Monitoring Arterial Blood Pressure: What You May Not Know

Arterial blood pressure (ABP), a basic hemodynamic index, is often used to guide therapeutic interventions in critically ill patients. Inaccurate ABP measuring may cause misdiagnosis and mismanagement. Because of a knowledge deficit related to hemodynamic and ABP monitoring, the authors discuss ABP physiology, factors that affect ABP, and the arterial pressure waveform and its interpretation in clinical situations.Arterial blood pressure (ABP) is a basic hemodynamic index often utilized to guide therapeutic interventions, especially in critically ill patients. Inaccurate ABP measuring creates a potential for misdiagnosis and mismanagement. The results of a recent pilot study1 at 2 university-affiliated hospitals suggested a knowledge deficit in arterial pressure monitoring and some of the most basic aspects of hemodynamic monitoring. A total of 391 critical care nurses practicing in various critical care specialties were invited to participate in the study. The response rate was 17.4% (n = 68). Most of the participants were between the ages of 30 and 39 years (56.1%) and had a baccalaureate degree as their basic (61.8%) and highest degree in nursing (58.8%). Most participants had more than 4 years of nursing experience (94.1%) and critical care experience (83.9%), and most did direct ABP monitoring at least once or twice each week (97.1%). The participants were asked to complete an 18-item, criterion-referenced questionnaire on ABP physiology, technical aspects of ABP monitoring, and ABP waveform interpretation in selected pathophysiological conditions. The mean score in this pilot study was 36.7% (SD, 11.8%). Total scores ranged from 11.1% to 61.1%.Literature on nurses’ knowledge of hemodynamic monitoring is limited, but several studies,2–5 published and unpublished, indicate a general knowledge deficit in pulmonary artery pressure monitoring. Because of these research findings, in this article, we focus on areas of particular knowledge deficit related to essential principles of hemodynamic monitoring and ABP monitoring. We discuss the physiology of ABP, physiological and pathophysiological factors that affect ABP, and the arterial pressure waveform and its interpretation in clinical situations common in critical care patients.The cardiovascular system has 3 types of pressures6,7: hemodynamic, kinetic energy, and hydrostatic. Hemodynamic pressure is the energy imparted to the blood by contraction of the left ventricle. This type of pressure is preserved by the elastic properties of the arterial system. Kinetic energy is the energy associated with motion and affects the pressure measured during direct ABP monitoring. Fluid density and gravity contribute to hydrostatic pressure, which is the pressure a column of fluid exerts on the container wall. For example, in a column of fluid, the pressure at a given level in the container is proportional to the height of the fluid column above that level. The pressure is highest at the bottom of the column. In the vascular system, hydrostatic pressure is proportional to the height of the column of blood between the heart and the peripheral vasculature. In a standing person, the pressure in the leg is higher than the pressure in the arm by the difference in hydrostatic pressure. In summary, arterial blood pressure represents the force exerted by the blood per unit area on the arterial wall6–8 and is the sum of hemodynamic, kinetic, and hydrostatic pressure.The arterial tree starts with the aorta and the major branches of this vessel. The aorta and its branches stretch to receive blood from the left ventricle and recoil to distribute the blood and to maintain arterial pressure. Arteries and arterioles control blood pressure through vasoconstriction or vasodilation.6 Arterioles are the primary sites that contribute to systemic vascular resistance (SVR).6,9–11 In addition, adrenergic control of the arterioles is a major determinant of blood flow into the capillaries. In the skin, for instance, blood can be shunted from the capillary beds to flow directly from arterioles into the venous system.9 Arteriovenous shunting occurs in shock states and helps to redirect blood flow to vital organs. Arteriovenous shunting is one reason measurements of blood pressure alone are not a good indicator of peripheral tissue perfusion.Arterial pressure is measured at its peak, which is the systolic blood pressure (SBP), and at its trough, which is the diastolic blood pressure (DBP). The SBP is determined by the stroke volume, the velocity of left ventricular ejection (an indirect indicator of left ventricular contractile force), systemic arterial resistance, the distensibility of the aortic and arterial walls, the viscosity of blood, and the left ventricular preload (end-diastolic volume).11–13 The blood pressure in the aorta during systole is a clinical indicator of afterload (the sum of the forces the