Hemodynamic monitoring considerations in the intensive care unit

Overview

In critical care, the goals of hemodynamic monitoring include mainly detection of cardiovascular insufficiency and diagnosis of the underlying pathophysiology. At the bedside, clinicians are faced with the challenge of translating concepts such as preload, contractility, and afterload into determinants of stroke volume and hence cardiac output. Ultrasound and echocardiography offer unique insight into ventricular filling and systolic function. In recent years there has been a general trend away from invasive hemodynamic monitoring. This was initially motivated by published data suggesting an association between the pulmonary artery catheter (PAC) and excess mortality in critically ill patients. Despite specific risks, subsequent randomized controlled trials have not sustained the concerns about excess mortality. The PAC should not be regarded as obsolete.

As already discussed in this text, ultrasound is proving useful in guiding safe and timely placement of many components of hemodynamic monitoring systems, including arterial, peripheral, and central venous access devices. Furthermore, because of its real-time nature, ultrasound, including echocardiography, offers the clinician a range of cardiovascular insights that are difficult or impossible to derive with other technologies. Ultrasound can be applied to a wide range of patients and is a safe, noninvasive, and reliable imaging method.

Hemodynamic monitoring devices

An overview of critical care hemodynamic monitoring would be incomplete without putting ultrasound in the context of the techniques available for estimating cardiac output, including nonultrasonic modalities. This broader topic is covered well in the literature and is outlined only briefly here. Demonstrating an association between any monitoring modality and improved outcome is challenging. Monitoring must be coupled with an effective change in therapy for a positive association to be observed. Clinical practice is characterized by the subtleties of interpretation, ongoing review, and titration of therapy to response. This does not translate easily into large-scale, randomized, controlled trial designs.

Clinicians differ in their preferences for particular hemodynamic monitoring techniques. Accuracy and degree of invasiveness are not the only considerations. Familiarity, availability of local expertise, cost (equipment and consumables), and applicability to a particular patient and the patient’s status must also be considered. Monitoring techniques tend to not be mutually exclusive and may be combined or changed to achieve the desired effect. For instance, initial hemodynamic evaluation with echocardiography may proceed to continuous monitoring, such as pulse waveform analysis.

Any form of hemodynamic monitoring ( Table 36-1 ) should be viewed as an adjunct to the clinical examination and must be interpreted as an integration of all available data. These may include the patient’s mental state, urine output, and peripheral perfusion (temperature and capillary refill time). Heart rate, arterial blood pressure, jugular venous pressure (or central venous or right atrial pressure [RAP]), and electrocardiography should also be incorporated. Other adjuncts to the interpretation of hemodynamic data might include Svo ₂ , Scvo ₂ , lactate, blood gases, capnography, gastric tonometry, or other assessment of the microcirculation.

Ultrasound indicator dilution is a novel application of ultrasound technology. Unlike transpulmonary thermodilution, which bases estimates of cardiac output on changes in blood temperature, ultrasound indicator dilution measures changes in ultrasound velocity. Normothermic isotonic saline is injected into a low-volume arteriovenous loop between arterial and central venous catheters. The change measured in ultrasound velocity (blood, 1560 to 1585; saline, 1533 m/sec) allows the formulation of an indicator dilution curve and calculation of cardiac output.

Invasive hemodynamic monitoring

As mentioned previously, observational studies raised questions about increased morbidity and mortality with the use of PACs ; however, subsequent randomized trials indicated that PACs are generally safe and may yield important information. The PAC has a trailblazing role in defining cardiovascular physiology and pathophysiology. The method provides “cardiodynamic insight” that other hemodynamic monitoring technologies still fail to elucidate. A PAC is not a therapy and cannot affect the prognosis, but it can be used to guide therapy. The usual clinical indications for placement of a PAC are shown in Box 36-1 .

Echocardiographic hemodynamic monitoring

A comprehensive echocardiographic examination is time-consuming. In the management of potentially unstable, critically ill patients, physicians will often prefer to focus their examination on pertinent variables. Several focused hemodynamic echocardiographic protocols have been developed and applied. Among others, these protocols include FOCUS (focused cardiac ultrasound ), ELS (Echo in Life Support ) and HART scanning (hemodynamic echocardiographic assessment in real time ).

As well as being minimally invasive (transesophageal [TEE]) or noninvasive (transthoracic [TTE]), echocardiography also offers unique diagnostic insight into a patient’s cardiovascular status. The presence of intracardiac shunts renders many hemodynamic monitoring devices invalid. Such shunts may be difficult to diagnose without echocardiographic techniques. Likewise, pericardial effusions, collections, and tamponade can also be difficult to diagnose without echocardiography.

In critical illness, cardiac function is not always globally affected. Echocardiography allows screening for and diagnosis of regional pathology, such as myocardial ischemia; furthermore, it allows evaluation of coronary arterial territories by regional wall motion abnormalities. Echocardiography may also disclose abnormalities such as dynamic left ventricular (LV) outflow obstruction and systolic anterior movement of the mitral valve. This may have particular therapeutic implications in critical care. Valvular dysfunction is also important to the critical care physician, and echocardiography is the clinical “gold standard” for detection and characterization (including grading). As an alternative to the PAC, echocardiography potentially offers important information about the right ventricle and pulmonic circulation.

Echocardiographically, cardiac output is calculated as the product of stroke volume and heart rate. Echocardiographic techniques for estimating stroke volume include linear techniques, volumetric techniques (two-dimensional [2D] and three-dimensional [3D] echocardiography), and Doppler. Guidelines have been developed for echocardiographic chamber quantification and should be applied for linear and volumetric assessments. Similarly, guidelines exist for Doppler measurements.