Single-Ventricle Physiology Dr. Chi-Hsiang Huang Department of Anesthesiology National Taiwan University Hospital

Introduction Physiology of the newborn (pre- and postoperative) Bidirectional cavopulmonary anstomosis (bidirectional Glenn or hemi-Fontan) Cavopulmonary anastomosis (Fontan)

The Newborn

Anatomy In CHD, anatomy dictates physiology Virtually all newborns with single-ventricle physiology have mixing of pulmonary and systemic venous return The most important anatomic issue: the outflow to and from the systemic ventricle and lungs

Systemic Outflow Obstruction Hypoplastic left heart syndrome (HLHS) Tricuspid atresia with transposed great arteries Double-inlet left ventricle Critical AS, severe CoA, or IAA DORV (some variations)

Systemic Outflow Obstruction Complete mixing of systemic and pulmonary venous return Ventricular outflow directed primarily to the PA Systemic blood flow (Qs) Largely by right-to-left ductal shunting Dependent on the relative PVR and SVR Systemic outflow obstruction is poorly tolerated Usually accompanied by signs or symptoms of shock

HLHS

Pulmonary Outflow Obstruction Tricuspid atresia Pulmonary atresia with IVS TOF with pulmonary atresia Severe Ebstein’s anomaly of the tricuspid valve Critical PS DORV (some variations)

Pulmonary Outflow Obstruction Complete mixing of systemic and pulmonary venous return Ventricular outflow predominantly directed out the aorta Low pulmonary blood flow (Qp) in single-ventricle patients implies an obligate right-to-left shunt (generally atrial level) Clinical consequences of low Qp are variable

Postoperative Anatomy Goal of initial palliative surgery to establish Unobstructed pulmonary and systemic venous return Unobstructed systemic outflow Limited Qp and PA pressure

Primary surgical options Norwood Operation CPB, cardioplegia, DHCA, ischemia-reperfusion Blalock-Taussig Shunt Low diastolic arterial pressure which may compromise coronary perfusion Unilateral PA obstruction Pulmonary artery band May increase the risk of subaortic obstruction and ventricular hypertrophy

Norwood Operation

Physiology Ratio of pulmonary blood flow to systemic blood flow Total blood flow partitioned into Qp and Qs Bases on the amount of anatomic obstruction or vascular resistance

Fick Principle Qs = VO2 / (CaO2 - CmvO2) Qp = VO2 / (CpvO2 – CpaO2) Qp/ Qs = (SaO2 – SmvO2) / (SpvO2 – SaO2) Qp/ Qs = 25 / (95 - SaO2) Estimation of Qp/ Qs based on SaO2

SmvO2 SmvO2 low, (SaO2 – SmvO2) > 25 Shock: ductal dependent Qs Myocardial dysfunction following surgery When the decrease in SmvO2 is offset by increased Qp/Qs, SaO2 will remain unchanged SmvO2 monitoring following Norwood procedure

SpvO2 SpvO2 likely to be normal in the absence of clinical or CXR evidence of pulmonary parenchymal disease Unexpected pulmonary venous desaturation occurred commonly, particularly with FiO2 < 0.3 Failure to account for decreased SpvO2 results in a falsely low calculation of Qp/Qs Maneuvers that decrease SpvO2 rather than Qp/Qs result in lower SaO2 and reduced DO2 because there is no increase in Qs

Implications Maximum DO2 occurs between a Qp/Qs of approximately 0.5 and 1 and dependent on the total CO Small changes in Qp/Qs can be associated with large changes in DO2 DO2 can be improved to a far greater degree by increasing total CO than by altering Qp/Qs Once SaO2 becomes critically low, further decreases can no longer compensated for by increases in Qs

Cardiac Output Low CO (Qp + Qs) Low Qs and low SaO2 Low SaO2 with clinical signs of low CO (anuria, poor capillary refill, high ventricular filling pressure, or metabolic acidosis out of proportion to the degree of cyanosis) suggests poor cardiac function

Ventricular Dysfunction Single ventricle is volume loaded Low Qs, particularly with low diastolic blood pressure (large PDA) or a high end-diastolic ventricular pressure (volume-loaded heart or after CPB) can cause coronary perfusion pressure to become critically low Compromise systolic ventricular function and further raise EDP and lower SAP  profound hemodynamic decompensation

Manipulation of Delivered Oxygen Goal pf management: Ensure adequate DO2, not to maximize SaO2 Optimization of DO2: Maintenance of cardiac inotropy while balancing Qp and Qs and maintaining adequate BP and SaO2

Management Manipulation of Qp/Qs by manipulation of PVR Management of total CO and SVR may be more effective Keeping Hb 13-15 mg/dL can have a positive influence on DO2 Increased Hb increases SmvO2 and SaO2 and decreases Qp/Qs in single-ventricle physiology

Manipulation of PVR and SVR

Manipulation of PVR and SVR Subatmospheric oxygen (FiO2 0.17-0.19) or respiratory acidosis can effectively raise PVR, decrease SVR, and thus decrease Qp/Qs in infants with unrestricted Qp Subatmospheric oxygen may be associated with PV desaturation (particularly postoperative) Inhaled CO2 in HLHS: increased cerebral and systemic DO2 ? Infants with low PVR and anatomically restricted pulmonary blood flow

