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The Single Ventricle
Karim Rafaat, M.D.
The title “single ventricle” includes those lesions designated as both
HLHS
HRHS
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 VSD
hypoplasia 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
Genetics
Familial 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
Epidemiology
Uniformly 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 ASD
Systemic venous blood mixes with pulmonary venous blood in the RA and RV
RV ejects blood into a large MPA
Systemic circulation is supplied in parallel with pulmonary circulation through a PDA
Multiple obstructions to systemic flow
Aortic valve atresia
Arch hypoplasiaPlace 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
Pathophysiology
Relative 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 ventricle
Remember 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 ago
Is still main option of treatment in many countries
Staged reconstructionStage I Norwood Procedure
Stage II Bi-directional Glenn or Hemi-Fontan
Stage III Fontan Procedure
Transplant
Goals of Surgery
Unobstructed systemic blood flowTo 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 dysfunction
Elevated PVR or pulmonary artery pressure
AV valve regurgitation
Ventricular 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 resistance
This 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 function
LCOS
Low Cardiac Output
Low systemic cardiac output can be due toGlobally decreased ventricular function
Elevated Qp:Qs
AV 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 ratio
The ratio can be altered by maneuvers discussed in my last talk
Total CO can be increased by careful selection of vasoactive agents
Want to avoid tachycardia and increasing afterload
Milrinone
Nesiritide
Dopamine
Hypoxemia
Pulmonary Venous desaturationAtelectasispulmonary 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 LV
When pulmonary flow is supplied by a shunt from a systemic artery, increases in SVR lead to increased pulmonary flow, and increased diastolic pulmonary run-off
This 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 1981
Reintroduced by Japanese surgeon Sano in the late 1990’s
The Sano modification
Directly 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 BTS
Improved 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-off
Less ventilatory manipulations were required for Qp:Qs management33/36 survived to BDG vs 14/20 in the BTS group
Other Considerations
Risk for shunt occlusionLow 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….
Stage II – Partial Cavopulmonary Anastomosis
After stage I, there are two problemsCyanosis
Excessive 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 months
Furthermore, 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 sensitive
A 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 remodeling
Also allows for adjustment of the upper body venous and lymphatic systems to deal with an increase in venous pressure prior to the Fontan
Usually performed around 4-6 months of age
Bidirectional Glenn
The RV/PA or BT shunt is removed
This volume unloads the ventricle
Critical in improving outcome in single ventricle palliation
SVC is anastomosed end to side with the RPA
Is more compatible with an extracardiac fontan procedure down the line
Hemi-Fontan
Similar to BDG physiologically
Has additional proximal SVC and inferior RPA anastomosis
RA communication closed with a patch
More suited for eventual lateral baffle Fontan
Stage II Physiology
Half the blood to the heart comes from the IVC, half from the pulmonary veins
Qp:Qs is now 0.5
SaO2 about 75-85%Infants with bigger heads have higher sats
Excessive volume load is now eliminated
Ventricle now pumps only Qs
Decreased 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 pressure
Qp must pass through two highly regulated vascular beds
Pulmonary and cerebral
Transpulmonary pressure gradient
Mean pulmonary arterial pressure – mean atrial pressure
Represents the driving force through the lungs
Low PVR allows for a low delta P
Which means lower SVC pressures
Pulmonary flow can be impaired by
High PVR
Increased atrial pressures
AV valve dysfunction
Ventricular 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 FRC
Remember 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 function
Elevated Qp:Qs
AV valve regurgitation
Loss of AV synchrony
Low Cardiac Output
Careful choice of inotropes
Passive pulmonary blood flow occurs mostly during diastole
Tachycardia is bad
Alpha agonists work on both pulmonary and systemic vascular beds
Increase in PVR will decrease preload
Increase in SVR increases afterload
Both bad in the post-op single ventricle
Low dose dopamine and milrinone are good choices
Elevated SVC Pressure
As 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 desaturation
Systemic venous desaturation
Decreased 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..
Stage III - Fontan
The 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 RA
If 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 bypass
Atrial 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 occurs
Maintains cardiac output when it is compromised by decreased pulmonary venous return
Fontan Physiology
Qp:Qs is equalVentricle 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 PEEP
Aim is usually to allow pt to do as much of the work of breathing as possible
Low set rate with high PS
Low Cardiac Output
Discerning cause made easier by “physiologic” RA and LA lines
SVC/RPA and common atrial pressure lines
Low Cardiac Output
CausesInadequate preload
Low SVC and atrial pressuresElevated PVR
High SVC and low atrial pressureAnatomic obstruction
Low atrial and high SVC pressuresPump failure
High 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
Milrinone
Dopamine
nesiritide
Cyanosis
AGAIN……
Pulmonary Venous desaturationAtelectasis
pulmonary edema
pneumothorax
Systemic venous desaturationAnemia
Low cardiac output
Decreased pulmonary blood flowElevated PVR in those with fenestration
Pulmonary venous hypertension
Arrhythmias
Atrial 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, 1998
Schwartz S et al, Single Ventricle Physiology, Critical Care Clinics 2003;19:393-411
Walker SG, et al, Single Ventricle Physiology – Perioperative implications, Seminars in Ped Surg, 2004, 188-202
