Qp/Qs in Single Ventricle

Date
August 22, 2025
URL
https://x.com/WithAScalpel/status/1958865669950161245
Categories
Single Ventricle
image

Qp/Qs in the Single Ventricle

Pulmonary–Systemic Flow Ratio

Definition

The Qp/Qs ratio describes the balance between pulmonary blood flow (Qp) and systemic blood flow (Qs). It is central to shunt physiology, oxygen delivery, and hemodynamic burden in congenital heart disease, particularly in single-ventricle circulation [5].

Formula

Qp/Qs = (SatPV − SatPA) / (SatAo − SatMix)

  • SatAo: Aortic oxygen saturation
  • SatMix: Mixed venous saturation (SVC + IVC)
  • SatPV: Pulmonary venous saturation (≈100% if no lung disease)
  • SatPA: Pulmonary arterial saturation

Normal physiology: SatAo − SatMix ≈ 20–25%, reflecting usual systemic oxygen extraction [2,3].

Assumptions in the Single Ventricle (e.g., HLHS)

  1. SatAo = SatPA — a common ventricle ejects to both aorta and PA (complete mixing assumption) [1–3].
  2. No lung disease → SatPV ≈ 100% — if pulmonary gas exchange is intact [2].

Examples

  • SpO₂ 80% → Qp/Qs = 25 / (100 − 80) = 1.25
  • SpO₂ 90% → Qp/Qs = 25 / (100 − 90) = 2.5

    • 🔎 Interpretation: Even SpO₂ ≈90% may imply Qp/Qs ~2.5, i.e., substantial left-to-right (pulmonary) dominance with risk of pulmonary overcirculation and systemic underperfusion [2].

Clinical Significance

  • Balanced circulation (Qp/Qs ≈ 1.0) optimizes oxygen delivery without overloading either circuit [3,5].
  • Qp/Qs > 1.5 suggests pulmonary overcirculation → ↑ pulmonary venous return, ↑ preload, heart failure risk [1,2].
  • Qp/Qs < 1.0 favors systemic output → more cyanosis but preserved systemic perfusion when needed [5].

Broader Context & Management Levers

  • In two-ventricle physiology, Qp/Qs quantifies intracardiac/extracardiac shunts (ASD, VSD, PDA). In single-ventricle, oximetry-based estimation is practical because direct flow measurement is difficult [5].
  • Targets: Many programs aim SpO₂ ~75–85% to avoid occult pulmonary steal; higher resting SpO₂ (≥90%) can paradoxically reflect excessive Qp at the expense of Qs [5].
  • Levers to balance Qp and Qs: Fraction of inspired oxygen and ventilation (PaCO₂), shunt/band sizing, afterload/vasoactive strategy; intermittent SVC/mixed venous saturation can guide postoperative management [4,5].

Practical Caveats (Keep in Mind)

  • SaO₂ alone is an imperfect surrogate for Qp/Qs because SvO₂ and SpvO₂ vary with cardiac output, metabolic demand, and lung function; integrate lactate, end-organ perfusion, and (when available) central venous oximetry [2,6].
  • Early after Norwood, unrecognized pulmonary venous desaturation (atelectasis, edema) can confound Qp/Qs estimates and compromise oxygen delivery despite “acceptable” SpO₂ [6].
  • Pulse oximetry error increases at low saturations in cyanotic patients; trend and context matter as much as absolute numbers [7].
  • In transitional/biventricular-conversion pathways, emerging LV contribution and changing resistances break the simple mixing assumptions, so oximetry-based Qp/Qs needs cautious interpretation [8].

References

[1] Barnea O, Austin E, Richman B, Santamore W. Balancing the circulation: theoretic optimization of pulmonary/systemic flow ratio in hypoplastic left heart syndrome. J Am Coll Cardiol. 1994. doi:10.1016/0735-1097(94)90123-6.

[2] Barnea O, Santamore W, Rossi A, Salloum E, Chien S, et al. Estimation of oxygen delivery in newborns with a univentricular circulation. Circulation. 1998. doi:10.1161/01.CIR.98.14.1407.

[3] Francis D, Willson K, Thorne S, Davies L, Coats A. Oxygenation in patients with a functionally univentricular circulation and complete mixing of blood: are saturation and flow interchangeable? Circulation. 1999. doi:10.1161/01.CIR.100.21.2198.

[4] Rossi A, Sommer R, Lotvin A, Gross RP, Steinberg L, et al. Usefulness of intermittent monitoring of mixed venous oxygen saturation after stage I palliation for hypoplastic left heart syndrome. Am J Cardiol. 1994. doi:10.1016/0002-9149(94)90293-3.

[5] Magoon R, Makhija N, Jangid S. Balancing a single-ventricle circulation: ‘physiology to therapy’. Indian J Thorac Cardiovasc Surg. 2020. doi:10.1007/s12055-019-00889-w.

[6] Taeed R, Schwartz S, Pearl J, Raake J, Beekman R, et al. Unrecognized pulmonary venous desaturation early after Norwood operation confounds Qp:Qs assessment and compromises oxygen delivery. Circulation. 2001. doi:10.1161/01.CIR.103.22.2699.

[7] Pritchett CM, O’Halloran CP, Lay AS, Monahan AM, et al. Pulse Oximetry Error in Patients With Single-Ventricle Palliation. JSCAI. 2022. doi:10.1016/j.jscai.2022.100394.

[8] Yuki K, Emani S, Dinardo J. A Mathematical Model of Transitional Circulation Toward Biventricular Repair in Hypoplastic Left Heart Syndrome. Anesth Analg. 2012. doi:10.1213/ANE.0b013e31825d36a1.