Respiratory Physiology

Respiratory Module: Understanding Ventilation/Perfusion Relationships

• Alveolar and blood gas partial pressures are calculated using the formula PB x %gas = 760 mmHg x 0.21 = 160 mmHg (PB - 47) x %gas = (760 mmHg - 47 mmHg) x 0.21 = 150 mmHg.
• Arterial blood gas (ABG) partial pressures are measured using arterial blood samples and a blood gas analyzer. Alveolar PCO2 values are measured approximately by measuring end-tidal values.
• The alveolar-capillary membrane is normally very thin (0.5 µm) and so there is rapid, complete equilibration of O2 and CO2 between the alveolar gas and the blood (perfusion rather than diffusion limited).
• Diffusing capacity (DL) or transfer factor (TL) is the extent to which a gas (e.g. oxygen or CO2) passes from the air sacs of the lungs into the blood. It is a distillation of all the factors which influence the diffusion of respiratory gases into one numerical representation.
• DLCO unit is ml/min/kPa, and TLCO unit is mmol/min/kPa. The expected value depends on hemoglobin, age, and sex. It is reduced in lung fibrosis, pneumonia, edema, and emphysema.
• The alveolar-arterial PO2 gradient is normally arterial (a) blood PO2 is slightly less (95 mmHg) than alveolar (A) PO2 (A-a PO2 gradient) because of venous admixture which is caused by anatomical shunt (bronchial and thebesian veins) and ventilation/perfusion mismatch.
• The alveolar gas equation is used to calculate alveolar oxygen partial pressure as it is not possible to collect gases directly from the alveoli. The arterial PO2 can be determined by obtaining an arterial blood gas.
• Ventilation/perfusion (V/Q) matching is essential for normal gas exchange in the lungs. For normal gas exchange, alveoli must be in close proximity to pulmonary capillaries. The average normal value of V/Q is 0.8 (ventilation is 80% of perfusion).
• In respiratory disease, the V/Q ratio may be increased (overventilation/underperfusion) or decreased (underventilation/overperfusion). An increased V/Q ratio means an increase in alveolar VD (= alveolar dead space) and wasted ventilation. A decreased V/Q ratio means shunting where deoxygenated venous blood bypasses the exchange area and enters the left heart causing arterial hypoxemia.
• A “true shunt” is where blood flows through a region with zero ventilation. Examples would be abnormal right-left shunts in the heart, atelectasis*, consolidation**