How is the transpulmonary gradient (TPG) measured and what are its units of measurement?

Transpulmonary Gradient Measurement and Units

The transpulmonary gradient (TPG) is measured as the difference between mean pulmonary artery pressure and mean pulmonary capillary wedge pressure, with the units of measurement being millimeters of mercury (mm Hg). 1

Measurement Procedure

• TPG is obtained during right heart catheterization, which is the gold standard procedure for assessing pulmonary hemodynamics 2
• The measurement requires:
  • Insertion of a pulmonary artery catheter (PAC) to measure pulmonary artery pressure (PAP) 1
  • Measurement of pulmonary artery occlusion pressure (PAOP), also known as pulmonary capillary wedge pressure (PCWP), which estimates left atrial pressure 1
  • Calculation of the difference between these two values (mean PAP - mean PCWP) 1

Formula and Units

• TPG = mean PAP - mean PCWP 1
• Units: millimeters of mercury (mm Hg) 1
• Normal TPG values are typically less than 12 mm Hg 1

Clinical Significance

• TPG helps assess the degree of pulmonary vascular abnormality, particularly in contexts where pulmonary hypertension may be present 1
• A TPG > 12 mm Hg has traditionally been used to identify "reactive" or "out of proportion" pulmonary hypertension in patients with left heart disease 1, 3
• TPG is particularly valuable in:
  • Evaluating patients with ARDS (Acute Respiratory Distress Syndrome) 1
  • Assessing candidates for heart transplantation 4, 5
  • Differentiating between passive and reactive post-capillary pulmonary hypertension 1

Limitations and Considerations

• TPG is sensitive to changes in cardiac output and both recruitment and distension of pulmonary vessels 3
• In some clinical contexts, the diastolic pulmonary gradient (DPG = diastolic PAP - mean PCWP) may be a more reliable indicator of pulmonary vascular disease than TPG 3, 6
• During ventilation with high PEEP (Positive End-Expiratory Pressure), calculating the transmural value of PCWP allows estimation of true left ventricular filling pressure 1
• In cases of tricuspid regurgitation with right ventricular dilatation, cardiac output measurements can be erroneous, affecting derived values 1

Recent Developments

• While TPG has been traditionally used, some recent evidence suggests that the diastolic pulmonary gradient (DPG) may be superior for diagnosing "out of proportion" pulmonary hypertension 3, 6
• Both TPG and pulmonary vascular resistance (PVR) have been shown to be predictive parameters for outcomes in heart failure patients with pulmonary hypertension 5, 7