left ventricle must overcome to eject blood).14,15 The diastolic pressure is affected by blood viscosity, arterial distensibility, systemic resistance, and the length of the cardiac cycle.11,16Pulse pressure is the difference between systolic and diastolic pressure. A normal pulse pressure in the brachial artery is approximately 40 mm Hg. An increased pulse pressure may be the result of increased stroke volume or ejection velocity and is common during fever, exercise, anemia, and hyperthyroidism.11 Other causes of increased pulse pressure include bradycardia (increased stroke volume), aortic regurgitation, and arterial stiffening, which is most noticeable after the age of 50 to 60 years.11,17–19 An acute decrease in pulse pressure may indicate an increase in vascular resistance, decreased stroke volume, or decreased intravascular volume.11–13Systemic mean arterial pressure (MAP) is defined as the mean perfusion pressure throughout the cardiac cycle. MAP is sensed by baroreceptors located in the carotid sinuses and the arch of the aorta. These receptors control arterial pressure mainly by adjusting heart rate and arteriolar vessel radius. MAP is also the basis for autoregulation by some organ systems such as the kidney, heart, and brain. Autoregulation is the automatic adaptation of the radius of an arteriolar vessel in an organ to maintain constant blood flow over a wide range of mean pressures (60–150 mm Hg) to protect functioning of the organ.7,20 MAP is the product of SVR and cardiac output (MAP=SVR × cardiac output).7,10As indicated previously, a main determinant of SVR is the radius of arterial, and particularly arteriolar, vessels. Changes in cardiac output are related to heart rate and stroke volume. Stroke volume, in turn, is determined by several factors, including heart rate, preload, after-load, cardiac contractility, and synergy of cardiac contraction (related to ventricular dilatation, abnormalities in ventricular wall motion, and ventricular arrhythmias).10,11 MAP is generally closer to diastolic pressure because diastole represents about two thirds of the cardiac cycle when the mean heart rate is close to 60/min. This relationship is expressed in the well-known formulas MAP = DBP + (SBP -DBP)/3 and MAP = [SBP + (DBP x 2)]/3.However, the proportion of diastole in the cardiac cycle changes with changes in heart rate. In calculations of MAP for a manually obtained ABP, these formulas must be used with caution, because they provide a good estimate of MAP only when the heart rate is close to 60/min.10 Fortunately, MAP is provided by most automatic ABP measuring devices and direct ABP monitoring systems, each of which uses a system-specific method to directly determine MAP.In summary, because of the multiplicity of factors that contribute to ABP and the complexity of their interrelationships, interpreting changes in arterial pressure and its components (SBP, DBP, MAP, and pulse pressure) as indicative of any single factor may lead to an erroneous assessment of a patient’s condition. When SBP and DBP are measured using different (oscillometric or direct) monitoring methods, the values can differ significantly. When MAP is measured using different monitoring methods, however, the values are very similar, because MAP is little affected by the phenomenon of wave reflection.21–25 Wave reflection and other factors that affect measurement of SBP and DBP are discussed later.For measurement of cardiovascular pressures, 3 conventions are observed6,7,20: (1) Cardiovascular pressures are expressed in millimeters of mercury, with the exception of central venous pressure, which may be measured in millimeters of mercury or in centimeters of water. For converting values in centimeters of water to values in millimeters of mercury, the value given in centimeters of water is divided by the factor 1.36. (2) Most cardiovascular pressures, such as ABP, central venous pressure, and pulmonary artery pressure, are referenced to the heart or, more specifically, to the atria, to eliminate hydrostatic pressure. (3) All cardiovascular pressure monitoring devices are zeroed to ambient atmospheric pressure, so that the actual pressure measured reflects the pressure above atmospheric pressure.A hemodynamic monitoring system contains 2 compartments: the electronic system and the fluid-filled tubing system. Although clinicians have little control over the electronic components such as the monitor, correct setup and maintenance of the tubing system and the pressure transducer are absolutely crucial to avoid error. With an improperly prepared or inadequately functioning monitoring system, not only the actively measured hemodynamic indices but also any derived variables will be erroneous,26 potentially invalidating a patient’s entire hemodynamic profile. Three procedural steps should be followed to prepare the monitoring tubing system and ensure its continued accuracy: priming of the pressure tubing, leveling