Single Ventricle Palliation #1: Pre–Stage I Physiology

Date
June 2, 2026
URL
https://x.com/WithAScalpel/status/2061772101707604097?s=20
Categories
Single Ventricle

Single Ventricle Palliation #1: Pre–Stage I Physiology

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Before Stage I palliation, neonatal single ventricle physiology is a fragile parallel circulation in which one functional ventricle must support both the pulmonary and systemic vascular beds. This differs fundamentally from normal biventricular physiology, where pulmonary and systemic circulations are arranged in series.

In pre–Stage I physiology, the central hemodynamic question is not simply whether the arterial oxygen saturation is “good.” The more important question is whether the circulation is balanced enough to provide adequate systemic oxygen delivery. In hypoplastic left heart syndrome (HLHS), this balance is particularly critical because systemic blood flow is often dependent on the ductus arteriosus, and pulmonary venous return must cross an adequate atrial communication to reach the systemic ventricle [1, 2].

The infant’s stability depends on several interacting factors:

  1. Ductal patency
  2. Adequate atrial-level communication
  3. Pulmonary vascular resistance
  4. Systemic vascular resistance
  5. Degree of intracardiac mixing
  6. Total output capacity of the single ventricle
  7. Adequacy of systemic oxygen delivery

1. One Ventricle Supporting Two Circulations

In normal physiology, the right and left ventricles work in series:

RV → pulmonary circulation → LV → systemic circulation

In pre–Stage I single ventricle physiology, the single ventricle ejects mixed blood into both the pulmonary and systemic circulations. Therefore, the total ventricular output is divided into pulmonary blood flow and systemic blood flow:

COsv = Qp + Qs

where:

  • COsv = total cardiac output of the single ventricle
  • Qp = pulmonary blood flow
  • Qs = systemic blood flow

This means that the single ventricle is not responsible only for systemic perfusion. It must generate enough total output to supply both the lungs and the body. As a result, the ventricle is chronically volume-loaded, and any excessive pulmonary blood flow can directly reduce effective systemic perfusion [3].

2. Complete Mixing and the Meaning of Oxygen Saturation

In many forms of pre–Stage I single ventricle physiology, systemic venous blood and pulmonary venous blood mix at the atrial, ventricular, or great arterial level. When mixing is complete, the oxygen saturation of blood entering the pulmonary and systemic circulations becomes similar:

O₂ saturation: PA = Ao

This equality does not mean that pulmonary blood flow and systemic blood flow are equal. It only means that the blood distributed to both vascular beds has similar mixed oxygen content.

This is a key clinical point. Arterial oxygen saturation alone cannot reliably define the adequacy of systemic perfusion. A neonate may have a relatively high saturation because Qp is excessive, while systemic blood flow is compromised. Conversely, a lower saturation may be acceptable if systemic perfusion and oxygen delivery are preserved [4, 5].

Thus, oxygen saturation should be interpreted together with systemic perfusion, lactate, acid-base status, urine output, blood pressure, and venous oxygen saturation when available.

3. Qp/Qs Balance: The Core Hemodynamic Problem

The ratio of pulmonary blood flow to systemic blood flow is expressed as:

Qp/Qs

In pre–Stage I single ventricle physiology, Qp/Qs is governed mainly by the relative balance between:

  • Pulmonary vascular resistance
  • Systemic vascular resistance

A useful conceptual framework is:

  • Low PVR → increased Qp → pulmonary overcirculation
  • High PVR → decreased Qp → cyanosis or pulmonary under-circulation
  • High SVR → relative diversion of flow toward the lungs
  • Lower SVR → improved systemic runoff, but only if systemic pressure and ventricular function remain adequate

Mathematical models of HLHS physiology suggest that systemic oxygen availability is often optimized when Qp/Qs is close to, or slightly below, 1.0 rather than when pulmonary flow is excessive [3]. Therefore, the goal is not maximal pulmonary blood flow. The goal is a balanced circulation that preserves systemic oxygen delivery.

