Pulse Dynamic Waveform Analysis
Blood pressure is the most common index of cardiovascular performance utilized today. Currently, blood pressure is used in the diagnosis of a wide variety of complications associated with the cardiovascular system. However, certain disorders may not be detectable, or quantifiable, by conventional blood pressure measurement. Therefore, the potential for further advances in cardiovascular monitoring lies in determining the presence and severity of these disorders by monitoring not only blood pressure, but also additional cardiovascular parameters. The Pulse Dynamic waveform may be used to determine some of these parameters, possibly providing the physician with more information regarding cardiovascular health.
This section provides a brief review of the cardiovascular system, outlines some of the major disorders that may impair system function, and describes the latest advances in this area using Pulse Dynamics.
The heart, lungs, and associated vasculature comprise the cardiovascular system. These components work together to fulfill the primary function of the cardiovascular system: to deliver oxygen to the body. Impairment of any component affects the rest of the system, so most disorders of the cardiovascular system have widespread effects. Therefore, each of those individual components, and the interaction that occurs within the vasculature needs to be described.
Central to the cardiovascular system is the heart, which pumps blood through the body in a pulsatile fashion. The cardiovascular cycle consists of a complex pumping process that occurs during each heart beat. This pumping takes place in four separate chambers, each with their own function and structure. A description of each step in the process in included below (approximate pressures and volumes are in units of mmHg and mL, respectively):
Ventricular Diastole (End)
The ventricle fills with blood.
Aortic Pressure: Decreases from 100
Ventricular Pressure: Increases slightly from 0
EKG: P wave
Ventricular Volume: Increases slightly from 100
Atrial Contraction: The atrium contracts, forcing more blood into the ventricle.
Aortic Pressure: Continues to decrease
Ventricular Pressure: Increases rapidly
EKG: QRS complex begins
Ventricular Volume: Increases
The ventricle increases contractile tone, but there is no change in volume.
Aortic Pressure: Decreases to 80
Ventricular Pressure: Increases rapidly to 80
EKG: QRS complex main spike
Ventricular Volume: Increases to maximum of 150
When ventricular pressure exceeds aortic pressure, the aortic valve opens and blood is ejected into the aorta.
Ventricular Diastole (Beginning):
Aortic Pressure: Increases rapidly to maximum of 120
Ventricular Pressure: Increases rapidly to maximum of 120
EKG: QRS complex ends, T wave begins
Ventricular Volume: Decreases rapidly to minimum of 50
Isovolumetric Relaxation: The ventricle relaxes its tone, but remains at the same volume. The dicrotic notch (see below) may be observed in pressure waveforms, and is caused by closure of the aortic valve. This notch may provide information regarding aortic valve regurgitation. The valve closes once ventricular pressure falls below aortic pressure.
Aortic Pressure: Dicrotic notch may be observed in waveforms, followed by a pressure decrease.
Ventricular Pressure: Decreases rapidly
EKG: T wave ends
Ventricular Volume: Begins to increase
Oxygenated blood is driven through the aorta into the complex network of arteries that carry blood into the systemic circulation. The flow of blood through an artery can be compared to the flow of water through a tube. This analogy has allowed the application of fundamental fluid dynamic principles for the development of mathematical models of the circulation, in order to better understand the interaction between arterial properties such as compliance (elasticity) and peripheral resistance.
In fluid mechanics, the two poles that characterize the spectrum of flow classification are laminar and turbulent flow. Ideal laminar flow is described as slow velocity, uniform flow without disturbances such as ripples or waves. At the other extreme is turbulent flow, in which waves created by high velocity flow increase flow resistance. Disturbances caused by turbulent flow generate vibrations in the arterial wall. The Korotkoff sounds may actually be audible vibrations created by systolic turbulent flow, and the periodic silences may be characteristic of laminar flow occurring during diastole. In the body, systolic blood flow in the central arteries is generally turbulent, whereas flow in the arterioles and capillaries is usually laminar.
The interdependence of flow and cardiovascular activity may be seen in the case of arterial remodeling. If the heart generates an abnormally high velocity flow over a sufficient length of time, hypertrophy of the arterial wall occurs, increasing peripheral resistance. The heart can respond by increasing pressure to overcome the additional resistance. A long term increase in pressure can lead to a vicious cycle that causes a decrease in arterial elasticity and increasing resistance to blood flow. This may lead to further, more serious, cardiovascular disorders.
