James M. Norton Advan Physiol Educ 25:53-61, 2001. You might find this additional information useful... This article has been cited by 5 other HighWire hosted articles: Interventricular Mechanical Asynchrony in Pulmonary Arterial Hypertension Left-to-Right Delay in Peak Shortening Is Related to Right Ventricular Overload and Left Ventricular Underfilling. J. T. Marcus, C. T.-J. Gan, J. J.M. Zwanenburg, A. Boonstra, C. P. Allaart, M. J.W. Gotte and A. Vonk-Noordegraaf J. Am. Coll. Cardiol., February 19, 2008; 51 (7): 750-757. [Abstract] [Full Text] [PDF] Miniature Intracardiac Assist Device Provides More Effective Cardiac Unloading and Circulatory Support During Severe Left Heart Failure Than Intraaortic Balloon Pumping REPLY J. M. Norton Advan Physiol Educ, June 1, 2003; 27 (2): 89-90. [Full Text] [PDF] Toward consistent definitions for preload and afterload--revisited C. ROTHE Advan Physiol Educ, March 1, 2003; 27 (1): 44-45. [Abstract] [Full Text] [PDF] Cardiovascular interactions: an interactive tutorial and mathematical model C. F. Rothe and J. M. Gersting Advan Physiol Educ, June 1, 2002; 26 (2): 98-109. [Abstract] [Full Text] [PDF] Medline items on this article's topics can be found at http://highwire.stanford.edu/lists/artbytopic.dtl on the following topics: Physiology .. Heart Wall Physiology .. Blood Pressure Medicine .. Etiology Education .. Textbooks Updated information and services including high-resolution figures, can be found at: http://ajpadvan.physiology.org/cgi/content/full/25/1/53 Additional material and information about Advances in Physiology Education can be found at: http://www.the-aps.org/publications/advan This information is current as of March 1, 2008 . Advances in Physiology Education is dedicated to the improvement of teaching and learning physiology, both in specialized courses and in the broader context of general biology education. It is published four times a year in March, June, September and December by the American Physiological Society, 9650 Rockville Pike, Bethesda MD 20814-3991. Copyright © 2005 by the American Physiological Society. ISSN: 1043-4046, ESSN: 1522-1229. Visit our website at http://www.the-aps.org/. Downloaded from ajpadvan.physiology.org on March 1, 2008 K. D. Reesink, A. L. Dekker, V. van Ommen, C. Soemers, G. G. Geskes, F. H. van der Veen and J. G. Maessen Chest, September 1, 2004; 126 (3): 896-902. [Abstract] [Full Text] [PDF] P E R S O N A L V I E W TOWARD CONSISTENT DEFINITIONS FOR PRELOAD AND AFTERLOAD James M. Norton Department of Physiology and Pharmacology; University of New England College of Osteopathic Medicine, Biddeford, Maine 04005 S ADV PHYSIOL EDUC 25: 53– 61, 2001. Key words: Law of LaPlace; wall tension; wall stress; cardiac remodeling; hypertrophy Recent changes in the basic science curriculum at the University of New England College of Osteopathic Medicine have included a reduction in the number of classroom contact hours devoted to the traditional lecture format. These reductions have prompted (i.e., required) faculty to be more creative and productive in their use of their formal classroom time with students, with much more attention being paid to outlining major concepts, establishing linkages among topics, utilizing classroom case presentations and breakout groups–a variety of techniques intended to foster student understanding. One outcome of this approach is that students, in turn, are more clearly required to master much of the basic knowledge (facts, definitions, etc.) on their own, with guidance from the faculty in the form of recommended and required readings in texts, review articles, and current literature. This seemingly rational use of published written materials by students to obtain the factual underpinnings for the concepts and relationships developed in class has one consequence that is very troublesome: published sources often have very different and sometimes conflicting definitions for important physiological terms. Because medical students generally dislike ambiguity and because faculty generally strive for accuracy, such discrepancies are annoying to both groups. But if conflicts among definitions of important terms and concepts remain unresolved, students may carry into their clinical training incomplete or inaccurate working definitions of these terms that 1043 - 4046 / 01 – $5.00 – COPYRIGHT © 2001 THE AMERICAN PHYSIOLOGICAL