4. Pulmonary Overcirculation

After birth, pulmonary vascular resistance normally falls. In a neonate with single ventricle physiology, this physiologic fall in PVR may cause excessive pulmonary blood flow.

When Qp becomes too high, several problems occur:

  1. Systemic blood flow decreases

    2. Blood is preferentially distributed to the pulmonary vascular bed, reducing effective systemic perfusion.

  2. Single ventricular volume load increases

    3. Excessive pulmonary venous return increases preload to the single ventricle.

  3. Myocardial oxygen demand rises

    4. The single ventricle must pump a larger total output.

  4. Systemic oxygen delivery may worsen despite higher saturation

    5. A high SpO₂ may reflect pulmonary overcirculation rather than healthy systemic perfusion.

Clinically, pulmonary overcirculation may present with:

  • Higher-than-expected oxygen saturation
  • Tachypnea
  • Increased work of breathing
  • Poor feeding
  • Low urine output
  • Cool extremities
  • Metabolic acidosis
  • Rising lactate
  • Worsening ventricular function

A commonly used clinical saturation range in balanced preoperative HLHS physiology is approximately 75–85%, although the target must be individualized based on perfusion and systemic oxygen delivery rather than saturation alone [1, 6].

5. Pulmonary Under-Circulation

The opposite problem is inadequate pulmonary blood flow. This may occur when PVR is high, pulmonary blood flow is anatomically restricted, the ductus is constricting, or mixing is inadequate.

Pulmonary under-circulation may result in:

  • Low arterial oxygen saturation
  • Cyanosis
  • Low systemic arterial oxygen content
  • Tissue hypoxia
  • Metabolic acidosis

However, low saturation does not always mean low systemic blood flow. In a completely mixed circulation, systemic oxygen delivery depends on both arterial oxygen content and systemic flow. Therefore, a patient with modest cyanosis but good perfusion may be more stable than a patient with higher saturation but poor systemic output [4, 5].

The clinical goal is oxygen delivery, not saturation alone.

6. Ductal Patency

In HLHS and other ductal-dependent systemic circulations, the ductus arteriosus provides the essential pathway from the pulmonary artery to the descending aorta and systemic circulation.

If the ductus constricts, systemic output can deteriorate rapidly. Clinical findings may include:

  • Weak or absent lower-extremity pulses
  • Hypotension
  • Worsening acidosis
  • Oliguria
  • Shock
  • Cardiovascular collapse

For this reason, prostaglandin E₁ is a central component of initial stabilization in ductal-dependent systemic circulation. The purpose is to maintain ductal patency and preserve systemic blood flow until surgical or catheter-based palliation can be performed [1, 2, 6].

In classic HLHS with mitral atresia and aortic atresia, the pulmonary artery effectively becomes the source of both pulmonary and systemic outflow, while the ductus arteriosus supplies the systemic circulation.

7. Atrial Communication

Adequate atrial-level communication is essential in HLHS. Pulmonary venous blood returns to the left atrium, but because the left-sided structures are hypoplastic, that blood must cross the atrial septum to reach the right atrium and systemic right ventricle.

If the atrial septum is restrictive or intact, pulmonary venous hypertension develops. This is a high-risk physiology.

A restrictive atrial septum may cause:

  • Severe cyanosis
  • Pulmonary edema
  • Pulmonary venous congestion
  • Respiratory failure
  • Profound hemodynamic instability

In this setting, urgent decompression of the left atrium may be required. Options may include catheter-based atrial septal intervention, atrial septal stenting, or surgical septectomy depending on institutional strategy and patient anatomy [7, 8].

This point is important because not all desaturation in HLHS is caused by low pulmonary blood flow. Desaturation may also reflect pulmonary venous desaturation from pulmonary edema or inadequate atrial egress.

