The Single Ventricle

Karim Rafaat, M.D.

The title “single ventricle” includes those lesions designated as both

HLHSHRHS

HLHS is far more common, and the strategy for palliation of both lesions similar, so I will not mention HRHS

HLHS - History

First described in 1952 by Lev as the pathologic complex “hypoplasia of the aortic tract”, included cases of:

hypoplasia of the aorta and VSDhypoplasia of the aorta with aortic stenosis or atresia, with or without mitral stenosis or atresia

In 1958, Noonan and Nadas termed these lesions as “hypoplastic left heart syndrome”.

Embryology

The embryologic cause is not fully understood.It probably results from a limitation of either LV inflow or outflow, such as the development of severe AS early

Decreased antegrade flow through LVmost common cause is mitral atresiadecreased division of cardiac myocytes

GeneticsFamilial inheritance:

Autosomal recessive and multifactorial inheritance have both been postulated.Sibling recurrence risk: 0.5%Sibling recurrence for all other cardiac malformations: 2.2%

Definable genetic disorder (28%):Turner Syndrome Noonan SyndromeTrisomy 13, 18, 21, or other microdeletion syndromes

EpidemiologyUniformly lethal prior to 1980Each year, approximately 1000 infants with HLHS are born in the US.Prevalence: 1 per 6000-7000 live births.In pathologic series, it accounts for 1.4-3.8% of congenital heart disease.Third most common cause of critical CHD in the newborn. 23% of all neonatal mortality from CHDMale predominance: 57-70%.

Anatomy

Underdevelopment of the left side of the heart

atresia of the aortic or mitral orifice hypoplasia of the ascending aorta.

The left ventricle may be small and nonfunctional or totally atretic

Pulmonary venous return from LA to RA through a large PFO or ASDSystemic venous blood mixes with pulmonary venous blood in the RA and RVRV ejects blood into a large MPA

Systemic circulation is supplied in parallel with pulmonary circulation through a PDAMultiple obstructions to systemic flow

Aortic valve atresiaArch hypoplasia

Place systemic flow at risk

Blood flow to the coronary and cerebral circulations is retrograde

Usually little or no flow through aortic valve

Postnatal decline in PVR places systemic, and especially the ductal dependant and retrogradely supplied coronary and cerebral vascular beds at risk for hypoperfusion secondary to pulmonary run-off

PathophysiologyRelative Qp and Qs determined via resistances of respective vascular bedsVentricle must supply both Qp and Qs

Single right ventricle has at least twice the volume load of an in series ventricleSignificantly volume overloaded

The aim of initial management is to optimize Qp and Qs in a manner that provides adequate end organ oxygen delivery without overloading the single ventricleRemember my last lecture?This balancing act is only temporizing and serves to allow pt to survive to definitive treatment

Treatment options

Supportive careOnly option up to 25 years agoIs still main option of treatment in many countries

Staged reconstructionStage I Norwood ProcedureStage II Bi-directional Glenn or Hemi-FontanStage III Fontan Procedure

Transplant

Goals of SurgeryUnobstructed systemic blood flow

To maximize oxygen delivery and minimize ventricular hypertrophy

Limited pulmonary blood flowTo minimize ventricular volume load and the risk of pulmonary hypertension

Unobstructed pulmonary venous returnTo minimize secondary pulmonary artery hypertension

Minimize likelihood of pulmonary artery distortionAvoid dysrhythmias

All these goals, achieved in a timely fashion, circumvent the major risk factors for poor outcome post-Fontan:

Ventricular hypertrophy causing diastolic dysfunctionElevated PVR or pulmonary artery pressureAV valve regurgitationVentricular systolic dysfunction

The reasons why the above hurt the post-fontan heart will be discussed later

Stage I – Norwood palliation

The goal of the Norwood is to stabilize and balance the parallel circuit, protect the pulmonary vascular bed and preserve ventricular function

Adequate oxygen delivery allows for the growth necessary for a hemi-fontan or BDG to be performed

Native ascending and transverse aortic arch is incorporated into a neo-aortaNeo-aorta created by augmenting native arch with autologous pulmonary homograftNeo-aorta is attached to the proximal pulmonary artery trunk

Neo-aorta provides systemic outflowImportant that the neo-aorta is free of obstructionObstruction is poorly tolerated by the single ventricle and is associated with increased interstage mortality

Distal MPA is closed Pulmonary flow is provided by a restrictive shunt from the right innominate artery to the RPA

