not participate in gas exchange.
C Dead space
Anatomic dead space is the volume of gas that ventilates conducting airways.
Alveolar dead space is the volume of gas not taking part in effective gas exchange at the alveolar level.
Physiologic dead space is the sum of anatomical and alveolar dead space.
The fraction of a tidal volume breath that occupies the physiologic dead space is commonly expressed as the VD/VT ratio.
In normal adult horses, the VD/VT ratio is 50–75%, a value significantly greater than the VD/VT ratio in humans (15–20%).
Figure 4.2 Lung volumes in an adult horse (500 kg). TLC, Total lung capacity (55 l); VC, Vital capacity (44 l); RV, Residual volume (11 l); IC, Inspiratory capacity (29.5 l); FRC, Functional residual capacity (25.5 l); IRV, Inspiratory reserve volume (23.5 l); TV, Tidal volume (6 l); ERV, Expiratory reserve volume (14.5 l).
D Measurement of dead space – Bohr equation
The ratio of physiologic dead space to the tidal volume (VD/VT) can be calculated using Enghoff's modification of the Bohr equation, which substitutes alveolar CO2 (PACO2) with arterial CO2 (PaCO2):
This measurement is based on the fact that all expired CO2 comes from perfused alveoli and none from dead space.PECO2 in this equation is the CO2 content in the mixed expired gas, which is obtained under experimental conditions by sampling a mixture of the expired gas collected from a large bag or through a device that measures of expired CO2 content and expired volume over time.In clinical practice, end‐tidal CO2 can be used to follow trends in the VD/VT ratio.
An increase in dead‐space ventilation necessitates an increase in minute ventilation in order to maintain alveolar ventilation. This results in an increase in the work of breathing required to maintain normal gas exchange in a spontaneously breathing patient. In a mechanically ventilated patient, an increase in dead space requires an adjustment in minute ventilation by increasing the respiratory rate or tidal volume settings. In a hemodynamically compromised patient, the latter changes may have negative cardiovascular consequences.
Factors that increase dead space include
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.
E Functional residual capacity (FRC)
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.
III Pulmonary compliance
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.
A Distribution of alveolar ventilation
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).
B Factors decreasing pulmonary compliance
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).
IV