8. Three Mechanisms of Desaturation

In preoperative single ventricle physiology, desaturation should be analyzed physiologically rather than treated reflexively with oxygen.

Major mechanisms include:

  1. Diminished pulmonary blood flow

    2. This may occur from elevated PVR, ductal restriction, or inadequate pulmonary blood supply.

  2. Low mixed venous saturation

    3. This reflects poor systemic oxygen delivery, increased oxygen extraction, low cardiac output, anemia, or increased metabolic demand.

  3. Pulmonary venous desaturation

    4. This may occur with pulmonary edema, atelectasis, lung disease, or pulmonary venous hypertension from restrictive atrial communication [2].

This framework is clinically useful because each mechanism requires a different response. Increasing FiO₂ may help in some cases, but in pulmonary overcirculation it can worsen systemic perfusion by further lowering PVR.

9. Ventilation Strategy and Vascular Resistance

Ventilation strongly influences Qp/Qs balance. In pre–Stage I physiology, respiratory management is not simply supportive; it is hemodynamic therapy.

Factors that tend to decrease PVR and increase pulmonary blood flow include:

  • High FiO₂
  • Respiratory alkalosis
  • Hyperventilation
  • Low PaCO₂
  • Improved lung recruitment when atelectasis is present

Factors that tend to increase PVR and reduce pulmonary blood flow include:

  • Hypercarbia
  • Acidosis
  • Hypoxia
  • Atelectasis
  • Excessive intrathoracic pressure
  • Pulmonary venous hypertension

Historically, strategies such as controlled hypoxia or permissive hypercarbia have been used to limit pulmonary overcirculation. However, these interventions must be applied carefully because they may also affect myocardial function, cerebral oxygenation, and systemic oxygen delivery. Studies comparing inspired gas strategies in preoperative HLHS suggest that the impact of manipulating FiO₂ and CO₂ should be assessed by systemic oxygen delivery rather than arterial saturation alone [9].

Thus, ventilation should be individualized. The goal is not a normal blood gas in isolation. The goal is a balanced Qp/Qs with adequate systemic perfusion.

10. Monitoring Systemic Oxygen Delivery

Because arterial saturation alone is incomplete, bedside assessment should integrate multiple markers.

Important clinical and laboratory markers include:

  • Arterial oxygen saturation
  • Systemic venous oxygen saturation, when available
  • Blood pressure
  • Pulse pressure
  • Capillary refill
  • Urine output
  • Lactate trend
  • Base deficit
  • Near-infrared spectroscopy
  • Ventricular function
  • Clinical work of breathing

Systemic venous saturation and arterial saturation together can provide a more complete estimate of oxygen extraction and systemic oxygen delivery [5]. A rising lactate, increasing base deficit, falling urine output, or declining systemic venous saturation should prompt concern for inadequate systemic oxygen delivery even if SpO₂ appears acceptable.

A practical bedside question is therefore:

Is this saturation associated with adequate systemic perfusion, or is it masking pulmonary overcirculation and systemic steal?

11. Surgical Meaning of Pre–Stage I Physiology

Stage I palliation is designed to convert an unstable ductal-dependent circulation into a more controlled surgical circulation.

In HLHS, Stage I reconstruction generally aims to establish:

  1. Reliable systemic outflow

    2. The native pulmonary root and reconstructed aorta become the systemic outflow pathway.

  2. Controlled pulmonary blood flow

    3. Pulmonary blood flow is supplied through a calibrated source, such as a modified Blalock–Taussig–Thomas shunt or an RV-to-PA conduit.

  3. Unrestricted atrial-level mixing

    4. The atrial septum is opened to allow pulmonary venous return to reach the systemic ventricle.

  4. Preparation for staged cavopulmonary palliation

    5. The circulation is stabilized until Stage II palliation, typically bidirectional Glenn, can reduce ventricular volume load.

