Editing Tetralogy of Fallot (section)

=== Hypercyanotic "Tet" spells ===
Infants and children with unrepaired tetralogy of Fallot may develop hypercyanotic,<ref>{{cite journal | pmc=5672507 | date=2017 | last1=Saini | first1=V. | last2=Samra | first2=T. | title=Persistent postoperative hypercyanotic spells in an adult with surgically untreated tetralogy of Fallot: Use of ketamine infusion | journal=Journal of Anaesthesiology Clinical Pharmacology | volume=33 | issue=3 | pages=412–413 | doi=10.4103/0970-9185.173324 | doi-access=free | pmid=29109649 }}</ref> or "tet," spells.<ref name="Abdulla_2011" /> Patients with prominent subvalvar muscle bundles and/or conal tissue in the right ventricular outflow tract are thought to be at higher risk for hypercyanotic spells. These are acute spells characterized by profound cyanosis, often in the setting of agitation or tachycardia, that may progress to loss of consciousness or cardiac arrest if not aborted.<ref name="Munoz_2010"/>{{rp|200}} This may be initiated by any event – such as anxiety, pain, dehydration, or fever<ref name="Munoz_2010">{{cite book | vauthors = Munoz R, Morell V, Cruz E, Vetterly C |title=Critical care of children with heart disease : basic medical and surgical concepts|date=2010|publisher=Springer-Verlag |isbn=9781848822627|location=London|pages= 18 |oclc=663096154}}</ref> – that leads to an increase in dynamic muscular obstruction of the right ventricular outflow tract. This in turn leads to decreased blood flow through the right ventricular outflow tract to the lungs and increased shunt of deoxygenated blood from the right ventricle to the left ventricle and, subsequently, to the systemic circulation.<ref name="Abdulla_2011" /> The pathophysiology of these episodes is multifactorial; increased sympathetic activation (from pain, agitation, fever, etc.) leads to increased myocardial contractility, which worsens dynamic muscular obstruction of the right ventricular outflow tract, and increased heart rate (tachycardia), which allows less time for right ventricular diastolic filling. Right ventricular outflow tract obstruction is more likely to occur in a relatively underfilled ventricle with increased contractility of the outflow tract myocardium. A relative decrease in systemic vascular resistance, as may be observed in distributive or neurogenic shock, may also precipitate hypercyanotic spells by increasing shunt from the right ventricle to the left ventricle.

Clinically, hypercyanotic spells are characterized by a sudden, marked increase in cyanosis and may progress to [[Syncope (medicine)|syncope]].<ref name="Munoz_2010" />{{rp|200}}

Older children will often [[Squatting position|squat]] instinctively during a hypercyanotic spell.<ref name="Abdulla_2011" /> This increases [[systemic vascular resistance]] and allows for a temporary reversal of the [[Cardiac shunt|shunt]]. It increases pressure on the left side of the heart, decreasing the right to left shunt. The decreased shunt volume results in a decrease in deoxygenated blood flow entering the systemic circulation and an increase in deoxygenated blood flow antegrade through the obstructed right ventricular outflow tract.<ref name="Squatting: the hemodynamic change i">{{cite journal | vauthors = Murakami T | title = Squatting: the hemodynamic change is induced by enhanced aortic wave reflection | journal = American Journal of Hypertension | volume = 15 | issue = 11 | pages = 986–988 | date = November 2002 | pmid = 12441219 | doi = 10.1016/S0895-7061(02)03085-6 | doi-access = free }}</ref><ref name="pmid5638470">{{cite journal | vauthors = Guntheroth WG, Mortan BC, Mullins GL, Baum D | title = Venous return with knee-chest position and squatting in tetralogy of Fallot | journal = American Heart Journal | volume = 75 | issue = 3 | pages = 313–318 | date = March 1968 | pmid = 5638470 | doi = 10.1016/0002-8703(68)90087-2 }}</ref>