Abstract
Importance: The goal of the cardiovascular system is to maximize delivery of oxygen to organs and tissues to meet their metabolic demands. Disruption of this normal cardiac function is common in a variety of pediatric disease states that require management in an intensive care unit. These include shock, heart failure, arrhythmias, and congenital heart disease. A thorough understanding of cardiovascular physiology is essential in the successful management of these cardiovascular derangements. This chapter reviews the hemodynamic principles of circulatory flow, the processes that regulate the mechanical function of the heart, the determinants of cardiac output, the cardiopulmonary interactions of the heart, and the relationship between the right and left side of the heart.
Observations: Our understanding of cardiovascular function and regulatory mechanisms as it relates to the management of pediatric patients in the intensive care unit has improved over recent years. The incorporation of these new physiologic principles into therapeutic and surgical interventions has been important in the management of pediatric cardiovascular disease by critical care providers.
Conclusions and Relevance: An understanding of cardiovascular physiology is the foundation of medical and surgical management of pediatric patients in the intensive care unit.
Key words
Cardiovascular physiology, hydrodynamics, myocardial contraction, cardiac output, cardiopulmonary interaction, ventricular interaction
The primary function of the cardiovascular system is to deliver sufficient amounts of oxygen and other nutrients to organs and tissues to meet their metabolic demands. Disruption of this normal cardiovascular function is commonplace in a variety of pediatric critical illnesses, including heart failure, cardiac arrhythmias, congenital heart disease, and shock. To manage these and other disease states, many pharmacologic and surgical interventions are routinely used in the intensive care unit that directly affect the cardiovascular system. A comprehensive understanding of cardiovascular physiology is therefore essential for all practitioners involved in the care of critically ill children.
The purpose of this chapter is to review essential physiologic concepts of the cardiovascular system. It will summarize hemodynamic principles that control circulatory flow, the basic processes that govern the mechanical function of the heart, and the critical determinants of cardiac output. It will also explore vital cardiopulmonary interactions and important crosstalk between the right and left sides of the heart.
Hemodynamic Principles
The foundation of cardiovascular physiology is based on the principles of fluid mechanics and hydrodynamics. These concepts are fundamental in explaining the relationship between blood flow, blood pressure, and the physical properties of the systemic circulation. An understanding of these principles is crucial to the care of critically ill children with cardiovascular disorders.
Flow, Velocity, and Area
The velocity of blood in the circulation is dependent on flow through a vessel and the vascular cross-sectional area of that vessel. Velocity describes the rate of fluid movement in terms of distance per unit time (e.g., cm/s). The flow through the circulation is determined by volume of blood per unit time (e.g., cm 3 /s or L/min). In a circulatory system with blood vessels of varying cross-sectional dimensions, velocity of blood is expressed by:
v = Q A
For a constant flow the velocity through a blood vessel is inversely proportional to the cross-sectional area of that blood vessel. Accordingly, the highest velocity in the circulatory system occurs in the aorta and the lowest velocity in the capillary bed, which has a very large total cross-sectional area ( Fig. 13.1 ).
Pressure, Flow, and Resistance
Blood flows through a vascular segment in the circulation only when the inflow pressure (P i ) and outflow pressure (P o ) are different, creating a driving pressure (ΔP) between the two ends. However, friction between the vessel walls and the moving fluid creates an intrinsic vascular resistance (R) in the circulatory system. The relationship between flow, driving pressure, and resistance can be described by the basic flow equation, a variation of Ohm’s law:
Q = ( P i − P o ) R
Flow through a vessel is inversely proportional to the vascular resistance and is dependent on the existence of pressure gradients in a vascular segment. This basic equation can be applied to not just a single blood vessel, but also the vascular bed of an organ or the entire systemic circulation.
French physiologist Jean Poiseuille derived one of the most fundamental laws describing the relationship between various physical properties of liquids and fluid flow, which can be extrapolated to the circulatory system. Poiseuille’s law describes the relationship of the various physical factors to flow as:
Q = ( P i − P o ) π r 4 8 η L
Even small changes in the diameter of a vessel will result in very large increases in flow. Changes in the contractile state of the vasculature can therefore be significantly altered and play an important regulatory role. Conversely, increases in blood viscosity and vessel length will result in a decrease in blood flow.
By applying Poiseuille’s law to Ohm’s law, it can be determined that resistance to flow is affected solely by vessel dimensions (length and radius) and blood viscosity, as explained by the hydraulic resistance equation:
R = ( P i − P o ) Q = 8 η L π r 4
Animal studies have confirmed these relationships. Targeted pressure measurements showed a progressive pressure decline from the aorta to the venous circulation, with the largest drop occurring in the arteriole system (see Fig. 13.1 ). Thus, given constant blood flow through the circulation, resistance must be greatest at the level of the arteriole system because of the smaller radius of the arterioles and the fact that the arteriole system exists in series. The total resistance of a system (R total ) in series is equal to the sum of the individual resistances:
R total = R 1 + R 2 + R 3 + … + R n
1 R total = 1 R 1 + 2 R 2 + 3 R 3 + … + 4 R n
This can be used to explain the physiologic concept of vessel recruitment. As more vessels are recruited to regional capillary beds in parallel—such as in skeletal muscle during active exercise—resistance to flow will decrease. Conversely, the loss of vessels in parallel—such as in end-stage pulmonary arterial hypertension—will result in increased pulmonary resistance.
Laminar and Turbulent Flow
Poiseuille’s law is most applicable to steady and streamlined flow of Newtonian fluids. Blood normally flows through the circulation in this orderly fashion—called laminar flow. In this type of motion, fluid moves as a series of layers with differing velocities. Velocity along the central axis of the vessel is the highest and decreases to zero at the wall of the vessel. The parallel layers of fluid with different velocities move in straight streamlined paths resulting in minimal energy expenditure.
When blood is forced to move at higher velocities through a narrow opening, irregular motions of blood flow develop in a vessel—called turbulent flow . In this type of motion there is rapid mixing and much friction. Therefore the vessel’s resistance to blood flow is substantially higher, an increased pressure is needed to drive blood flow, and the cardiovascular system must expend more energy to generate the increase pressure.
A dimensionless number in fluid mechanics called the Reynolds number (R e ) can predict whether blood flow will conform to a laminar or turbulent flow state. This number represents the ratio of inertial forces to viscous forces within a fluid flowing through a cylindric tube:
R e = 2 ρ vr η
Vessel Wall Shear Stress
Alterations in the hemodynamic conditions inside blood vessels can lead to the development of stresses near the vessel walls: (1) circumferential stress due to the phasic pressure of blood flow and (2) shear stress on the vascular endothelium of a vessel wall due to the changes in blood flow mechanics. These stresses, in combination with vascular wall compliance and endothelial-induced vasodilation, also help to regulate blood vessel diameter.
Shear stress is the force applied by the blood against a vessel wall and is determined by the following equation:
τ = γ ˙ η
is shear rate (in s -1 ).The determination of shear stress on a surface is based on the fundamental assumption of fluid mechanics that the velocity of fluid on the surface is zero (no-slip condition). This assumption leads to the establishment of a velocity gradient, with the velocity of the fluid particles increasing the further they move away from the vessel wall. The maximum velocity of fluid particles is then reached at the location in the vessel that is furthest removed from the vessel walls. If we consider the blood vessel as a straight cylindric tube with nonelastic walls, then the shear rate, the rate at which neighboring layers of fluid move with respect to each other, can defined as:
γ ˙ = du / dr
Through mathematical derivation the shear stress on the wall can be related to blood flow (Q) by the Hagen-Poiseuille equation:
τ = 4 η Q π r 3
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