SOCIETY VOLUME 25 : NUMBER 1 – ADVANCES IN PHYSIOLOGY EDUCATION – MARCH 2001 53 Downloaded from ajpadvan.physiology.org on March 1, 2008 ignificant differences exist among textbook definitions for the terms preload and afterload, leading to confusion and frustration among students and faculty alike. Many faculty also chose to use in their teaching simple terms such as “end-diastolic volume” or “aortic pressure” as common-usage approximations of preload and afterload, respectively, but these are only partial representations of these important concepts. Straightforward definitions both of preload and afterload that are concise yet still comprehensive can be developed using the Law of LaPlace to describe the relationships among chamber pressure, chamber radius, and wall thickness. Within this context, the term “preload” can be defined as all of the factors that contribute to passive ventricular wall stress (or tension) at the end of diastole, and the term “afterload” can be defined as all of the factors that contribute to total myocardial wall stress (or tension) during systolic ejection. The inclusion of “wall stress” in both definitions helps the student appreciate both the complexities of cardiac pathophysiology and the rationale for therapeutic intervention. P E R S O N A L may be adequate most of the time but may fail the students in novel or complicated clinical situations. V I E W ical definition and what might serve as an approximation of preload clinically according to common usage (9, 16). A similar degree of variability was observed among the textbook definitions of afterload. This term was defined as the load against which a muscle exerts force (9), the pressure in the arteries leading from the ventricles (1, 4, 8, 9, 14, 16, 17, 21, 22, 24, 25, 27–29), aortic and ventricular pressures (assumed to be identical) (15, 16, 31), myocardial wall tension or stress (15, 16, 31), peripheral resistance (6, 11, 12, 16), force needed to overcome opposition to ejection (18), output impedance (19, 20), and diastolic aortic pressure (26). As was the case for preload, some texts gave multiple definitions (16, 19) and some gave specific definitions but allowed for afterload to be approximated as aortic pressure according to common clinical usage (16, 18, 20, 27). This is where I first encountered the discrepancy between my own physiological approach to defining afterload and the choice by some clinical faculty during their lectures to use simpler terms such as “arterial pressure” or “peripheral resistance” to define afterload, terms that are only indirect and incomplete representations of the real concept. Compilations of textbook definitions of preload and afterload. To assess the extent of variability in the definitions of preload and afterload, I decided simply to compile a list of definitions for these two terms from all of the comprehensive physiology texts, cardiovascular physiology monographs, and physiology review books in a representative faculty collection (namely, the books sitting on my office shelves) as well as from two selected websites. Most, but not all, of the 29 texts I surveyed were the current editions, but for such basic concepts as preload and afterload, I did not feel that an edition or two would make much of a difference. With the use of the index, I searched in each text for the clearest statement that defined either term and generated a table summarizing my findings (Table 1). In the quotes provided in this table, words in italics or bold print appeared as such in the original text. Definitions of preload and afterload. The basis for the definitions of both preload and afterload is the Law of LaPlace (also known as the surface tension law or the Law of Young-LaPlace), stated as follows for a thin-walled spherical structure: T ⫽ PR/2, where T is wall tension, P is chamber pressure, and R is chamber radius. For a thick-walled structure such as the left ventricle, a more appropriate form of the equation would be ⫽ PR/2w, where wall stress () is related to T and wall thickness (w) as follows: T ⫽ w. With the use of the format of LaPlace’s equation, preload for the left ventricle can be best described as the left ventricular or T at the end of diastolic filling, as follows: preloadLV ⫽ (EDPLV)(EDRLV)/2wLV, where EDPLV is left ventricular end-diastolic filling pressure, EDRLV is left ventricular end-diastolic