Gorlin Formula — Cath Lab Worksheet

Aortic Valve Area (AVA)

AVA (cm²) = CO × 1000 44.3 × SEP × √ΔP

COCardiac output (L/min) SEPSystolic ejection period (sec/min) ΔPMean transaortic gradient (mmHg) 44.3Gorlin constant (empirical)

Severity	AVA (cm²)
Normal	> 2.0
Mild AS	1.5 – 2.0
Moderate AS	1.0 – 1.5
Severe AS	< 1.0
Very Severe / Critical	≤ 0.6

Mitral Valve Area (MVA)

MVA (cm²) = CO × 1000 37.7 × DFP × √ΔP

COCardiac output (L/min) DFPDiastolic filling period (sec/min) ΔPMean transmitral gradient (mmHg) 37.7Gorlin constant for AV valves

Severity	MVA (cm²)
Normal	4 – 6
Mild MS	> 1.5
Moderate MS	1.0 – 1.5
Severe MS	< 1.0

Determining SEP & DFP

Cycle length (sec/beat) = 60 ÷ HR

SEP — Systolic Ejection Period (Aortic)

From tracing: AV opens (LV > Ao) to dicrotic notch (AV closes)
SEP (sec/min) = ejection time/beat × HR
Typical: ~0.30–0.35 sec/beat → 20–25 sec/min
Prolonged in: severe AS, low EF, bradycardia

DFP — Diastolic Filling Period (Mitral)

From tracing: MV opens (LA/PCWP > LV) to MV closes (LV > LA)
DFP (sec/min) = filling time/beat × HR
Typical: ~0.40–0.50 sec/beat → 30–40 sec/min
Shortened dramatically in tachycardia
In AFib: varies beat-to-beat → average multiple cycles

Memory Anchors

AVA = flow ÷ (time × √gradient)
AS hates low flow — low CO → falsely small AVA
MS hates tachycardia — short DFP → overestimates severity
Fick > TD for valve area when accuracy matters

Cardiac Output: Fick vs Thermodilution

	Thermodilution	Fick
Pros	Easy, reproducible, less operator-dependent	Physiologically grounded; preferred in low CO, valvular disease, structural heart
Cons	Inaccurate in severe TR, low output, shunts	VO₂ often estimated → propagation error
Gorlin literature	Commonly reported in modern labs	Original Gorlin studies used direct Fick; guidelines favor Fick for valve areas

Pearl: If TD & Fick differ meaningfully, use Fick for Gorlin — especially in AS or MS.
Always document: CO method used + whether VO₂ was measured or assumed.

Common Pitfalls & Interpretation

Pitfall	Effect & What to Do
Low cardiac output	Falsely small valve area. Consider dobutamine stress if low-flow low-gradient AS suspected.
Tachycardia	Minimal effect on AVA. Major effect on the MITRAL GRADIENT — a short DFP raises the mean gradient sharply. Gorlin MVA is theoretically DFP-invariant (DFP sits in the denominator and also drives the gradient), so the practical risk is measurement error, not bias: DFP is short, so a small timing error moves the answer a lot.
Atrial fibrillation	DFP varies beat-to-beat. Average ≥ 5–10 consecutive cycles.
Echo–Cath discordance	Often due to: different CO methods, SEP/DFP assumptions, or pressure recovery (echo < cath area).
PCWP for LA pressure	PCWP may overestimate true LA pressure (V-wave, overdamping). Consider direct LA measurement if available.
Estimated VO₂	125 mL/min/m² assumption may be ±25% off. Small VO₂ error → large AVA error. Measure when possible.

Worked Example — AVA

Given: CO = 4.2 L/min | HR = 72 bpm | Ejection time = 0.32 sec/beat | Mean gradient = 45 mmHg

Step 1: SEP = 0.32 × 72 = 23.0 sec/min
Step 2: √45 = 6.71
Step 3: AVA = (4.2 × 1000) ÷ (44.3 × 23.0 × 6.71) = 4200 ÷ 6836 = 0.61 cm² → Severe AS

Worked Example — MVA

Given: CO = 4.8 L/min | HR = 80 bpm | Filling time = 0.42 sec/beat | Mean gradient = 12 mmHg

Step 1: DFP = 0.42 × 80 = 33.6 sec/min
Step 2: √12 = 3.46
Step 3: MVA = (4.8 × 1000) ÷ (37.7 × 33.6 × 3.46) = 4800 ÷ 4383 = 1.10 cm² → Moderate MS

Safe Report Documentation Language

"Valve area calculated using Gorlin equation with Fick / thermodilution cardiac output"
"SEP/DFP measured from simultaneous LV-Ao / LV-LA tracings" (or: "estimated based on HR")
"Findings interpreted in the context of flow conditions"