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

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

3. The Newborn

4. 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

5. 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)

6. 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

7. HLHS

8. 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)

9. 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

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

11. 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

12. Norwood Operation

13. 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

14. 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

15. 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

17. 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

21. 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

22. 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

23. 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

24. 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

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

26. Manipulation of PVR and SVR

27. Manipulation of PVR and SVR
Subatmospheric oxygen (FiO ) 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

28. 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

29. 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

30. 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

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

32. 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

33. Bidirectional Cavopulmonary Anastomosis

34. 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

35. Hemi-Fontan

36. Bidirectional Glenn

37. 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

38. 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

39. 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

40. 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

41. 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

42. Total Cavopulmonary Anastomisis

43. 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

44. Fontan Operation

45. Fontan Operation

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

47. 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

48. 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

49. 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

50. THE END