radius, and wLV is left ventricular w. The preload for the right ventricle would be described mathematically in an analogous fashion. Defined in words, therefore, preload represents all the factors that contribute to passive ventricular wall stress (or tension) at the The variability among the textbook definitions was surprising. For example, preload is variously defined as: muscle fiber tension (7, 9, 20, 22), muscle fiber stretch (1, 6, 25), muscle fiber length (3, 18), enddiastolic volume (2, 5, 12, 19, 24, 27–29), end-diastolic filling pressure (7, 9, 12, 14, 17, 21, 26, 28, 29, 31), force (3), or, as part of a description of basic muscle mechanics, weight (15). A number of texts describe preload as either volume or pressure (24, 29), and some distinguish between a strict physiolog- VOLUME 25 : NUMBER 1 – ADVANCES IN PHYSIOLOGY EDUCATION – MARCH 2001 54 Downloaded from ajpadvan.physiology.org on March 1, 2008 Such is the case with the concepts of preload and afterload, major determinants of cardiac function along with heart rate and myocardial contractility. Each year, after the completion of the pathophysiology segment of our second-year Cardiovascular System course, students begin turning up at my office door claiming that some of the clinical faculty defined preload and afterload much differently, and more simply, than I had. As I routinely do when students bring to my attention evidence of differing opinions or contrary definitions provided by faculty, I suggested that they check the textbooks and the biomedical literature to resolve the differences. This year, because of an inordinate number of complaints and confusion about the definition of afterload, in particular, I decided to follow my own advice. P E R S O N A L V I E W TABLE 1 Summary of published definitions of preload and afterload Reference Afterload Definition Textbook of Medical Physiology (9) “. . . the degree of tension on the muscle when it begins to contract . . . is the preload . . .” (p. 115) “For cardiac contraction, the preload is usually considered to be the end-diastolic pressure when the ventricle has become filled.” (p. 115) “. . . the load against which the muscle exerts its contractile force . . . is called the afterload.” (p. 115) “The afterload of the ventricle is the pressure in the artery leading from the ventricle.” (p. 115) Physiology (2) “. . . the resting muscle is stretched by a preload, which in the intact heart represents the end of filling of the left ventricle during diastole (in other words, it represents the end diastolic volume).” (p. 366) “During ejection, the afterload is represented by aortic and intraventricular pressures, which are virtually equal to each other.” (p. 366) Best and Taylor’s Physiological Basis of Medical Practice (31) “The end-diastolic pressure in the ventricle can also be equated to the preload.” (p. 220) “In the whole heart the preload should constitute the tension in the wall at the end of diastole (which determines the resting fiber length), but for practical purposes the ventricular end-diastolic volume of the ventricular end-diastolic pressure is used to indicate the preload.” (p. 227) “Afterload ⫽ wall tension (stress). Diagram showing how the systolic wall tension or wall stress (which represents the afterload in the myocardial fibers during left ventricular ejection) is affected by the geometry of the left ventricle (LV).” (p. 220, legend of Figure 2.111) Review of Medical Physiology (6) “In vivo, the preload is the degree to which the myocardium is stretched before it contracts . . .” (p. 546) “. . . and the afterload is the resistance against which blood is expelled.” (p. 546) Essential Medical Physiology (14) “The filling pressure is often termed the preload because this is the load on the muscle fibers before contraction.” (p. 192) “The afterload for the contraction is the aortic pressure . . .” (p. 192) Medical Physiology (23) “At the end of diastole the intraventricular pressure is analogous to the ‘preload’ in a simple muscle strip preparation (i.e., the weight that is suspended from such a strip to stretch it to the desired initial length).” (p. 994) “During ejection the aortic pressure is related to the ‘afterload,’ or weight that the muscle [strip] is required to lift.” (p. 994) Human Physiology (26) “The preload is given by the end-diastolic pressure . . .” (p. 387) “. . . and the