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Basic Hemodynamics for Pediatric and Congenital Cardiac Care

John E. Mayer, Jr, MD

Key Points

  • Fundamental Hemodynamic Principles: Understanding the pathophysiology of congenital heart disease requires a solid grasp of hemodynamic concepts such as fluid pressure, vascular resistance, and the relationship between blood flow and pressure. These principles form the foundation for evaluating and managing pediatric and congenital cardiac patients.
  • Ohm’s law: Ohm’s law for electricity states that “current equals voltage divided by resistance”, where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms: I = V/R In physiology, this equation translates into “blood flow equals pressure divided by vascular resistance”: Q = P/R.
  • Total Fluid Energy and Gravitational Effects: Blood flow is driven by differences in total fluid energy, which includes pressure, gravitational, and kinetic energy components. Gravitational effects are particularly significant in venous return and must be considered when measuring pressures with fluid-filled catheters.
  • Vascular Resistance and Poiseuille’s Law: Vascular resistance is calculated as the pressure drop across a vascular bed divided by blood flow. Poiseuille’s law highlights the exponential impact of vessel radius on resistance, emphasizing that small changes in diameter can lead to large changes in pressure gradients, especially in areas of narrowing.
  • Laminar vs. Turbulent Flow: Blood flow can be laminar or turbulent, with turbulence more likely at higher velocities or in areas of stenosis. The Reynolds number is used to predict the transition from laminar to turbulent flow, which is clinically relevant for the detection of murmurs and bruits.
  • Calculation of Blood Flow and Shunts: The Fick principle is commonly used to calculate pulmonary and systemic blood flows, as well as shunt ratios (Qp/Qs), in congenital heart disease. Accurate calculation depends on representative blood sampling and consideration of factors affecting oxygen consumption.

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Last updated: August 11, 2025