Haemodynamic monitoring - Clinical Tree

Essentials

1
Haemodynamic monitoring includes observation of the complex physiology of blood flow, with the aim of providing data that can be used to improve patient management and outcomes.
2
Numerous methods are available that should be considered in a stepwise fashion, from simple clinical assessment to highly technical, invasive procedures, such as use of the pulmonary artery catheter.
3
Effective use of haemodynamic monitoring devices requires an understanding of cardiovascular physiology.
4
Currently there is a move away from simple blood pressure measurements towards targeting end-organ perfusion and the adequacy of cardiac output.
5
Use of any monitoring technology in the emergency department (ED) must consider the time associated with its introduction, the skill levels required, and the clinical benefits provided.
6
No monitoring modality improves outcome unless it is linked to a valid treatment pathway.
7
The pulmonary artery (Swan–Ganz) catheter was for many years considered a ‘gold standard’ for haemodynamic monitoring, but evidence suggests no improvements in patient outcome. It should therefore not be used in the ED.
8
Less invasive devices have been developed in recent years. Their role in the ED is yet to be fully elucidated.
9
Further developments will likely result in greater use of less invasive methods for haemodynamic monitoring, with an increased ability to monitor at the microcirculation and/or cellular level and better correlation between observed events and final diagnosis.

Introduction

Haemodynamics is concerned with the physiology of blood flow and the forces involved within the circulation. Haemodynamic monitoring involves the study of this complex physiology using various forms of technology to understand these forces and put them into a clinical context that can be used to direct therapy. The utility of basic monitoring is universally accepted. However, the maxim that ‘ not everything that counts can be counted and not everything that can be counted counts ’ (Albert Einstein, 1879–1955) should be borne in mind.

This is particularly salient in the emergency department (ED), where the pressure of work and the diversity of patients do not allow the unlimited use of complex and expensive monitoring systems.

This chapter provides an outline of current approaches to the various technologies available for haemodynamic monitoring and their applicability in the ED. Many methods are available, which should be thought of in a stepwise progression from simple clinical assessment to invasive, highly technical methods using sophisticated devices.

Historical background

As recently as 100 years ago, only temperature, pulse and respirations were measured and used to manage patients. The technology for auscultatory blood pressure measurement was available but did not come into regular use until the 1920s.

Intensive care as a medical/nursing specialty evolved in tandem with the electronic revolution of the 1960s. At the same time, increasingly sophisticated haemodynamic and laboratory techniques vastly improved diagnosis and provided a way to evaluate therapy further. Despite these major advances in the ability to monitor multiple physiological variables, there is little evidence to suggest that they have resulted in tangible improvements in patient outcome.

Practical use of monitoring

The practical use of any monitoring device must be appropriate to the individual clinical environment. Thus it may be reasonable to insert a pulmonary artery Swan–Ganz catheter in the intensive care unit (ICU), where the necessary time can be taken, yet impractical and potentially unsafe in a busy ED. Another consideration is that haemodynamic monitoring should be used only when the clinical outcome may be influenced and potentially improved. Once irreversible cellular damage has occurred, there is no benefit no matter how far therapy is maximized.

Further, haemodynamic monitoring may not improve patient outcome unless it is linked to a valid clinical management pathway. Clinicians should introduce monitoring equipment only if it will have a direct influence on their choice of therapy, as the use of invasive monitoring carries potential risks of harm to the patient. The injudicious use of physiologically based treatment protocols may lead to worse outcomes. All monitored variables must be evaluated and applied in a manner proven to lead to benefit in terms of both the diagnosis and management.

Overview of cardiovascular physiology

One possible reason that haemodynamic monitoring has not been associated with improvements in outcome is the inability to understand and manipulate patients’ physiology effectively.

Circulatory model

Haemodynamic data are traditionally considered in the context of a circulatory model. This model varies but usually consists of a non-pulsatile pump and a hydraulic circuit with discrete sites of flow resistance alongside the Frank–Starling mechanism with its concepts of preload, contractility and afterload.

Cardiac output

Cardiac output (CO) is the volume of blood pumped by the heart per unit of time, usually expressed in litres per minute (L/min). The heart operates as a pump and ejects a bolus of blood known as the stroke volume (SV) with each cardiac cycle. CO is the product of SV and heart rate (HR).

A complex set of interrelated physiological variables determines the magnitude of CO, including the volume of blood in the heart (preload) , the downstream resistance to the ejection of this blood (afterload) and the contractility of the heart muscle. However, it is the metabolic requirements of the body that are the most potent determinant of CO.

Regulation of cardiac output

The regulation of CO is therefore complex. A single measurement represents the summation of many interacting physiological processes. Basal CO is related to body size and varies from 4 to 7 L/min in adults. This value divided by the body surface area enables comparison between patients with different body sizes, giving the cardiac index (CI).

Bedside methods do not measure CO directly, meaning that the values obtained are only estimates. Assessment of CO is therefore not done routinely. Indeed, misuse of CO data may worsen outcomes. The European Society of Intensive Care Medicine Consensus on Circulatory Shock and Haemodynamic Monitoring (2014) did not recommend routine measurement of CO for patients with shock with a clear diagnosis or in patients responding to initial therapy.

Cardiac index

CI measurement is valued over simple blood pressure recording as it describes the total volume of blood flow in the circulation per unit of time and hence serves as an indicator of oxygen delivery to the tissues. The CI is also useful for understanding and manipulating the pump activity of the heart.

Role of haemodynamic monitoring in the emergency department

The role of haemodynamic monitoring in the ED is even less well defined. Given the plethora of devices but the lack of a ‘gold standard’, there are insufficient data to recommend any one method over another.

Nevertheless, the Surviving Sepsis Campaign Guidelines (2016) emphasize that resuscitation of a patient with severe sepsis, for example, should begin as soon as the diagnosis is made and not be delayed until ICU admission. The use of such an approach based on strict treatment protocols has been shown to reduce morbidity and mortality (see Chapter 2.5). Obstacles include a lack of skill to perform the initial procedures and difficulty in providing the required higher level of care due to ED staffing and patient flow constraints. However, with a potential patient stay in an ED (with finite critical care resources) of up to 24 hours, approximately 15% of critical care is already being provided in this setting.

Clinical assessment

Current guidelines on haemodynamic monitoring recommend frequent measurement of blood pressure and physical examination variables, including signs of hypoperfusion, such as reduced urine output and abnormal mental status. Clinical examination is ‘low risk’ yet may yield much important information, but the sensitivity and specificity are low, even when individual elements are interpreted in isolation. Also, clinical assessment of the circulatory state may be misleading.

Nevertheless, clinical assessment still has an important role in the initial assessment of a critically ill patient. Paradoxically, the development of haemodynamic measuring devices was driven by the poor ability to assess the critically ill patient clinically, yet patients managed simply by clinical assessment may do better than those managed with invasive, complex devices.

Key properties of an ‘ideal’ haemodynamic monitoring system include the following:

Measurement of variables that are clinically relevant
Measurements that are accurate and reproducible
Measurements that are continuous
Generation of data that are clinically interpretable and useful for guiding therapy
Operation that is simple and user-independent
Operation and utility that result in clinical benefit to the patient
Operation and utility that cause no harm to the patient
Operation and utility that are cost-effective

Clinical markers of cardiac output

The underlying issue is not what a patient’s CO is but rather whether this CO is effective for that particular patient. Trends are more important than specific single-point values in guiding therapy. An effective CO should need no compensation; therefore a patient should have warm toes simultaneously with a normal BP and HR. One of the advantages of clinical end points is that they remain the same whatever the phase of the illness.

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