and zeroing, and dynamic response testing.The generation and recording of all arterial waveforms (systemic arterial pressure and pulmonary artery pressure) are based on the same basic principles. The invasive catheter provides access to the arterial system being monitored and is designed to pick up the pressure waves generated in the arterial system by cardiac contractions. The catheter is connected to the fluid-filled tubing of the monitoring system. The fluid column in the tubing system carries the mechanical signal created by the pressure wave to the diaphragm of the electrical pressure transducer. The transducer creates the link between the fluid-filled tubing system and the electronic system, and converts the mechanical signal into an electrical signal. The electrical signal is transmitted to the monitor and then is amplified and displayed as an waveform and as a mechanical than in the tubing system are one of the most and of in hemodynamic monitoring. most often or of the mechanical a waveform and erroneous pressure only mm in can cause waveform priming of the entire tubing system is one of the most steps to avoid technical priming starts with of all from the to from into the as a result of the pressure by the pressure the entire tubing system should be and the transducer are common of and throughout priming and of the catheter system. In to maintain the after the setup of the system, the are arterial pressure monitoring system must be referenced to heart the level of the left and at atmospheric pressure as the These can be through leveling and or of the catheter system is by the of the monitoring system the on of the with the of the The of the heart is the which can be located by the of 2 a from the at the and a through the of a from the to the of the to level the is a of The on which vascular is to be In most clinical the central arterial pressure is the pressure of because is a determinant in cardiac and In to monitor central arterial pressure, the monitoring system must be to the heart by using the that the most reflects the level of the in and The which has also used as an for the heart, is not in all and is not When the monitoring system is referenced to the of the then the pressure of a particular in the arterial tree is monitored and not central arterial pressure. measured pressure is increased by hydrostatic pressure the is of 2 the is to atmospheric pressure. the or is in this has several for the monitor atmospheric pressure as the atmospheric the level as the hydrostatic also is the but transducer or over are related to and and In to ensure leveling and must be the relationship between the and the is The reason is hydrostatic pressure. For the is above or the actual level of the left mm of hydrostatic pressure is or to the measured pressure. the is the the measured pressure will be an of the actual hemodynamic pressure by mm Hg. This that and are for all hemodynamic monitoring, they more crucial when hemodynamic pressures, such as pulmonary artery pressure mm Hg) and central venous pressure mm Hg) are being In these from the and the can cause and may to ensure once the has should be on the for correct of in a patient’s hemodynamic the same must be used for measurements may be Although several on pulmonary artery pressure monitoring indicated changes in pulmonary artery pressures related to to a recent research ABP values obtained with various when the was used as the The of in ABP values for monitoring systems referenced to the was not in the to determine a hemodynamic monitoring system can a patient’s cardiovascular pressures, the dynamic response of the catheter system must be when system also has can the waveform be as an reflection of a patient’s dynamic response of a hemodynamic monitoring system is defined by its and the The the pressure monitoring system when shock by a signal such as the arterial pressure pulse or the pressure signal by a The of a monitoring system is a of the of a system and to response is a the of 2 an indirect of the and the dynamic response of the monitoring system or These steps are in response may at experience has however, that with dynamic response can be in than 2 A provide as and may as a close (1) the between 2 of the is than then will be at least and the system will most be the between 2 is mm or then the is or and the system will with any degree of a the higher the the the the is the dynamic response of the monitoring system. (2) The system is the or the wave or occurs after the of the the system is most and indicate that the dynamic response of the monitoring system are of the arterial the and generated by the has suggested as a method for the dynamic response of a monitoring system. the waveform and may when the and of the system the system or the arterial waveform may because of physiological the dynamic response of the system a recording of the arterial When is for clinical the method of the measured ABP, or any