (iii) between the blood and tissues.

       The surface area available for diffusion.

       The physical properties of the gas.

       The thickness of the air‐blood barrier.

        The driving pressure of the gas between the alveolus and capillary blood as described by Fick's law of diffusion.Fick's Law Vgas = the volume of gas transferred across a membrane or barrier.A = the area available for diffusion.T = the membrane thickness.D = a diffusion constant that is dependent on the physical properties of the gas.P1‐P2 = the partial pressure difference of the gas across the membrane.Note: CO2 is approximately 20 times more soluble than O2, and therefore its diffusion across a membrane is less likely to be impaired, relative to O2, by a change in membrane thickness.

       In the normal lung, equilibration of O2 and CO2 across the alveolar capillary membrane occurs within 0.25 seconds; approximately one third of the time the blood is in the capillary.

       Carbon dioxide is the end product of aerobic metabolism. There is a continuous gradient of CO2 from the mitochondria in peripheral cells to venous blood and then to the alveolar gas.

       Carbon dioxide is transported in the blood in several forms including:Dissolved in physical solution (~5%).As carbonic acid (~90%).Combined with proteins (~5%) such as carbaminohemoglobin.

       Carbon dioxide moves from the blood to the alveoli in its dissolved form only.

       Alveolar CO2 partial pressures are directly proportional to CO2 production and indirectly proportional to alveolar ventilation. VaCO2 = Rate of CO2 production.VA = Alveolar ventilation.

       Clinically, the adequacy of alveolar ventilation is evaluated by measuring arterial CO2 partial pressures (PaCO2).

       The normal values for PaCO2 and PACO2 are between 35 and 45 mmHg.

       The partial pressure of oxygen in the alveoli can be determined using a simplified version of the alveolar gas equation:FiO2 = Fraction of inspired oxygen ≈ 0.21.PB = Atmospheric pressure (760 mmHg at sea level).PH2O = Water vapor pressure (mmHg) in airway (~50 mmHg at body temp of horse).R = Respiratory gas exchange ratio (~0.8).

       This calculation emphasizes the significance of FiO2 and PaCO2 on the alveolar gas partial pressure.

       Clinically, this equation highlights the significance of O2 supplementation for patients with impaired ventilation.