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Background and Aim The microcirculation consists of arterioles, capillaries, and venules <150 µm, a major terminal vascular network of the systemic circulation. However, little is known about the microvascular structure and function in the ACHD population.Methods We evaluated the microcirculation in 22 adults with a Fontan circulation (26±6 years), 20 adults post childhood Tetralogy of Fallot (TOF) repair (31±10 years), and 25 healthy adult controls (31.7±10 years). Handheld Capillary Microscopy (HVCS, Honiton, UK) was used to assess the vessels in the hands, feet, nailfolds, and oral cavity. Basal and Maximal Capillary Density (BCD & MCD) were recorded & measured on the dorsum of the hand and feet after venous congestion. Central blood pressure, augmentation index, and pulse wave velocity (PWV) were measured using Mobil-O-Graph® device (Stolberg, Germany).Results Significant functional and structural capillary rarefaction was observed in both TOF (BCD-mean difference -11 capillaries/field, p = 0.014; MCD-mean difference -17 capillaries/field, p = 0.002) and Fontan patients (BCD-mean difference -18 capillaries/field, p = 0.001; MCD-mean difference -24 capillaries/field, p = 0.001) compared to healthy controls ( figure 1). Both Fontan and TOF adults have visible thickening of the capillaries, increased looping, distortion, and widening of venous limb of the capillary loop in the sublingual/nailfold/buccal microcirculation (figure 1). These adverse effects were more accentuated in the Fontan group (figures 2 & 3). Fontan patients tended to have lower Systolic BP (-8.6 mmHG, p<0.008) and Central BP (-11.7 mmHg, p<0.010), and PWV (-0.56 millisecond, p<0.029) compared to those with Tetralogy.Conclusions Both Fontan and TOF adults have significant structural capillary rarefaction, with the adverse microvascular changes more accentuated in Fontan patients. Comparison with other CHD groups may clarify whether this is a casual association with CHD or a response to circulatory challenges such as high systemic venous pressure.Abstract 494 Figure 1The microvascular changes in the skin, nailfold, buccal, and the sublingual area in a healthy volunteer, TOF adult, and a Fontan adult. Fontan adult shows structural skin capillary rarefaction, thickening of the capillaries, increased looping, distortion, and widening of venous limb of the capillary loop in the sublingual/nailfold/buccal microcirculation. TOF adult show similar changes, but to a lesser extent compared to the Fontan adultAbstract 494 Figure 2The box plot showing the statistically significant dilatation of the venous limb of the capillaries in the nailfold of the Fontan adults compared to the healthy adults and the TOF adults