How to interpret Pulmonary Vascular Resistance (PVR) via ultrasound (USG) guidance?

PVR Interpretation: Two Distinct Clinical Contexts

I need to clarify that "PVR" has two completely different meanings in clinical medicine, and your interpretation depends entirely on the clinical context.

Context 1: Pulse Volume Recording (PVR) - Peripheral Arterial Disease

PVRs provide qualitative waveform analysis of limb perfusion to detect peripheral arterial disease and localize stenoses. 1

How PVR Works

• PVRs are created by inflating pneumoplethysmography cuffs at predetermined levels on each limb, measuring miniscule volume changes with each pulse to create waveform tracings. 1
• The technique relies on limb volume change rather than pressure, making it particularly valuable in patients with noncompressible vessels (common in diabetes and chronic renal insufficiency where ankle-brachial index fails). 1

Waveform Interpretation

• Compare waveforms at sequential levels to determine segmental disease and assess arterial blood flow quality at each station simultaneously. 1
• Normal waveforms show sharp upstrokes and prominent dicrotic notches. 1
• Abnormal waveforms demonstrate blunted upstrokes, absent dicrotic notches, and decreased amplitude, indicating hemodynamically significant stenosis proximal to that level. 1

Clinical Application

• Use PVR in conjunction with ankle-brachial index and segmental pressure measurements for comprehensive peripheral arterial disease assessment. 1
• PVR is especially useful when ankle-brachial index >1.3 indicates noncompressible vessels. 1
• The test provides insight into disease location but cannot determine precise severity or number of lesions. 1

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Context 2: Pulmonary Vascular Resistance (PVR) - Pulmonary Hypertension

PVR >3 Wood units combined with mean pulmonary artery pressure >25 mmHg is required to diagnose pulmonary arterial hypertension after excluding left heart disease, lung disease, and thromboembolic disease. 1

PVR Calculation Formula

• PVR = (mean pulmonary artery pressure - pulmonary capillary wedge pressure) / cardiac output 2
• Units: Wood units (normal <2-3 Wood units) or dynes·s·cm⁻⁵ (multiply Wood units by 80 for conversion). 2
• All measurements must be obtained via right heart catheterization—echocardiographic estimates are unreliable for diagnosis. 1

Required Measurements

• Mean pulmonary artery pressure (mPAP) measured via right heart catheterization 2
• Pulmonary capillary wedge pressure (PCWP) obtained by wedging balloon catheter in pulmonary arterial branch, measured at end-expiration 1, 2
• Cardiac output via thermodilution (triplicate measurements) or Fick method (preferred if severe tricuspid regurgitation present) 1, 2

Diagnostic Interpretation

• PVR distinguishes passive pulmonary hypertension (elevated mPAP, normal PVR) from pulmonary vascular disease (elevated mPAP, elevated PVR). 1
• Normal PVR with elevated mPAP suggests left heart disease, valvular disease, or high-flow states (anemia, pregnancy, sepsis, thyrotoxicosis). 1
• Transpulmonary gradient (mPAP - PCWP) >20 mmHg with PVR >3 Wood units indicates disproportionate pulmonary vascular involvement even in left heart disease. 1

Critical Clinical Caveats

• In patients with tricuspid regurgitation and right ventricular dilatation, thermodilution cardiac output measurements can be erroneous, compromising PVR calculation accuracy. 1, 2
• During mechanical ventilation with high PEEP, calculate transmural PCWP to estimate true left ventricular filling pressure. 1, 2
• In ARDS, calculated PVR may underestimate true pulmonary vascular resistance due to extended West zones 1 and 2; transpulmonary pressure gradient remains valuable. 1

Units Confusion Warning

• Significant variability exists in PVRI (PVR index) reporting—only 45.4% of published literature uses correct units ending in m² (meters squared). 3
• Always verify whether reported values are PVR (Wood units) or PVRI (Wood units·m²) to avoid life-altering misinterpretation. 3

Emerging Evidence on Borderline PVR

• Patients with precapillary pulmonary hypertension and PVR 2-3 Wood units (below diagnostic threshold) still demonstrate adverse outcomes with 5-year survival of 84%. 4
• These borderline patients appear to respond to pulmonary arterial hypertension therapy, though this requires validation in randomized trials. 4

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Determining Which PVR You Need

If the clinical question involves:

• Leg pain, claudication, wounds, or post-revascularization assessment → Pulse Volume Recording (peripheral arterial disease context) 1
• Dyspnea, right heart failure, elevated pulmonary pressures, or pre-transplant evaluation → Pulmonary Vascular Resistance (cardiopulmonary context) 1, 2