Ridderbos FS, Bonenkamp BE, Meyer SL, Eshuis G, Ebels T, van Melle JP, Willems TP, Berger RMF.
Heart. 2020 Feb;106(3):233-239. doi: 10.1136/heartjnl-2019-314972. Epub 2019 Sep 6.
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Take Home Points:
- The Nakata index = (left pulmonary artery cross sectional area + right pulmonary artery cross sectional area) by body surface area (mm2/m2).
- Single centre study of 39 (child and adult) patients with a Fontan circulation – mean age at time of CMR study 18 ±7 yrs.
- Cross sectional, single centre study of patients with a Fontan circulation exploring whether the Nakata index is associated with longer term functional clinical status – subjective and objective.
- The Nakata index in patients with a Fontan was significantly smaller compared to normal control subjects (n=40) – 239±79 mm2/m2 vs 330±30mm2/m2 (p<0.001).
- Nakata index was positively correlated to higher peak VO2.
- Nakata index was negatively correlated to NYHA functional class.
- Nakata index was an independent predictor of peak VO2.
- Patients with a Fontan circulation and higher Nakata index have a higher peak VO2 and are subjectively less breathless compared to similar patients with a smaller Nakata index.
Commentary from Dr. Damien Cullington (Liverpool, UK), section editor of ACHD Journal Watch: The Fontan circulation is a palliative procedure with no shortage of long-term problems. We are continually striving to understand in more detail what constitutes the ‘good Fontan’ – good ventricular function, a predominantly systemic LV, no significant valvular dysfunction and an unobstructed Fontan pathway are four key elements.
It seems relatively intuitive that patients with a Fontan circulation and larger pulmonary arteries (PAs) would be a ‘better Fontan’ compared to patients with relatively smaller PAs. This analysis from Ridderbos et al sought to investigate whether relative PA size post Fontan completion correlates to longer term subjectively reported symptoms (NYHA) and objective measurement of functional capacity (peak VO2). The authors state that no CMR based study has assessed this relationship previously. The primary end points to assess subjective and objective clinical status were NYHA class and the standard cardiopulmonary measure of peak VO2 respectively. The authors nicely illustrate the comparative difference in PA size (measured by Nakata index) in patients with a Fontan compared to the Nakata indices of normal subjects (Figure 1).
The baseline characteristics of the cohort are shown in Table 1. The mean age of patients was 18±7 yrs., 98% of the cohort had a lateral tunnel or extra-cardiac conduit type Fontan and 82% had a systemic left ventricle. This was a mixed cohort of paediatric and adult patients – the majority of patients were <20 years old with a mean duration between Fontan completion and CMR of 12 ±7 years.
Correlation between Nakata Index, baseline characteristics and functional status
Nakata index correlated negatively with age at CMR (r= -0.39, p=0.013) and time since Fontan completion (r = -0.34, p=0.034). Patients with an extracardiac conduit had a significantly higher Nakata index (p=0.03) compared to those with a lateral tunnel (269 ± 90 mm2/m2 vs 214 ± 57 mm2/m2).
The Nakata index was negatively correlated with NYHA class (Figure 2) and positively correlated with peak VO2 (indexed for weight and % of predicted peak VO2) (Figure 3). The Nakata index was a significant univariable predictor of peak VO2 and remained so in a multivariable model incorporating maximum heart rate and O2 pulse at peak exercise.
Patients with previous central PA banding (n=13) or a systemic-to-PA shunt (n=15) had smaller Nakata indices than patients who had not had such interventions (p=0.045).
Based on these results, what can we do better?
This study found that patients with a Fontan circulation who have a higher Nakata index, therefore, larger PAs, have higher functional capacity (measured by peak VO2) and lower self-reported NYHA class.
So, what can the congenital cardiology team do to help achieve better long-term functional outcomes for our Fontan patients? It is clear that some factors are easily modifiable, others not so. There may be some signal to suggest an extracardiac conduit is associated with a higher Nakata index and PAs are larger in patients who avoid PA banding or BT shunts, but naturally, some surgical interventions are a necessity. Pulmonary distensibility is an important component of functional capacity so meticulous surgical technique throughout the palliative pathway is required to avoid PA injury/scar and achieve high quality Fontan conduit anastomoses.
Patients with a Fontan circulation, compared to normal subjects have a significantly smaller pulmonary artery cross sectional area. This is probably to be due to an attenuation of growth of the pulmonary vasculature due to, atypical, non-pulsatile flow through the pulmonary arteries. Further, sequential analyses need to be performed to understand how PA size (and therefore the Nakata index) changes as patients age. This is now easily achievable since most patients with a Fontan have cross sectional imaging repeated as part of a standard protocol every 3-5 years. Repeated cross sectional imaging will help to serially reappraise whether proactive catheter interventions are required to help optimise Fontan flow. Application of 4D flow analysis may also be ancillary to decision making.