afterload by the diastolic arterial pressure.” (p. 387) Human Physiology: Foundations and Frontiers (21) “In the case of the heart, the preload is the right atrial pressure . . .” (p. 376) “In the case of the heart, . . . the afterload is the aortic pressure.” (p. 376) Textbook of Physiology: Circulation, Respiration, Body Fluids, Metabolism, and Endocrinology (24) “The end-diastolic filling pressure or maximal diastolic volume (preload) is the most important determinant of stroke volume.” (p. 973) “The second intrinsic factor that determines the stroke volume is the aortic pressure or afterload.” (p. 974) Physiology (3) “The force required to stretch the muscle . . . is called the preload. The term preload is also used to indicate the length of the sarcomere or muscle before contraction.” (p. 43) “The afterload represents an impediment to the shortening of muscle fibers or to ejection in the heart. The afterload for the ventricles is the arterial pressure . . .” (p. 163) VOLUME 25 : NUMBER 1 – ADVANCES IN PHYSIOLOGY EDUCATION – MARCH 2001 55 Downloaded from ajpadvan.physiology.org on March 1, 2008 Preload Definition P E R S O N A L V I E W TABLE 1—Continued Summary of published definitions of preload and afterload Reference Afterload Definition Circulatory Physiology (28) “. . . during diastole a greater influx of blood into the ventricle will cause the ensuing contraction to be more forceful. This may be thought of as a ‘preload’ stimulus since it was applied before contraction began.” (p. 76) “. . . he [Starling] controlled the right atrial pressure (and thereby the right ventricular diastolic pressure or preload) . . .” (p. 76) “He [Starling] also controlled the aortic pressure (or afterload) by means of an artificial aortic resistance.” (p. 76) Cardiovascular Physiology (22) “During diastolic ventricular filling, for example, the progressive increases in ventricular pressure and volume combine to increase muscle tension (T ⫽ P 䡠 r) . . .” (p. 55) “End-diastolic pressure is referred to as ventricular preload because it sets the resting tension of the cardiac muscle fibers at the end of diastole.” (p. 55) “Systemic arterial pressure is often referred to as the ventricular afterload because it determines the tension which must be developed by cardiac muscle fibers before they can shorten.” (p. 56) Cardiovascular Physiology (1) “. . . the preload refers to the stretch of the left ventricle just before the onset of contraction (the so-called end-diastolic volume) . . .” (p. 65) “. . . and the afterload refers to the aortic pressure during the period when the aortic valve is open.” (p. 65) Physiology of the Heart and Circulation (18) “It is customary to refer to this length [just before contraction] in terms of the force or preload required to stretch the muscle to its precontraction length.” (p. 77) “In summary, cardiac afterload . . . is the left ventricular myocardial force necessary to overcome opposition to ventricular ejection. In the clinical setting, as a rough index, it is frequently related to aortic pressure.” (p. 177) An Introduction to Cardiovascular Physiology (15) “To study the effect of stretch, the relaxed muscle is stretched to a known length by means of a small weight or preload . . .” (p. 78) “The afterload is the stress, S [force per unit cross-sectional area of wall], during systole, and from the statement S ⫽ Pr/2w we see that it depends not only on the arterial pressure but also on chamber radius and wall thickness.” (p. 89) Modern Cardiovascular Physiology (11) “This volume [end-diastolic volume] is often termed preload because it is a load applied to the muscle fibers before they contract.” (p. 9) “This resistance [to outflow] is termed the afterload, since it is applied after contraction is initiated.” (p. 11) Cardiovascular Physiology (20) “. . . the upper end of the muscle is anchored, and a weight (preload) is suspended from the lower end. The resting force is equal to the weight attached . . .” (p. 94) “Preload, the force just prior to contraction (Chapter 3), is related in the ventricle to enddiastolic pressure.” (p. 118) “The afterload on an intact ventricle is consequently not a simple quantity, and authorities do not agree on how it should be measured or expressed.” (p. 118) “The input impedance of the systemic or pulmonary arteries is the most appropriate measure of ventricular afterload, but