The purpose of Stage I palliation is not to create normal physiology. It is to create a survivable and more controllable circulation that supports growth and end-organ perfusion while preparing the patient for the next stage [10].

Key Points

  • Pre–Stage I single ventricle physiology is a parallel circulation.
  • The single ventricle must support both pulmonary and systemic blood flow.
  • Total single ventricular output is: COsv = Qp + Qs.
  • Under complete mixing, pulmonary artery and aortic oxygen saturations may be similar.
  • Oxygen saturation alone does not define systemic perfusion.
  • Qp/Qs is governed mainly by the PVR/SVR balance.
  • Excessive pulmonary blood flow can produce high saturation but poor systemic oxygen delivery.
  • Ductal patency is essential in ductal-dependent systemic circulation.
  • Adequate atrial communication is mandatory for pulmonary venous decompression.
  • The clinical target is balanced systemic oxygen delivery, not maximal oxygen saturation.
  • Stage I palliation converts ductal-dependent physiology into a surgically controlled circulation.

Concise Summary

Pre–Stage I single ventricle physiology is a fragile parallel circulation in which one ventricle supplies both the lungs and the body. In complete mixing, pulmonary artery and aortic saturations may be similar, but this does not mean pulmonary and systemic blood flows are equal. The key determinant of stability is the balance between Qp and Qs, largely governed by PVR and SVR. Excessive pulmonary blood flow may produce deceptively high oxygen saturation while compromising systemic perfusion. Therefore, management should focus on ductal patency, adequate atrial communication, controlled pulmonary blood flow, and preservation of systemic oxygen delivery.

References

[1] Salmon AP. Hypoplastic left heart syndrome—outcome and management. Arch Dis Child. 2001;85(6):450-451.

[2] Donnellan A, Justice L. Preoperative stabilization of infants with hypoplastic left heart syndrome before Stage I palliation. Crit Care Nurse. 2016;36(1):52-59.

[3] Barnea O, Austin EH, Richman B, Santamore WP. Balancing the circulation: theoretic optimization of pulmonary/systemic flow ratio in hypoplastic left heart syndrome. J Am Coll Cardiol. 1994;24(5):1376-1381.

[4] Francis DP, Willson K, Thorne SA, Davies LC, Coats AJ. Oxygenation in patients with a functionally univentricular circulation and complete mixing of blood: are saturation and flow interchangeable? Circulation. 1999;100(21):2198-2203.

[5] Barnea O, Santamore WP, Rossi A, Salloum E, Chien S, Austin EH. Estimation of oxygen delivery in newborns with a univentricular circulation. Circulation. 1998;98(14):1407-1413.

[6] Graham EM, Bradley SM, Atz AM. Preoperative management of hypoplastic left heart syndrome. Expert Opin Pharmacother. 2005;6(5):687-693.

[7] Atz AM, Feinstein JA, Jonas RA, Perry SB, Wessel DL. Preoperative management of pulmonary venous hypertension in hypoplastic left heart syndrome with restrictive atrial septal defect. Am J Cardiol. 1999;83(8):1224-1228.

[8] Rychik J, Rome JJ, Collins MH, DeCampli WM, Spray TL. The hypoplastic left heart syndrome with intact atrial septum: atrial morphology, pulmonary vascular histopathology and outcome. J Am Coll Cardiol. 1999;34(2):554-560.

[9] Tabbutt S, Ramamoorthy C, Montenegro LM, Durning SM, Kurth CD, Steven JM, Godinez RI, Spray TL, Wernovsky G, Nicolson SC. Impact of inspired gas mixtures on preoperative infants with hypoplastic left heart syndrome during controlled ventilation. Circulation. 2001;104(12 Suppl 1):I159-I164.

[10] Tabbutt S, Tweddell JS, Ghanayem NS. Hypoplastic left heart syndrome and other shunt-dependent single ventricle lesions. Pediatr Crit Care Med. 2016;17(8 Suppl 1):S318-S322.