Clinical Handbook of Arterial Haemodynamics

Author:    Mr S.R.Dodds MA,MS,FRCS.

Preface - Arterial Haemodynamics

Haemodynamics is the study of the forces that control the flow of blood in the vascular system and therefore forms one of the basic sciences of vascular surgery. Haemodynamics is principally concerned with blood pressure and flow; which in turn are the result of a complex interaction between the physical properties of blood and tissues, and the physiological and biochemical processes underlying homeostasis and cellular metabolism.

Patients presenting with symptomatic arterial disease represent a situation where the limits of the homeostatic mechanisms have been reached: they can no longer compensate for changes in the demand for blood by the tissues. The management of patients with peripheral arterial disease requires careful assessment and close collaboration between vascular surgeons, physicians, radiologists, anaesthetists and technologists.

A wide range of techniques based on haemodynamic theory have been developed for assessing, treating and monitoring patients with arterial disease. It is important that all the specialists involved the management of these patients understand the indications and limitations of the techniques available and how to apply them to clinical problems. However, despite the extensive published literature, there is still little practical application of haemodynamic theory to clinical problems.

In order to communicate effectively everyone involved in the management of patients with arterial disease need a common working knowledge of the principles of haemodynamics. Unfortunately much of the published work is in specialist texts or non-medical journals and there is currently no up-to-date introductory text suitable for surgeons, physicians, radiologists and vascular technologists.

Haemodynamics is a branch of fluid mechanics and requires sophisticated mathematics for a full and rigorous analysis. However, the principles that govern blood flow in normal and diseased arteries can be simplified and presented in a largely non-mathematical way.  These principles can be applied to the clinical problems of diagnosis, pre-operative assessment, treatment planning and post-operative surveillance. The management of a patient with occlusive arterial disease of the leg is a common problem that is amenable to the application of haemodynamic theory. The haemodynamics of the cerebral and coronary circulations is more complex but follow the same principles.

Research into the application of fluid dynamic theory to the vascular system is progressing rapidly and in the future powerful computer-based tools will be available to assist in interpreting haemodynamic information in the management of individual patients. The primary goal is to be able plan and predict the outcome of a given procedure for an individual patient with good reliability.

There are many excellent texts on the haemodynamics of the normal vascular tree in which the pressure/flow behaviour is approximated to that of a linear electrical circuit. However, the presence of occlusive arterial disease changes the pressure/flow behaviour in a fundamental way.  An arterial stenosis represents an element which is fundamentally non-linear in behaviour and it is therefore erroneous to assume that the diseased arterial tree acts like a linear electrical circuit.  The terms resistance and impedance that are commonly used to describe the normal arterial tree, must be used with more care when referring to diseased arteries. In addition, the essentially non-linear behaviour of diseased arteries means that numerical approximation techniques (i.e. simulations) must be used rather than purely analytical methods (i.e. formulae). It is for this reason the basic principles of numerical modelling are presented in parallel with the theoretical principles of haemodynamics. An understanding of these methods requires about the same level of mathematical skill as basic statistics, and the necessary background information is included in the appendices for those who wish to experiment with these techniques.

In addition to covering the basic haemodynamic principles, the most important technical details of the many available investigative techniques are included. In order to make most appropriate use of these investigations, it is important to appreciate their inherent limitations and to correctly interpret their results.

The main thrust of the book is to present the management of the common peripheral arterial conditions from a haemodynamic view-point, drawing on the theoretical principles to interpret clinical observations and measurements and introducing the concept of modelling as a tool to assist with the clinical decision making process. The rapid development of interventional vascular radiology over the last two decades means that radiologists are adopting both diagnostic and therapeutic roles and the boundary with surgery is becoming increasingly blurred. It is for this reason that the target audient includes vascular radiologists, vascular surgeons and vascular technologists.

As the title suggests, the scope of this book is directed mainly to peripheral arterial haemodynamics. The haemodynamic interaction between the heart and the pulmonary/systemic circulations is of great importance to cardiac surgeons and anaesthetists but falls outside the scope of this text. The structure, function and pathology of the microcirculation is only covered in outline because, although important, routine clinical assessment of the microcirculation is still largely a research technique. Similarly, the haemodynamics of the venous system is not covered, not because the principles are different, but because the the relationship between large vein pressure, flow, posture and exercise are more complex and less well understood, as are the effects of chronic venous hypertension, the aetiology of venous ulcers and and the rationale for their treatment.

In an attempt to appeal to a diverse range of readers, the book is organised in chapters that  cover the historical and epidemiological background, outlines the fundamental physical principles of haemodynamics in normal and diseased arteries and introduces the principles of evidence based clinical management. Later chapters apply these principles to the management of patients with arterial disease, starting with the assessment of the patient and moving on to the haemodynamic aspects of specific treatment options.  References to the principles of mechanics and more detailed descriptions of the analytical numerical techniques are attached as appendices for those who wish to explore in more depth.

Widespread use of worked examples through the text is used to illustrate the simple application of haemodynamic principles to clinical practice.  More complex examples are demonstrated using the results of computer simulations and  interactive simulation software is available for readers to work with and explore the more complex relationships in their own way.

It is not our goal to present a summary of the latest research but to provide a broad overview of how haemodynamic theory can be applied in daily clinical practice. Despite the inherent complexity of arterial haemodynamics the basic principles are straightforward and can be usefully applied in an empirical way.

As this approach is new and untested in a formal setting the material is presented here solely for information and education.  The interactive haemodynamic simulation tools are not intended for use in daily clinical practice and the author can accept no responsibility for their use in that context.

S.R.Dodds
Sutton Coldfield
December 2001.