What is the normal trans‑pulmonary gradient (mean pulmonary artery pressure minus pulmonary capillary wedge pressure)?

Normal Transpulmonary Gradient

The normal transpulmonary gradient (TPG), defined as mean pulmonary artery pressure minus pulmonary capillary wedge pressure, is ≤12 mmHg.

Hemodynamic Definition

The TPG is calculated as:

• TPG = mean PAP - mean PAWP (or PCWP)

According to the most authoritative ESC/ERS guidelines, a TPG ≤12 mmHg defines "passive" or isolated postcapillary pulmonary hypertension, while a TPG >12 mmHg indicates "reactive" or combined pre- and postcapillary pulmonary hypertension 1.

Clinical Context and Normal Values

In healthy individuals:

• Mean pulmonary artery pressure averages 14 ± 3 mmHg with an upper limit of normal around 20 mmHg 1
• Pulmonary capillary wedge pressure has an upper limit of normal of 13-15 mmHg 2
• This yields a normal TPG in the range of 1-7 mmHg in most circumstances

Important Clinical Distinctions

The 12 mmHg Threshold

A TPG >12 mmHg is suggestive of intrinsic changes in the pulmonary circulation that override the passive increase in wedge pressure 3. This threshold has been used in multiple guidelines to distinguish:

• Isolated postcapillary PH (Ipc-PH): TPG ≤12 mmHg
• Combined postcapillary and precapillary PH (Cpc-PH): TPG >12 mmHg 1

Critical Limitations of TPG

The TPG has significant limitations because it is influenced by cardiac output, stroke volume, and changes in PAWP 4, 5. The 2015 ESC/ERS guidelines acknowledge these shortcomings and recommend using diastolic pressure gradient (DPG) and pulmonary vascular resistance (PVR) in combination for more accurate classification 4.

The updated 2015 definitions specify:

• Isolated postcapillary PH: DPG <7 mmHg and/or PVR ≤3 Wood units
• Combined postcapillary and precapillary PH: DPG ≥7 mmHg and/or PVR >3 Wood units 4, 6

Practical Application

When interpreting right heart catheterization:

1. Calculate TPG (mean PAP - mean PAWP)
2. If TPG ≤12 mmHg: Suggests purely passive/postcapillary component
3. If TPG >12 mmHg: Consider intrinsic pulmonary vascular disease, but verify with:
   • DPG (diastolic PAP - mean PAWP): abnormal if ≥7 mmHg 4, 5, 7
   • PVR: abnormal if >3 Wood units 4

The DPG is less dependent on flow and left atrial pressure changes than TPG, making it a more reliable marker of true pulmonary vascular disease 5, 7. In patients with left heart disease and elevated PAWP, the DPG better identifies those with significant pulmonary vascular remodeling and worse prognosis 7.

Prognostic Significance

An elevated TPG (>12 mmHg) in the setting of left heart disease identifies patients at higher risk for:

• Worse postoperative outcomes 8
• Progression to overt pulmonary hypertension 9
• Increased mortality 10, 7

However, DPG ≥7 mmHg is a superior predictor of mortality and pulmonary vascular disease compared to TPG alone in patients with postcapillary pulmonary hypertension 7.