Читать книгу Manual of Equine Anesthesia and Analgesia - Группа авторов - Страница 220

 Intrapulmonary shunts can result in the delivery of poorly oxygenated blood into the pulmonary venous blood.

 The fraction of cardiac output that passes through a shunt is expressed as the shunt fraction (Qs/Qt), and can be calculated using the Berggren equation:

 CcO2 = the O2 content in pulmonary capillary blood (Calculated based on PaO2, assuming 100% saturation of Hb).CaO2 and CvO2 = the O2 content in arterial and mixed venous blood (obtained from a pulmonary artery catheter), respectively.In order for this calculation to be accurate, measurements must be performed when the horse is breathing 100% O2.Figure 4.5 The oxy‐hemoglobin dissociation curve illustrates the relationship between the partial pressure of oxygen in arterial blood and the percent of hemoglobin that is bound by oxygen. The sigmoid shape of the curve illustrates the cooperative binding of oxygen to hemoglobin. The P50 indicates the partial pressure of oxygen at which hemoglobin is 50% saturated.Estimates of (Qs/Qt) can be determined using the F‐shunt equation where a fixed arterial to mixed venous oxygen content difference [C(a‐v) O2] of 3.5 ml/dl is assumed.

 Normal shunt fraction is <5%.Clinically insignificant shunts are 10–19%.Clinically significant shunts are 20–30%.Potentially fatal shunts are >30%.

 Physiologic shunts are the most common type of shunt and they arise secondary to atelectasis or consolidation of alveoli.

 Anatomic shunts include bronchial, mediastinal, pleural, and coronary veins.

 Pathologic anatomic shunts include shunts secondary to congenital or traumatic anomalies and intrapulmonary tumors.

 Shunts and V/Q inequalities have a greater impact on O2 uptake than CO2 removal from the lungs due to the shapes of their respective dissociation curves.Specifically, blood passing under‐ventilated alveoli tends to retain its CO2, and blood passing over‐ventilated alveoli gives off an excessive amount of CO2.The amount of the retained CO2 and the excessively lost CO2 are proportional due to the relatively linear relationship of CO2 to VA.On the other hand, blood passing under‐ventilated alveoli does not take up enough O2, and blood passing over‐ventilated alveoli cannot take up a proportionately increased amount of O2 owing to the flatness of the O2‐Hb dissociation curve in this region.