Decreased pulmonary artery pressure (e.g. decreased cardiac output).

       Loss of perfusion to ventilated alveoli despite normal pulmonary artery pressure (e.g. pulmonary embolus).

       Increased airway pressure.

       Equipment (e.g. endotracheal tubes (ET) protruding excessively beyond the lips).

       Rapid, short inspirations.

       At the completion of a tidal volume breath, the lung does not empty. The volume of gas remaining in the lung is referred to as the FRC (see Figure 4.2).

       The volume of the FRC is important as it acts as a reservoir for gas exchange and is estimated at 51 ml/kg in the horse.

       A reduction in FRC alters lung mechanical properties (see pulmonary compliance below) as well as increasing pulmonary vascular resistance.

       In the horse, the FRC is reduced with recumbency and general anesthesia.

       If the FRC is very low, one can see from the shape of the pressure‐volume curve (see Figure 4.3), that the change in lung volume for a given change in pressure is very small.

       Compliance (ΔV/ΔP) is the change in lung volume (ΔV) per unit change in transpulmonary pressure (ΔP) and it has been estimated as 22.7 l/kPa (3.0 l/mmHg).

       Lung compliance can be measured in an awake spontaneously breathing horse in a laboratory setting by measuring the mouth to intrathoracic pressure and lung volume change using a pneumotachograph (device that measures airflow over time) attached to a facemask. In anesthetized horses, pulmonary compliance and pressure volume curves are determined using positive pressure to achieve a change in lung volumes from FRC to total lung capacity. (see Figure 4.3)

       Compliance is the slope of the pressure‐volume curve and, due to the non‐linear shape of the curve, it fluctuates with lung volume.

       In the spontaneously breathing animal with normal lungs, tidal volume breathing occurs on the steep portion of the curve. As a result, for a given change in intrapleural pressure, there is a greater change in lung volume than would occur if tidal volume breathing occurred at the extremes of the curve.

       The distribution of alveolar ventilation, or the change in alveolar size with each breath, is not uniform throughout the lung due to differences in the mechanical properties of the lung and chest wall.

       In the standing horse, the intrapleural pressure is more sub‐atmospheric in the dorsal part of the lung relative to the ventral part, due to the effect of gravity on lung tissue. The alveoli in the dorsal part of the lung are therefore more distended, and less compliant than in the ventral part of the lung. As a result, alveolar ventilation per unit change in pressure is greater in the ventral compared to the dorsal part of the lung.The alveoli in the dorsal aspect of the lung would be at a location further to the right on the curve. (see Figure 4.3)Figure 4.3 Pulmonary pressure‐volume curve; illustrating greater pressure difference required for inspiration than for expiration (squares = inspiration; circles = expiration).

       Atelectasis resulting in a loss of lung volume.

       Pulmonary edema and/or pulmonary surfactant dysfunction.

       Pleural, interstitial or alveolar disease.

       Airway occlusion.

       Pleural and/or pericardial effusion.

       Rigidity of the chest wall or diaphragm (e.g. secondary to abdominal distension).

       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