Modified BTS

Post-Norwood Anatomy

Post-Norwood issues

The hope is that nowRBTS + Rp = Rs

So the circulations are balanced and volume work is minimized

Meaning for a given required Qs, total Q can be less as the ratio is more favorable

But……

The ventricle has just been through hypothermic cardiopulmonary bypass with myocardial ischemia/arrestVascular endothelium of the systemic and pulmonary circulations have also been subjected to bypass and injuryCombined effect is a systemic inflammatory and adrenergic stress responseThe ventricle can also exhibit a low cardiac output syndrome in the first 12-24 hours post op

All vascular beds show signs of endothelial dysfunction

Evidenced by increased resistanceThis may tip the balance of flow towards the pulmonary circulation

Systemic oxygen demands may be unable to be met by the post-op ventricle

Leading to anaerobic metabolism, acidosis and worsening functionLCOS

Low Cardiac Output

Low systemic cardiac output can be due toGlobally decreased ventricular functionElevated Qp:QsAV valve regurgitation

How to discern between the above?

EchocardiographyEvaluates pump function and rules out AV valve regurg

Arterial-venous oxygen saturation difference

An A-V DO2 more than 40% suggests inadequate tissue delivery of oxygen and low systemic cardiac output

OR…Lactate level plus base deficitGood echo function plus high A-V DO2 = Qp>Qs

TreatmentOne must pay attention to both TOTAL CO and the Qp:Qs ratioThe ratio can be altered by maneuvers discussed in my last talkTotal CO can be increased by careful selection of vasoactive agents

Want to avoid tachycardia and increasing afterloadMilrinoneNesiritideDopamine

HypoxemiaPulmonary Venous desaturation

Atelectasispulmonary edemapneumothorax

Systemic venous desaturationAnemiaLow cardiac output

Decreased pulmonary blood flowElevated PVRPulmonary venous hypertensionPulmonary artery distortionRestrictive systemic to pulmonary shunt

Gotta rule out the top two, then, think about echo or cath to rule out the anatomic causes

Which need a surgeon….

Coronary circulation

Single right ventricle coronary blood flow occurs predominantly in diastole

Like an in series LVWhen pulmonary flow is supplied by a shunt from a systemic artery, increases in SVR lead to increased pulmonary flow, and increased diastolic pulmonary run-offThis can lead to myocardial ischemia….and sudden death

Which is why

“Leaving a kid in Norwood physiology is like taking a walk through Watts at midnight”

Dr. Cocalis The Wall of Wo

Sudden death post NorwoodUnpredictable and suddenExperienced centers report survival between 63-94%1

Inter-stage mortality of 10-15%2

Rapid fall in PVR, or increase in SVRSteal from coronary arteries

lower pressure in pulmonary circulation throughout cardiac cycle

The Journal of Thoracic and Cardiovascular Surgery 2003;126(2) 504-509Arch Dis Child Fetal Neonatal Ed 2005;90:F97-102.

• Bartram et al, Causes of Death after the Modified Norwood procedure: A study of 122 postmortem cases, Ann Thorac Surg, 1997

122 cases over 15 yearsThe leading causes of death

largely correctable surgical technical problems associated with perfusion of the lungs (36%), of the myocardium (27%), and of the systemic organs (14%).

The proposed solution to surgical manipulation of the coronary arteries the pulmonary diastolic run-off through the modified BTS

Is an RV to PA conduitFirst described by Norwood in 1981Reintroduced by Japanese surgeon Sano in the late 1990’s

The Sano modificationDirectly supplies pulmonary flow via the RV

Aortic diastolic runoff does not occurPost-op diastolic BP is higherCoronary perfusion is improved

Blood flows only during systole

Reducing total pulmonary blood flow Improves Qp:Qs, thus protecting pulmonary vascular bed and decreases volume load on the RV, giving it a greater chance to return to normal size and function

Less distortion of the pulmonary arteries than is seen with a BTSImproved growth of PA’s

Trade off’sVentriculotomy

Increases potential for low cardiac output syndromeThe damage to the ventricular wall may be offset by the better coronary perfusion…..

