Chapter 3: Hemodynamics
3.1 Hemodynamic Parameters
The behavior, health, and status of the cardiovascular system can be described in many ways. One of the most popular techniques is to evaluate certain cardiovascular parameters that may be used as an index to cardiovascular fitness. They are as follows:
Cardiac Output (CO):
Cardiac output is one of the most basic measurements of cardiovascular function. It is usually defined as the volume of blood ejected per minute by either ventricle, and since the two ventricles are usually in balance, the two outputs are essentially equal.
It is generally recognized that cardiac output increases approximately in proportion to an increase in body metabolism. This is demonstrated by the fact that cardiac output increases during exercise and fever and is decreased during sleep and with conditions such as hypothyroidism.
In the healthy resting human adult, mean cardiac output is approximately 5.6 L/min. Moreover, age has an important influence on cardiac output, as is demonstrated by the fact that from its maximum value at age 27, cardiac output decreases about 1% per year. Thus, by age 65 it can be expected that cardiac output has decreased to about 60% of its value in early adulthood.
Cardiac Output (CO) is
calculated as: CO = SV X HR [L/min]
Additional Information:
Peripheral Resistance (PR):
The peripheral resistance is the effect of the vessels resisting flow. The peripheral resistance is primarily a function of vessel size and of the number of vessels open. It can be calculated by the following:
PR = MAP/CO [mmHg/(L/min)]
Compliance (C):
The compliance is a relatively new cardiovascular parameter that was developed to assess the elasticity or rigidity of the heart and arteries. The compliance is calculated as:
Change in Volume/Change in Pressure
In order to assess the systemic compliance, which is the compliance of the heart and large arteries, the change in volume of the heart (stroke volume) is divided by the change in pressure within the heart (systolic pressure minus the diastolic pressure):
C =
SV/(SBP-DBP) x [ml/mmHg]
Additional Compliance Links:
Arterial Compliance vs. Distensibility
LV dP/dt:
This is an index that is used clinically to characterize the contractile
ability of the heart. It is believed
that maximum dP/dt is a reasonable index of the initial velocity of myocardial
contraction. The maximum left ventricular
dP/dt, which is normally about 1600 mm Hg/sec, tends to be less than 1200 mm
Hg/sec in patients with disorders of the left ventricular myocardium.
3.2 Simple Fluid Mechanics
Pipe Mechanics:
To understand blood flow in the cardiovascular system, one must understand simple fluid mechanics. Pressure, flow, and resistance are all fundamental elements of fluid mechanics. The relationship between these parameters clearly defines the behavior of the blood in the heart and vessels of the human body. The fundamental equation of fluid mechanics is a derivative of Ohm’s Law. Simply stated:
Pressure = Flow x Resistance
Pressure is measured in millimeters of mercury (mmHg), flow is measures in liters per minute (L/min), and resistance is measured in millimeters of mercury per liter per minute (mmHg/L/min).
Artery Mechanics:
In order to understand blood flow at the local level, one must examine a single straight arterial segment. Therefore, the fundamental equation becomes:
Mean Arterial Pressure = Arterial flow x Peripheral Resistance
The local equation considers only the flow of blood through a single arterial segment. The arterial flow is the volume of blood flowing through the segment per minute which is dependent on both the cross sectional area of the arterial segment (a) and the velocity (u).
Arterial flow = cross sectional area (a) x velocity (u)
Therefore, as an example, a large artery with a slow flow velocity or a small artery with rapid flow velocity can maintain the same flow rate.
The peripheral resistance is the resistance of the artery to the flow and it is dependent on both the cross sectional area (a) and the elasticity (ke) of the arterial segment.
Peripheral Resistence = 1/a x 1/kehard
For example, if an artery is hard and thin it will provide great resistance to blood flow.
Cardiovascular Mechanics:
In contrast, in order to evaluate the behavior of the complete cardiovascular system one must examine the heart and all the vessels of the body. Therefore, our fundamental equation becomes:
Mean Arterial Pressure = Cardiac Output x Systemic Vascular Resistance
The cardiovascular system equation takes into account not just a single artery, but the entire vascular system. In this case, the cardiac output reflects the volume of blood exiting the heart and entering the arterial system per minute. The cardiac output is dependent on both the stroke volume (SV), which is the volume of blood ejected per contraction, and the heart rate (HR).
