Respiratory Medicine

The upper limit of pulmonary arterial pressure is 30 mmHg (mean is 15)

Pulmonary vascular resistance is 1/10th of systemic vascular resistance

1 kPa = 7.5 mmHg

FEV1 low = smaller breath volume

FVC low =  restrictive lung expansion (low TLC is the hallmark).

FEV1/FVC <70 => Obstructive

FEV1/FVC normal, but FVC low = Restrictive

The diffusing capacity of the lungs for carbon monoxide (DLCO), also known as the transfer factor for carbon monoxide (TLCO), measures the amount of carbon monoxide (CO) transferred per minute from alveolar gas to red blood cells (RBCs). This test provides critical insights into the lungs' ability to transfer oxygen from inhaled air to the bloodstream—principle: Fick equation. DLCO reflects the integrity of the alveolar–capillary membrane + pulmonary capillary blood volume. It depends on 1. alveolar surface area, 2. membrane thickness, 3. pulmonary capillary blood volume, 4. haemoglobin content

Normal DLCO = ~25 mL/min/mmHg.  <80% predicted = abnormal.  <60% predicted = significant diffusion defect

↓DLCO in ILD = hallmark of alveolar–capillary membrane thickening and loss of gas-exchanging surface area.

DLCO distinguishes between fibrotic restriction (↓decreased DLCO) and extrapulmonary restriction (normal DLCO), such as scoliosis or neuromuscular disease.

In Asthma: DLCO normal or increased — because alveoli are intact (because it's an airway disease only. Similarly, for chronic bronchitis. In COPD (emphysematous): DLCO decreased — hallmark of alveolar destruction. Therefore, DLCO differentiates asthma (reversible, intact alveoli) from emphysema (irreversible, destructive parenchymal disease).