it is complicated to analyze and takes the form of a frequency-dependent spectrum (Chapter 6). The choice is thus between a simple variable like mean aortic pressure, which is an indirect, partial representation of the real afterload, and the more complete but complicated analysis involving in computing impedance.” (p. 118) VOLUME 25 : NUMBER 1 – ADVANCES IN PHYSIOLOGY EDUCATION – MARCH 2001 56 Downloaded from ajpadvan.physiology.org on March 1, 2008 Preload Definition P E R S O N A L V I E W TABLE 1—Continued Summary of published definitions of preload and afterload Reference Preload Definition Afterload Definition n/a “The Frank-Starling Law was established in animal studies in which a constant aortic pressure (afterload) and a constant contractility were maintained.” (p. 23) Fundamental Cardiovascular and Pulmonary Physiology (7) “The ventricle begins to contract . . . at a measurable end-diastolic pressure that represents the initial load of the ventricle or preload.” (p. 73) “Ejection begins . . . and the ventricular pressure at this point (equal to aortic pressure) represents ventricular afterload.” (p. 73) Pathophysiology of Heart Disease (16) “. . . the preload can be thought of as the amount of myocardial stretch at the end of diastole, just prior to contraction.” (p. 195) “preload: the ventricular wall tension at the end of diastole. In clinical terms, it is the stretch on the ventricular fibers just prior to contraction, often approximated by the enddiastolic volume or end-diastolic pressure” (p. 196, Table 9.1) “afterload: the ventricular wall tension during contraction; the resistance that must be over come in order for the ventricle to eject its contents. It is often approximated by the systolic ventricular (or arterial) pressure.” (p. 196, Table 9.1) “It [afterload] is more formally defined as the ventricular wall stress that develops during systolic ejection.” (p. 196) Pathophysiology of Disease: An Introduction to Clinical Medicine (19) “ ‘Preload’ is the amount of filling of the ventricle at end-diastole.” (p. 227–229) “The impedance against which the heart must work is termed ‘afterload;’ increased afterload (aortic pressure for the left ventricle) will cause a decrease in stroke volume.” (p. 227) Cardiopulmonary System (25) “In terms of muscle performance, the preload is the stretch on a muscle fiber prior to contraction.” (p. 39) “Aortic pressure (PAo, the afterload) . . .” (p. 39) Harrison’s Online (10) “In the heart-lung preparation the stroke volume within limits correlates directly with the diastolic fiber length (preload) . . .” (Chap. 232) “. . . the stroke volume of the intact ventricle [is] determined by three influences: (1) the length of the muscle at the onset of contraction, i.e., the preload; . . .” (Chap. 232) “In the intact heart the afterload may be defined as the tension or stress developed in the ventricular wall during ejection. Therefore, the afterload is determined by the aortic pressure as well as the volume and thickness of the ventricular cavity.” (Chap. 232) “ . . . the tension that the muscle is called upon to develop during contraction, i.e., the afterload.” (Chap. 232) Integrated Medical Curriculum Online (13) “Preload is end diastolic volume (EDV).” “Afterload . . . can be defined as ‘the force the heart has to overcome to eject blood.’ ” Physiological Medicine: a clinical approach to basic medical physiology (17) “Preload is the venous pressure that results in filling of the heart in diastole.” (p. 322) “Afterload is the pressure against which the heart must work to pump blood.” (p. 322) Human Physiology: From Cells to Systems (27) “The extent of filling is referred to as the preload, because it is the workload imposed on the heart before contraction begins.” (p. 292) “The arterial pressure is referred to as the afterload because it is the workload imposed on the heart after the contraction has begun.” (p. 294) VOLUME 25 : NUMBER 1 – ADVANCES IN PHYSIOLOGY EDUCATION – MARCH 2001 57 Downloaded from ajpadvan.physiology.org on March 1, 2008 Clinical Cardiology (4) P E R S O N A L V I E W TABLE 1—Continued Summary of published definitions of preload and afterload Reference Preload Definition Afterload Definition “The preload is the ventricular end-diastolic volume or pressure.” (p. 83) “The third factor that determines stroke volume is the afterload, or pressure load, i.e., the