PEEP PEEP increases PVR by compressing the interalveolar pulmonary arterioles in normal lung compliance The nadir of PVR occurs at FRC rather than at zero PEEP Initial PEEP applies radial traction forces and aids vascular recruitment Increases PEEP may prevent PV desaturation by optimizing lung gas exchange and therefore decrease Qp/Qs

Manipulation of SVR Intravenous vasodilator Relatively greater effect on the systemic vasculature in poor systemic perfusion and low PVR Nitroprusside, phenoxybenzamine, inamrinone, milrinone b-stimulation of myocardium with vasodilation can further increase total CO without associated vasoconstriction Particularly valuable after DHCA Inappropriate SVR  Qp, Qs (BP, SaO2 ), masking potential warning signs of low Qs

Inotropic Support Inotropic support that increases Qs may also increase SaO2 simply by increasing SmvO2 Dobutamine (5 and 15 mg/kg/min):  Qp/Qs Epinephrine (0.05 and 0.1 mg/kg/min):  Qp/Qs Dopamine (5 and 15 mg/kg/min):  Qp/Qs Low-dose epinephrine (0.05 mg/kg/min): greatest  in PVR/SVR ratio, largely because of  SVR DO2 is increased dramatically by increasing total CO and is optimized by adjusting Qp/Qs

Combination of inotropic support and decreasing SVR is potentially the optimal strategy to maximize DO2.

High PVR Not all pulmonary overcirculation Very low Qp (PaO2 < 30 mmHg)  pulmonary dead space and impair minute ventilation Respiratory acidosis further  PVR Alveolar recruitment strategies of ventilation in atelectasis or pulmonary disease Minimum airway pressure, high-frequency jet ventilation Supplemental inspired oxygen, hyperventilation, and alkalosis Inhaled NO, iv PGE1  BP by vasoconstriction may  Qp and usually  SaO2 but at the expense of some systemic perfusion

Bidirectional Cavopulmonary Anastomosis

Anatomy Second stage of single-ventricle palliation SVC connected directly to the PA and other sources of Qp are either eliminated or severely restricted Bidirectional Glenn and hemi-Fontan anstomoses

Hemi-Fontan

Bidirectional Glenn

Physiology The driving force for Qp is SVC pressure Qp must pass through two separate and highly regulated vascular beds: cerebral and pulmonary vasculature Removes the left-to-right shunt and thus the volume load from the single ventricle

SVC Pressure Acute rise in SVC pressure Selection of patients with low PVR minimizes the risk from elevated SVC pressure Failure to maintain low SVC pressure lead to problems maintaining adequate SaO2 Small veno-venous collateral vessel contribute to arterial desaturation

Minimize SVC Pressure Minimize use of positive pressure, including PEEP, following surgery Allow the end-expiratory lung volume to approximate FRC Minimal mean airway pressure and early extubation in patient with healthy lungs Negative-pressure ventilation associated with increased Qp Higher airway pressure to maintain FRC in pneumonia or ARDS Aprotinin and modified ultrafiltration:  transpulmonary pressure gradient, less pleural drainage, improved SaO2

Vascular Resistance Qp largely dependent on resistance of 2 highly but differentially regulated vascular beds Cerebral and pulmonary vasculatures Opposite responses to changes in CO2, acid-base status, and O2 Qp dependent on venous return through SVC (largely cerebral blood flow) Hyperventilation following bidirectional cavopulmonary anastomosis impair cerebral blood flow and decrease SaO2 Inhaled NO may be the best treatment for high PVR and low SaO2 after bidirectional cavopulmonary anastomosis

Volume Unloading The right-to-left shunt is eliminated and all Qp is effective pulmonary flow An acute increase in wall thickness and decrease in cavity dimension has been associated with improved tricuspid valve function Preload and afterload are both decreased Change in ventricular geometry may increase risk for systemic outflow obstruction in some

Total Cavopulmonary Anastomisis

Anatomy Most common current approach to the Fontan operation Intracardiac lateral tunnel Less thrombogenic, possibility for growth Extracardiac conduit Without cardioplegia, less arrhythmogenic May be fenestrated

Fontan Operation

Fontan Operation

Physiology Hybrid of bidirectional cavopulmonary anastomosis and normal cardiovascular physiology Qp dependent on systemic venous pressure, and all Qp is effective Elevated PAP (> 10-15 mmHg) is associated with poor outcome, largely because it is difficult to maintain CVP in this range without large third-space losses of fluid

Fontan Fenestration Providing a source of Qs that is not dependent on passing through the pulmonary circulation Decrease PAP enough to reduce third-space losses of fluid

Low CO state Essential to determine and treat the underlying cause Obstruction to Qp Low LAP, high CVP (or large third-space fluid losses) Significant cyanosis in Fenestrated Fontan If high PVR: O2, hyperventilation, alkalosis, inhaled NO

Low CO state Myocardial dysfunction High LAP, high CVP Ischemia-reperfusion injury Poor preoperative myocardial function Inotropic agents not increase ventricular afterload Phosphodiesterase inhibitors Dobutamine Low-dose epinephrine ( 0.05 mg/kg/min) Mechanical circulatory support

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