other directly monitored hemodynamic is is to determine the dynamic response of the monitoring of the of the response are discussed in more ventricular contraction creates a pressure pulse or pulse is the pressure pulse that a when a patient’s pulse by The pressure pulse is also is sensed by the The normal arterial pressure waveform is in The systolic or mainly reflects the pressure pulse by left ventricular The pressure pulse is followed by the flow wave by the actual of blood volume. The that the at the of the reflects volume The systolic pressure is measured at the of the The is by the of the aortic and The of the to the of aortic in the cardiac cycle. For example, aortic is in with the occurs on the in patients. The also on the when arterial pressure is measured at more sites in the arterial tree The and proportion of the diastolic wave that the changes with arterial and heart rate. pressure is measured the of the systolic waveform on the monitor or on a is not only by the pressure pulse but is also a result of a phenomenon as wave Wave reflection is related to the of blood flow by the and of the arterial the to or reflection of the pressure In a to waves on the the or pressure waves and the waves The of the 2 types of waves the SBP the the blood pressure is measured in the arterial In other the of waves to the measured systolic pressure occurs in the particularly in the and the measured SBP may be to mm higher than central aortic normal of the wave is to be wave reflection an in left ventricular and cardiac In with elastic the wave to the heart during the diastolic of the cardiac cycle and artery In or with the wave to the heart during systole and systolic pressure and left ventricular vasoconstriction can increase wave reflection and cardiac of waves to the measured systolic pressure is during the and In such as occurs with for instance, measured systolic pressure may not in proportion to the actual degree of in central aortic The of may be in the of waves after the systolic but aortic pressure, and cardiac may be more through clinical of the shock with wave reflection can lead to the of central aortic pressure, because peripheral SBP may be mm higher than aortic measured SBP in this provide a of that the is perfusion A systolic and a diastolic waveform with waves may be of shock with common when with arterial is the blood pressure by the arterial catheter with the blood pressure obtained manually or with an a the arterial catheter is often to be with the that pressure obtained with the catheter be an interpretation may be and several between (1) and (2) and pressure waveforms by or monitoring systems and waveforms that the physiological of a and (3) blood pressures obtained direct indirect monitoring hemodynamic monitoring systems are is because the of the fluid column by the arterial pressure pulse on and waveform be and systems and their are With an system, the waveform its and with a or results in systolic and diastolic pressure and An waveform can also be the result of aortic or cardiac output states such as or In to determine the waveform is a result of an system or is an reflection of a patient’s the dynamic response must be systems to have a and are often of of an often not up in clinical the of hemodynamic monitoring systems on the is than An system will systolic pressures mm Hg) and diastolic most often in the of systolic (the of systolic must be in with aortic regurgitation, or states such as In these a in the systolic the dynamic response of the monitoring system. A heart rate than may also cause systolic because of the of of monitoring systems with different monitoring systems, different hemodynamic pressures can be measured the patient’s has not The often between directly and obtained SBP and DBP are also by of and waveforms and an of physiological situations when each type of waveform may are the to be is the cause of the waveform is the patient’s or the monitoring system. the for and systems is the are a main factor in waveform or In to other factors may the of a monitoring system and the signal. These factors include and of and into the monitoring system should (1) be (2) of tubing with its length to (3) not be with tubing or and have In addition, the should be of any and be and the pressure the should be at a pressure of mm Hg. This will not only from into the but also catheter is a but of monitoring. of the of may be a waveform that waveform is the should be for and of or cardiac the potential should be by blood from the any for the dynamic response with higher than the viscosity of normal also lead to of the hemodynamic For example, blood in the arterial catheter to blood or should be from the system. a general the system and should be any hemodynamic measurement is especially when clinical are to be The the most for the of the monitoring system and dynamic response and affect SBP and MAP is to these of waveform and is on the dynamic response of the