Increased volume load secondary to reversed diastolic flow in a non-valved conduitPossibility of shunt occlusionConcern of RV arrhythmias post ventriculotomy

Not confirmed by present studies, though

Januszewska et al, RV to PA shunt and modified BTS in preparation for hemi-Fontan procedure in children with HLHS, European Jour Cardiac Surg, 27, 2005

78 children – 27 underwent Norwood with BTS 51 underwent Sano modificationThose who underwent Sano, at time of hemi-fontan

Larger pulmonary arteriesWhich means lower resistance to the passive flow that will be supplying the lungs after the BDG or Fontan

Less RVHBetter diastolic function, and so lower filling pressures required

Lower Qp:Qs (0.8 vs 1.27)Less pulmonary vascular remodeling and less ventricular volume load

Pizzaro et al, Right Ventricle to Pulmonary Artery Conduit Improves Outcome after stage I Norwood for HLHS, Circulation, 2003; 108Retrospective cohort review

36 RV to PA conduits20 BTS

Those with RV to PA conduitsHigher diastolic BPLower PaO2

Indicating lower Qp:Qs secondary to less diastolic run-offLess ventilatory manipulations were required for Qp:Qs management33/36 survived to BDG vs 14/20 in the BTS group

Other ConsiderationsRisk for shunt occlusion

Low sats lead to high Hct’s, which increases risk of thromboembolic complicationsNeed to be well hydrated

Shunt failureSlowly occurs as pt grows, but shunt does not

Leads to slowly progessive cyanosis as oxygen consumption increases in a growing pt

VENOUS ACCESSAny venous embolus may reach systemic vascular beds Watch for air bubbles, clots, meticulously….

Glenn Hughes – A Village PersonBidirectional?

Stage II – Partial Cavopulmonary Anastomosis

After stage I, there are two problemsCyanosisExcessive ventricular volume load

The Fontan fixes both of the above, but must come after an intermediate step…Why?

Reasons for a staged repairThe fontan requires low PVR to allow for passive pulmonary flow

PVR does not reach nadir until 6-8 monthsFurthermore, following the high Qp:Qs state of pre-norwood, the pulmonary vasculature can be reactive

Which is exacerbated by the stress of bypass

The parallel circulation single ventricle is relatively hypertrophied and dilated secondary to volume overload

Shifts Frank-Starling curve down and to the right

Means the norwood ventricle is very volume sensitiveA loss of ventricular filling secondary to increases in PVR would lead to critically decreased CO

The solution is a staged procedure that allows for more gradual ventricular unloading and remodelingAlso allows for adjustment of the upper body venous and lymphatic systems to deal with an increase in venous pressure prior to the FontanUsually performed around 4-6 months of age

Bidirectional GlennThe RV/PA or BT shunt is removed

This volume unloads the ventricleCritical in improving outcome in single ventricle palliation

SVC is anastomosed end to side with the RPAIs more compatible with an extracardiac fontan procedure down the line

Hemi-Fontan

Similar to BDG physiologicallyHas additional proximal SVC and inferior RPA anastomosisRA communication closed with a patchMore suited for eventual lateral baffle Fontan

Stage II PhysiologyHalf the blood to the heart comes from the IVC, half from the pulmonary veinsQp:Qs is now 0.5SaO2 about 75-85%

Infants with bigger heads have higher sats

Excessive volume load is now eliminated

Ventricle now pumps only QsDecreased cavity dimension and increased wall thickness improves tricuspid function

Preload is not critically dependant upon unimpeded pulmonary flow

Increases in PVR won’t significantly affect systemic circulation

Qp driving force is now SVC pressureQp must pass through two highly regulated vascular beds

Pulmonary and cerebral

Transpulmonary pressure gradient

Mean pulmonary arterial pressure – mean atrial pressureRepresents the driving force through the lungsLow PVR allows for a low delta P

Which means lower SVC pressures

Pulmonary flow can be impaired by

High PVRIncreased atrial pressures

AV valve dysfunctionVentricular diastolic dysfunction

A low transpulmonary gradient with a good CO means good things for sleep….

Post-Op issues – Ventilator Management

Excessive Paw will limit systemic venous return via increased intrathoracic pressure increase PVR, potentially decreasing pulmonary flow AND increases SVC pressure

Minimize iT, PIP and choose PEEP that allows for maintenance of FRCRemember that Qp comes through the cerebral vascular bed…

So maneuvers like alkalosis and hyperventilation to decrease PVR may INCREASE cerebral vasc resistance, decreasing flow and further exacerbate hypoxemia

Low Cardiac Output

Low systemic cardiac output can be due toGlobally decreased ventricular functionElevated Qp:QsAV valve regurgitationLoss of AV synchrony

Low Cardiac Output

Careful choice of inotropesPassive pulmonary blood flow occurs mostly during diastole

Tachycardia is badAlpha agonists work on both pulmonary and systemic vascular beds

Increase in PVR will decrease preloadIncrease in SVR increases afterloadBoth bad in the post-op single ventricle

