McGoon Ratio
The McGoon ratio is calculated as the sum of the right and left pulmonary artery diameters (measured at their origins before branching) divided by the diameter of the descending aorta at the diaphragm, with a threshold of ≥1.5-1.6 traditionally considered adequate for Fontan candidacy, though modern evidence suggests smaller pulmonary arteries should not preclude the operation.
Definition and Calculation
The McGoon ratio quantifies pulmonary artery size relative to systemic circulation. The measurement involves:
- Numerator: Right pulmonary artery diameter + Left pulmonary artery diameter (measured at the prebranching point)
- Denominator: Descending aortic diameter (measured at the diaphragm level)
This ratio is typically obtained via cardiac catheterization angiography or cross-sectional imaging (CMR, CT) 1, 2.
Traditional Fontan Candidacy Criteria
Historically, strict selection criteria for Fontan operation included adequate PA size, with McGoon ratio ≥1.5-1.6 considered necessary 3. The ESC guidelines specify that "adequate PA size" is among the critical factors for optimal outcomes, alongside low pulmonary vascular resistance (PVR), mean pulmonary artery pressure <15 mmHg, preserved ventricular function, minimal AV valve regurgitation, and normal rhythm 3.
The ACC/AHA guidelines similarly emphasize that successful Fontan outcomes depend on meeting defined hemodynamic criteria, though they do not specify an absolute McGoon ratio cutoff 4.
Modern Evidence: Reconsidering Size Thresholds
Recent high-quality research demonstrates that small pulmonary arteries should not be considered an absolute contraindication to Fontan operation. A 2014 study of 146 patients found that those with McGoon ratios ≤1.6 (or modified ratios ≤1.2) were not at higher risk for:
- Prolonged mechanical ventilation (p=0.87)
- Extended ICU stay (p=0.52)
- Longer hospital stays (p=0.08)
- Pleural effusions (p=0.25)
The authors concluded there is "no need to postpone the FO due to smaller PAs" 5.
Clinical Implications and Pitfalls
Key Considerations:
Pulmonary arteries naturally decrease in relative size after Glenn and Fontan procedures 1. In one study, the McGoon ratio decreased from 2.5 pre-Glenn to 2.14 post-Glenn (p<0.008), and further declined to 1.4 at mean 7.4 years post-Fontan (p<0.0001). This suggests PA size should not be an absolute limiting factor.
Discontinuous pulmonary arteries are not a contraindication. A series of 12 patients with discontinuous PAs who underwent centralization achieved 100% survival post-Fontan with median McGoon ratio of 1.65 (range 1.0-2.1), with all patients achieving NYHA class I status 6.
Modified measurements matter. Some centers measure the narrowest PA diameters rather than prebranching diameters, yielding lower "modified" McGoon ratios. The threshold for modified measurements is approximately ≤1.2 5.
Common Pitfalls to Avoid:
Do not delay Fontan solely based on small PA size. Prolonging palliative stages to "grow" pulmonary arteries comes at the expense of chronic volume overload on the single ventricle and complications from prolonged cyanosis 5.
Recognize that McGoon ratio is only one factor. The ratio weakly correlates with post-repair hemodynamics (R²=0.4 in tetralogy repair) 2. Other factors—particularly PVR, ventricular function, and AV valve competence—are equally or more important 3.
Consider the Nakata index as complementary. The Nakata index (sum of PA cross-sectional areas divided by body surface area) provides additional information, with traditional threshold of ≥150 mm²/m² (modified ≥100 mm²/m²) 1, 5.
Algorithmic Approach to Fontan Candidacy
When evaluating Fontan suitability:
Measure McGoon ratio via catheterization or CMR (Class I indication prior to Fontan if no contraindications to non-invasive assessment) 7
If McGoon ratio <1.5-1.6: Do not automatically exclude patient; assess comprehensively:
- Mean PA pressure (<15 mmHg preferred) 3
- PVR (low essential)
- Ventricular function (preserved required)
- AV valve regurgitation (none to mild)
- Presence of PA distortion requiring intervention
Consider intervention for PA stenosis even without measurable gradients, as obstruction can suppress flow without creating pressure differences 8
Proceed with Fontan if hemodynamics favorable, regardless of borderline McGoon ratio, given modern evidence showing small PAs do not predict poor early outcomes 5
The decision should prioritize overall hemodynamic suitability rather than fixating on a single anatomic measurement, as modern surgical techniques and perioperative management have made the operation safer even with smaller pulmonary arteries.