catheter When all steps have to the of a system, the dynamic response or then MAP should be followed or an method of monitoring blood pressure should be arterial catheter of the ABP monitoring system The blood contains kinetic When the blood is by the of the the kinetic energy of the blood is into pressure. This pressure may 2 to mm to the systolic pressure measured by an monitoring The of directly monitored systolic pressure by kinetic energy is to as the or the of the tubing system the fluid of the system. Although the clinical of is not is that of the tubing system be at an mm Hg) between the pressure output displayed on the monitor and pressures directly from or are potentially These can lead to erroneous and are especially in with or The monitoring system between pressure during zeroing, blood and and arterial pressure all are into pressure monitoring by of the and in a monitoring system for example, electrical or may also be on the patient’s pressure clinicians can and eliminate some of these and a of waveform on a recording to the most assessment of a patient’s hemodynamic the method for hemodynamic pressure measurement is to the to changes in pressure, which affect cardiac output and systemic pressure. With pressure The decreased pressure is on the ABP as a of the waveform during venous to the of the heart is ventricular stroke volume. The increase in ventricular stroke volume is by increased pulmonary vascular and blood during left ventricular stroke volume is rate and SVR increase as a The result of this of is the phenomenon as physiological decrease in ABP of mm during An decrease in ABP than mm may indicate cardiac or also occurs in with pulmonary and heart and can be by mechanical the increase in pressure can be in the of the of the arterial pressure a mechanical is the increase in pressure may lead to of directly monitored monitoring systems determine the mean of pressure at lead to erroneous output The of on arterial pressure provides a for and recording arterial pressure, any other hemodynamic index, at the of by using a or, a is to arterial stiffening, or all of which increase the of In these physiological waves with the systolic in a pulse pressure and systolic often as a systolic in the peripheral ABP waveform In addition, the diastolic wave may be or 4 and peripheral ABP in with or In each instance, the and of the waveform may be an of systolic pressure and central aortic pressure. and on the monitoring system. The arterial waveforms in 4 and also be the result of systolic to dynamic response of the monitoring system. The result of the dynamic response provides that the ABP is is measured as the area the pressure divided by the of the of the pressure (the of a single cardiac A and systolic such as the in 4 and little to the total area the pressure can to measured the measured MAP is affected by wave reflection and the response of the monitoring system than is measured The MAP constant when measured at different sites throughout the arterial measured SBP and DBP may In MAP is a more hemodynamic and provides a more interpretation of a patient’s hemodynamic of the various of indirect ABP measurement and they with direct ABP monitoring is the of this several principles should be in this ABP monitoring pressure all indirect of ABP measurement are related to blood relationship between these 2 because they different of and In such as and indirect pressure than direct ABP SVR is in with for instance, the flow results in an indirect ABP that is higher than the directly measured such as and wave reflection contribute to direct systolic pressure that are often higher than obtained pressure In other when ABP by using different measuring methods, different results should be specifically, a between the and the arterial pressure monitoring system is not a for the functioning of the pressure monitoring system. MAP, on the is when and direct measurements are the and the direct ABP monitoring are generally in clinical direct ABP monitoring has a ABP monitoring is the only and method that and monitoring of a patient’s With of a patient’s ABP is This may be of clinical when the of pressure during mechanical or the of stroke volume in or other monitoring is a and with to nursing to ensure functioning of the monitoring system and correct interpretation of the response is the method of the of a monitoring system to hemodynamic MAP is a hemodynamic because is least affected by monitoring catheter the dynamic response of the catheter system, and wave MAP provides the estimate of central aortic pressure and is the main hemodynamic monitored by the system to control blood pressure. The value of MAP provides for its in clinical especially when of is or the of these are ABP or MAP values are not related to peripheral tissue perfusion and organ system For of obtained from assessment such as hemodynamic monitoring devices must be with from clinical assessment of to for and for and in this