Cardiac Output = Stroke Volume (SV) x Heart Rate (HR)
The systemic vascular resistance is the resistance of the vasculature to the flow. Much like the peripheral resistance, the systemic vascular resistance is dependent on the cross sectional area. However, in this case it is dependent on the total arterial cross sectional (A). In addition, the elasticity of the entire arterial system (KE) must be evaluated.
Systemic Vascular Resistance =1/A x 1/Ke
Height Dependent Pressure Variations:
A fundamental property of pressure is that the pressure is the same for all points at a certain level. For example, the pressure at the surface of a pool is the same for all points. However, as the depth increases the pressure also increases as a result of hydrostatic pressure. Hydrostatic pressure is defined as the pressure due to a fluid. Therefore, the pressure at depth is defined as:
PDepth = PSurface + p x g x h
In which P Depth is defined as the pressure at a depth (h) below the surface, measured in millimeters of mercury (mmHg), r is the density of the fluid which is defined as mass of the fluid per volume, (Water = 1.0 kilograms/Liter), g is the acceleration due to gravity (m/sec 2), and h is the depth below the surface (meters).
Hydrostatic pressure not only applies to simple physical examples but also to the human body. For example, the cardiovascular system must constantly adjust to changes in pressure due to hydrostatic pressure. As one moves from a sitting to standing position, the vessels of the legs must constrict to counterbalance the additional pressure caused by an increase in height. A similar reflex may occur in the arm as it is raised, however, in this case the vessels relax to adjust to a decrease in height. In the first case, the vessels of the legs constricted due to the increase in hydrostatic load, and in the second case the vessels of the arm dilated due to the decrease in hydrostatic load.
3.3 Hemodynamic Pulse Pressure Waveforms
The arterial blood pressure pulse is one of the most fundamental physical quantities related to haemodynamics. Because of the perceived need to quantify pressure levels throughout the cardiovascular system, measurement techniques have been developed which, at times have obscured the true nature of this pulsatile phenomenon. In quantifying blood pressure oscillations, these techniques emphasize the mean value, peak values or both. Blood pressure is, however, a composite wave that undergoes significant changes in its contour while traveling within the arterial system. Thus the term "blood pressure", when used alone, does not express this wave nature.
Various civilizations in the past have used the arterial pulse as a guide to diagnose and treat various diseases. This would obviously require the classification of pulses according to its features and relating them to various disease states. The Chinese art of pulse feeling, which is still practiced, recalls 33 different patterns associated with pathology. The features that are used in classification of pulse pressure waveforms are depth, rate, force, rhythm, etc.
The adoption of arterial pulse feeling techniques by western medicine resulted in construction of intricate mechanical devices to display the arterial pulse graphically. There are five time-relative points on the waveform from which parameters relating to the heart and arterial system can be determined. They form the positions describing the foot, first shoulder, second shoulder, incisura and the duration of the pulse.
After the foot of the pulse indicating the onset of ejection—systole—the pressure wave rises to an initial peak where it forms a shoulder. It then proceeds to a second shoulder that often constitutes the peak pressure in the elderly. The former point is related to timing of peak flow while the second shoulder to reflected waves. The end of ejection is associated with closure of the aortic valve that is often seen as a distinct incisura on the aortic pressure pulse. After this there is a gradual decline in the pressure during diastole due to absence of flow from the ventricle.
Using these feature points, several indices describing the properties of the arterial system and the ventricular/vascular coupling parameters can be calculated. Some of these are heart rate and well-known pressures such as systolic and diastolic pressures, measured at time T2 and Tf, respectively. Indices related to the hardening of the arteries can also be determined from the waveform. The pulse wave velocity, arterial compliance and the steepness of the wavefront are some of these indices. For a given flow waveform, the harder the arteries get the steeper the wavefront becomes, the transmission time of the pulse to periphery decreases and the diastolic portion of the curve decays more rapidly. In elderly people with stiff arteries the pulse pressure increase, resulting paradoxical reduction in the diastolic pressure with accompanying increase in systolic pressure, Systolic Hypertension.
The DynaPulse Educational package provides a unique tool for the acquisition and analysis of arterial pressure waveforms. The following information is presented at a basic level and is intended to help you learn to appreciate hemodynamic waveforms.
Learn More About Arterial Pressure
Waveforms
Examine Abnormal EKG’s and
Corresponding Arterial Waveforms