Heart failure is often the ultimate result of cardiovascular complications, and occurs when either the left or right ventricle fails to pump blood into the circulation at a sufficient rate to meet body requirements. Failure of one ventricle (right heart or left heart failure) may result in ischemia (inadequate perfusion of tissues), edema (blood congestion in one branch of the circulation), total heart failure (failure of both right and left ventricles), or ultimately, death. Some of the causes of heart failure are described below.
Ischemic heart disease is characterized by occurrence of ischemia in the myocardium (heart muscle). This may be due to any one of several factors, and indicates insufficient coronary perfusion. Common causes of ischemic heart disease are coronary atherosclerosis, aortic valvular disease, or coronary embolism. If perfusion is not restored quickly, an acute myocardial infarction may occur. An infarction is an ischemic area of cells that has not received oxygenated blood flow for some time, and is characterized by massive cell necrosis (cell death) in the ischemic area, in addition to severe impairment of tissue function in the regions surrounding the afflicted zone. In the case of acute myocardial infarction, the subsequent impairment of cardiovascular function may lead to heart failure.
Another cause of heart failure may be valvular disease. There are many potential causes for heart valve impairment, including congenital factors, bacterial infection, and other events that propagate structural or functional heart valve damage.
Valve dysfunction commonly involves the mitral or aortic valves. Failure for both of these types of valves is similar, and so a description of aortic valve failure should be sufficient to characterize the disorder.
Aortic valve failure may occur by either stenosis or regurgitation, or a combination of the two. Stenosis refers to an impairment of the valve opening, resulting in dysfunction. Regurgitation, on the other hand, occurs when the valve itself does not open or close correctly. Regurgitation often causes significant backflow, or reflux, into the ventricle during systole. A sufficiently large reflux will result in incomplete ventricular emptying.
Aortic stenosis is characterized by a narrowing of the aortic opening, and may be due to calcification caused by an abnormally-shaped valve (congenital case), by time and use (degenerative case), or by an externally caused disease such as rheumatic fever. Degenerative aortic stenosis occurs exclusively in the elderly. If the opening deteriorates sufficiently, cardiac output drops, followed by left heart failure.
Aortic regurgitation, on the other hand, is characterized by a reflux of blood from the aorta to the left ventricle during diastole. There are many causes of aortic regurgitation, including rheumatic fever, bacterial infection, and blunt chest trauma. Chronic regurgitation causes decreases in cardiac output and ventricular elasticity, and may lead to low cardiac output, edema, and heart failure. A sudden, severe, occurrence of regurgitation (due to trauma, for example) results in an immediate decrease in cardiac output. The decrease in cardiac output results in insufficient blood flow to adequately perfuse all tissues. The body responds to this situation by initiating widespread peripheral vasoconstriction, to maintain adequate perfusion to vital organs such as the brain and heart. The increased resistance created by peripheral vasoconstriction may lead to ischemia and shock. The decreased cardiac output also can cause sudden-onset pulmonary edema.
In addition to providing information regarding cardiac function, Pulse Dynamics may also be used to obtain properties of the peripheral circulation. Peripheral resistance and arterial compliance both describe properties of the peripheral circulatory system.
Arterial compliance, which is the inverse of the elastic modulus, measures the degree of arterial wall stiffness. Arterial compliance (or simply "compliance") is affected by a wide variety of circumstances, including age and certain cardiovascular diseases, such as hypertension. Therefore, a determination of a patient's compliance may facilitate the diagnosis of hypertension.
Peripheral resistance is a property common to all fluid filled "pipe" systems. There will always be a downstream resistance to the flow of a fluid through a pipe. This resistance is due to a number of properties, including properties of the fluid, the diameter of the pipe, and any turns or branching that takes place in the system. For the purposes of circulation the pipe would be the arteries and other associated vessels and the fluid would be blood.
Recently, a technique has been validated for the quantitative determination of arterial compliance. This was accomplished by comparing the Pulse Dynamic waveform compliance values to those values derived in the invasive catheterization laboratory. A method for determining peripheral resistance from the waveform is currently under development, as well. These parameters will provide the physician with additional information to diagnose hypertension and determine cardiovascular performance, without taking any additional measurements.