Definition: either natural or palliated situation where blood to the pulmonary circulation is supplied in parallel rather than serial to the systemic circulation by the same pumping chamber(s).
Anatomy: A single ventricle (SV) physiology or parallel circulation (PC) exists in:
a. True SV anatomy maintained by natural (ductus arteriosus or MAPCAS) or artificial systemic-to-pulmonary artery shunts (classic or modified Blalock-Taussig shunt/central shunt/Sano shunt).
b. Two ventricle anatomy with obstructed pulmonary or systemic outflow (PA/VSD, IAA, critical PS, AS, coarctation) maintained by natural (ductus arteriosus or MAPCAS) or artificial systemic-to-pulmonary artery shunts (classic or modified Blalock-Taussig Shunt, central shunt).
c. Two ventricle anatomy with unobstructed pulmonary or systemic outflow, but large, non-restrictive intra- or extracardiac shunt lesion (ASD, VSD, AVSD, AP window, truncus arteriosus, PDA).
Nature foresees a maximally dilated pulmonary vascular bed (e.g. nitric oxide pathway) compared to a variably constricted systemic vascular bed (PVR«SVR), therefore, the common pathophysiologic feature in PC is an unrestrictive pulmonary blood flow with increased pulmonary (Qp) compared to systemic (Qs) blood flow – Qp/Qs>1 – resulting in volume overload, and subsequent failure of the systemic ventricle(s).
The acuity of the latter is dependent on the underlying anatomy (a>b>c), age of the infant (physiologic drop in PVR after birth), associated cardiac lesion (AV valve competency), and concurrent illness (bronchiolitis etc).
Further specific pathophysiologic consideration:
Ad a. More critical heart failure in single right ventricle anatomy. Deep cyanosis due to mixing at various levels. Potential for acute pulmonary stealing in the distressed child, with sudden loss of systemic cardiac output.
Ad b. Moderate cyanosis. Impaired gut perfusion with increased risk of NEC in the lesions with systemic outflow obstruction.
Ad c. Pulmonary vascular endothelial dysfunction with vascular hyper-reactivity and mainly postoperative risk of PHT.
Whether pre- or postoperatively, core concept in managing SV physiology or PC is to balance the circulation, i.e. to optimise systemic output and control pulmonary blood flow. The optimal Qp/Qs for a balanced circulation is 0.8-1. The latter can be estimated from formula (SaO2-SmvO2)/(SpvO2 – SpaO2).
Simplification 1: SmvO2 = SpaO2, and SpvO2 = 100.
Simplification 2: if SaO2 >80 in room air, then Qp : Qs >1.0
General considerations: anticipate low cardiac output syndrome (clinical, lactate, ΔSmvO2 [30 ± 5], ΔpCO2 [7 ± 1]), optimise oxygen balance (DO2↑ [HR x Hb x SaO2 x PL x Contr x AL] ≈ VO2↓ [MV, analgesia, sedation, paralysis, normo-/hypothermia]) early, and avoid resuscitating inadequate haemodynamic indices late.
Manipulation of Qp: PVR↑ (SaO2 ≤80, pCO2↑, PEEP↑, IT↑, Hct↑).
Manipulation of Qs: SVR↓ (vasodilatation).
 Cardiol Young 2003;13:316-322. Lawrenson J et al. Manipulating parallel circuits.
 Cardiol Young 2004;14(Suppl 1):52-60. Nelson DP et al. Neonatal physiology of the functionally univentricular heart.
 Arch Dis Child Fetal Neonatal Ed 2005;90:F97-F102. Theilen U et Shekerdemian L. The intensive care of infants with hypoplastic left heart syndrome.
 NeoReviews 2011;10(5):e239-e244.Mastropietro CW et al. Parallel Circultations: Managing Single-Ventricle Physiology.
 Congenit Heart Dis. 2012 Sep-Oct;7(5):466-78. Lowry: Resuscitation and perioperative management of the high-risk single ventricle patient: first-stage palliation.
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