total peripheral resistance. This is the load which the heart must pump against in order to eject blood, and its magnitude is best represented by the [arterial] diastolic pressure.” (pp. 84–85) Physiology (5) “The preload for the left ventricle is left ventricular end-diastolic volume, or enddiastolic fiber length; that is, the resting length from which the muscle contracts.” (p. 127) “The afterload for the left ventricle is aortic pressure.” (p. 127) Physiology: An Illustrated Review with Questions and Answers (29) “Preload, the ventricular end-diastolic volume (or pressure), reflects how much the heart is stretched before contraction.” (p. 45) “Afterload, the pressure against which the heart must pump to eject blood, is a function of the total peripheral resistance.” (p. 45) Blond’s Medical Guides: Physiology (8) EDV (or preload) is directly affected by the amount of blood that enters the ventricle while the heart is in diastole.” (p. 156) “While afterload relates mainly to the blood pressures in the arteries . . .” (p. 157) Taber’s Cyclopedic Medical Dictionary (30) preload. In cardiac physiology, the end-diastolic stretch of the muscle fiber. In the intact ventricle, this is approximately equal to the end-diastolic volume or pressure. (p. 1585) afterload. In cardiac physiology, the stress or tension that develops in the ventricular wall during systole. (p. 51) Definitions of preload and afterload drawn from 31 different sources including textbooks, monographs, and websites. end of diastole. From this expression, one can see that end-diastolic filling pressure or end-diastolic volume (manifested in the equation above as radius) contribute to preload, but should not be equated with preload. A summary flow chart of factors contributing to preload is provided in Fig. 1. here.) From the expression above, it is clear that anything that increases left ventricular output impedance and therefore requires a greater ventricular pressure during systole (aortic stenosis, hypertension, increased total peripheral resistance, hypertrophic cardiomyopathy, etc.) will cause an increase in afterload. Also, if the chamber radius is increased as the result of increased filling during diastole or ventricular remodeling in response to chronic increases in filling pressures, afterload will be increased even if arterial pressure is normal. Arterial pressure and total peripheral resistance contribute to afterload but should not be equated with afterload. A summary flow chart of factors contributing to afterload is provided in Fig. 2. Similarly, with the use of LaPlace’s equation again, left ventricular afterload can be best described as the left ventricular or T during systolic ejection: afterloadLV ⫽ (SPLV)(SRLV)/2wLV, where SPLV is left ventricular systolic pressure and SRLV is left ventricular systolic radius. The afterload for the right ventricle would be described mathematically in an analogous fashion. Defined in words, therefore, afterload represents all the factors that contribute to total myocardial wall stress (or tension) during systolic ejection. (In vivo, both systolic pressure and systolic volume are changing constantly during the ejection phase of the cardiac cycle, and, therefore, so is afterload; but this variability during systole doesn’t significantly affect the basic arguments presented These definitions for preload and afterload fit the psychological need for conciseness and brevity, yet by their mention of wall stress, these definitions force a consideration of the Law of LaPlace and of the complex relationships among pressure, volume, and wall tension in the beating heart. The importance of focusing on wall stress in the definitions of preload VOLUME 25 : NUMBER 1 – ADVANCES IN PHYSIOLOGY EDUCATION – MARCH 2001 58 Downloaded from ajpadvan.physiology.org on March 1, 2008 Physiology: A Review with Questions and Explanations (12) P E R S O N A L V I E W and afterload (afterload, in particular) relates to the metabolic costs associated with the development of myocardial wall tension and, therefore, chamber pressure. The greater the tension requirement during systole, the greater the demand for oxygen and metabolic substrate by the myocardium. In the presence of cardiac disease, both physiological compensatory mechanisms and therapeutic regimens have as their goals the reduction of myocardial wall tension (and, therefore, myocardial