Low dose dopamine and milrinone are good choices

Elevated SVC PressureAs evidenced by upper compartment plethora and edemaDDx

Obstruction at anastomosisPulmonary artery distortionElevated PVR

Elevations in SVC pressure limit cerebral blood flowCPP = MAP – SVC pressure

Combined with hyperventilation / alkalosis to maintain low PVR, perfusion decrease is more markedProlonged SVC pressure elevation can lead to cerebral edema, worsening the above

3% NaCl / Mannitol

Hypoxemia

Three broad categories of cause:Pulmonary Venous desaturationSystemic venous desaturationDecreased pulmonary blood flow

I just brought these up again because I like the organization of thinking here…I’m trying to ram this one home..

Dis ees a fontan

Stage III - FontanThe final step in single ventricle palliationWhen is the stage II patient ready?

Usually about 6 months following stage IIIncreasing growth and activity increases venous return from lower limbsWhen the ventricle has remodeled and displays good function on echoGood AV valve functionSmall transpulmonary pressure gradient Low ventricular end diastolic pressureMost centers aim for completion between 12-24 months of age

Lateral baffleBlood from IVC directed into RPA via a baffle in the lateral portion of the RAIf the preceding operation was a hemi-fontan, then IVC to RPA continuity is achieved by removing the RA patch

Extracardiac FontanA conduit of PTFE tubing or aortic allograft is placed between IVC and RPA

AdvantagesLimited bypassAtrial arrythmias less common

DisadvantageConduit cannot increase in size as pt. grows

In both operations, poor ventricular compliance or increased PVR is a concern

Both lead to decreased pulmonary flow, and, possibly, decreased preload

Often a fenestration is left in the baffle or conduit

When systemic venous pressure increases, increased shunting of blood through the fenestration occursMaintains cardiac output when it is compromised by decreased pulmonary venous return

Fontan PhysiologyQp:Qs is equal

Ventricle supplies QsSystemic venous pressure drives Qp

Shunt through fenestration lowers SaO2 slightlyPulmonary flow is non-pulsatile

Increases pulmonary vascular bed impedanceDecreases capillary recruitment

Flow through pulm vasc bed dependant upon those factors that lend to a low transpulmonary pressure gradient

Good ventricular systolic and diastolic functionAV valve competenceLow PVR

Elevated pulmonary artery pressures leads to higher fluid replacement to maintain high SVC pressures

Third spacing leads to effusions, ascitesAcsites increases required ventilator pressures, decreases renal perfusion

Post-Op Issues - Ventilator Management

Pulmonary flow is impeded by a high PVRPositive pressure ventilation increases PVRPEEP in excess of that required to maintain FRC increases PVR

Minimal PEEPAim is usually to allow pt to do as much of the work of breathing as possible

Low set rate with high PS

Low Cardiac OutputDiscerning cause made easier by “physiologic” RA and LA lines

SVC/RPA and common atrial pressure lines

Low Cardiac OutputCauses

Inadequate preloadLow SVC and atrial pressures

Elevated PVRHigh SVC and low atrial pressure

Anatomic obstructionLow atrial and high SVC pressures

Pump failureHigh SVC and atrial pressuresCan be due to

AV valve regurgitationVentricular dysfunctionLoss of A-V synchronyVentricular outflow obstruction

Poor CO can lead to acidosisAcidosis contributes to increased PVR, leading to further desaturation, worse O2 delivery, more acidosis….etc

Careful use of vasoactive agents that increase pump function without increasing afterload

MilrinoneDopaminenesiritide

Cyanosis

AGAIN……Pulmonary Venous desaturation

Atelectasispulmonary edemapneumothorax

Systemic venous desaturationAnemiaLow cardiac output

Decreased pulmonary blood flowElevated PVR in those with fenestrationPulmonary venous hypertension

ArrhythmiasAtrial and ventricular pacing wires are in placeLoss of AV valve synchrony is bad

Decreases CO, increases required transpulmonary pressure gradient

Given the extent of this lecture, and how vast a subject arrythmias are……

You got the wires….and the box…figure something out.Usual bothersome arrhythmias are atrial in nature, so pacing the atria at a rate higher than the intrinsic rate will fix the issueSame goes for a fast junctional rate….

References

Chang et al, Pediatric Cardiac Intensive Care, LWW, 1998Schwartz S et al, Single Ventricle Physiology, Critical Care Clinics 2003;19:393-411Walker SG, et al, Single Ventricle Physiology – Perioperative implications, Seminars in Ped Surg, 2004, 188-202