Distensibility Index Calculation
The distensibility coefficient (DC) is calculated as DC = ΔA/(A × ΔP), where ΔA is the change in arterial cross-sectional area, A is the baseline area, and ΔP is the pulse pressure (systolic minus diastolic pressure). 1
Standard Formula
The distensibility coefficient represents the fractional change in arterial cross-sectional area per unit change in local pulse pressure. 1 The formula can be expressed in two equivalent ways:
- Area-based formula: DC = ΔA/(A × ΔP) 1
- Diameter-based approximation: DC ≈ 2ΔDia/(Dia × ΔP), where ΔDia is the change in diameter and Dia is the baseline diameter 1
The more precise diameter-based formula accounts for the relationship between area and diameter: DC = (2ΔDia·Δ + ΔDia²)/(D² × ΔP), though the simplified approximation is commonly used in clinical practice. 1
Critical Measurement Requirements
Pressure Measurements
Use local blood pressure at the measurement site rather than brachial cuff values, as pulse pressure amplifies significantly as it travels from central to peripheral arteries. 1 This distinction is crucial because brachial pressure can differ substantially from carotid or femoral pressures, leading to inaccurate distensibility calculations. 1
Diameter/Area Measurements
Arterial diameter or area changes can be measured using: 1
- B-mode guided M-mode ultrasound for superficial vessels (carotid, brachial, radial, femoral arteries) 1
- A-mode radiofrequency tracking for more precise wall tracking 1
- MRI for larger vessels throughout the vasculature, though spatial resolution limits accuracy in smaller vessels 1
Important Technical Considerations
Wall Tracking Method Impact
Ultrasound algorithms that track the media-to-adventitia transition (rather than lumen diameter) will yield smaller ΔDia and larger Dia values, systematically underestimating distensibility. 1 Ensure your measurement protocol specifies which boundary is being tracked for consistency.
Relationship to Pulse Wave Velocity
The distensibility coefficient directly relates to pulse wave velocity through the Bramwell-Hill equation: PWV = √(1/ρDC), where ρ is blood density (approximately 1050 kg/m³). 1 This allows calculation of a single PWV value (PWV_DC) from distensibility measurements rather than requiring two-site measurements. 1
Clinical Application Sites
The most commonly measured arteries for distensibility assessment include: 1
- Carotid artery: Most frequently used for research and clinical assessment 1
- Brachial artery: Can be measured with or without ultrasound using specialized devices 1
- Femoral artery: Used in conjunction with carotid measurements 1
- Radial artery: Accessible peripheral site 1
Validation Considerations
Non-invasive distensibility measurements using echocardiographic diameters and brachial pressure correlate strongly (r = 0.949) with invasive measurements using angiographic diameters and direct aortic pressure. 2 This validates the clinical utility of non-invasive approaches when proper technique is employed. 2
The formula has been validated to detect clinically meaningful differences, such as significantly reduced distensibility in patients with coronary artery disease compared to normal subjects, using both invasive and non-invasive techniques. 2