oxygen consumption) and the restoration of a balance between oxygen supply and demand, especially important in patients with impaired coronary blood flow. A comprehensive definition of afterload, such as that provided here, would help students appreciate this therapeutic rationale. preload and increased afterload. If filling pressures, output pressures, and stretch (factors in the numerators of the equations described above) are loads imposed on the heart by conditions within the circulatory system, then a change in myocardial wall thickness (in the denominator) can be considered as a major myocardial response to these externally imposed perturbations. For increased preload, the additional wall stress caused by a larger chamber radius is normalized by increasing the wall thickness enough to restore the ratio EDR LV/wLV in the equation above for preload. Likewise, for an increased afterload generated by a greater output impedance requiring higher ventricular pressures during systole, the systolic wall stress is normalized by hypertrophy that restores the ratio SPLV/wLV. The relationships among pressure, radius, and wall thickness described above provide a clear physiological explanation for the different patterns of hypertrophy and remodeling seen in response to increased In conclusion, the tendency clearly exists in texts, in conversation, and even in formal lectures to use short, simple definitions of preload and afterload. Preload is defined variously as “filling pressure” or end-diastolic VOLUME 25 : NUMBER 1 – ADVANCES IN PHYSIOLOGY EDUCATION – MARCH 2001 59 Downloaded from ajpadvan.physiology.org on March 1, 2008 FIG. 1. Factors determining preload: a flow diagram illustrating the various factors within the cardiovascular system that determine preload and therefore end-diastolic myocardial passive wall stress based on the parameters in the Law of LaPlace: chamber radius, chamber pressure, and wall thickness. P E R S O N A L V I E W volume; afterload is often simplified as “total peripheral resistance” or arterial pressure. According to the above analysis, these are only components of preload and afterload and don’t tell the whole story. If, in the mind of a student, afterload is defined only as aortic pressure, then that student will not be able to appreciate fully the increases in afterload (left ventricular wall stress) and, therefore, oxygen consumption that would accompany aortic stenosis, obstructive cardiomyopathy, or ventricular remodeling associated with increased chamber radius. pathophysiology and the therapeutic approaches to heart disease. It is my contention that preload and afterload should be consistently defined in terms of myocardial wall stress (or tension) and that the definitions should always include the major factors affecting wall tension for each, namely, chamber pressure, chamber radius, and wall thickness. If you keep wall stress or “wall tension” built into your definitions of preload and afterload, you will be better able, in my opinion, to help your students understand cardiovascular Received 22 June 2000; accepted in final form 30 October 2000 The author acknowledges the students in the College of Osteopathic Medicine and in Physician Assistant and Nurse Anesthesia programs of the University of New England for many constructive criticisms and comments regarding what really works in the classroom. Address for reprint requests and other correspondence: J. M. Norton, Dept. of Physiology and Pharmacology, Univ. of New England College of Osteopathic Medicine, 11 Hill’s Beach Rd., Biddeford, ME 04005 (E-mail: [email protected]). REFERENCES 1. Berne RM and Levy MN. Cardiovascular Physiology. St. Louis: Mosby, 1997, p. 1–324. 2. Berne RM, Levy MN, Koeppen BM, and Stanton BA. Physiology. St. Louis: Mosby, 1998, p. 1–1131. 3. Bullock J, Boyle J, and Wang MB. Physiology. Philadelphia: Williams & Wilkins, 1995, p. 1– 641. VOLUME 25 : NUMBER 1 – ADVANCES IN PHYSIOLOGY EDUCATION – MARCH 2001 60 Downloaded from ajpadvan.physiology.org on March 1, 2008 FIG. 2. Factors determining afterload: a flow diagram illustrating the various factors within the cardiovascular system that determine afterload and therefore myocardial wall tension during systole based on the parameters of the Law of LaPlace: chamber radius, chamber pressure, and wall thickness. 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