Hemodynamic monitoring (2024)

Preload

Preload refers to the amount of myocardial fiber stretch at the end of diastole, it also refers to the amount of volume in the ventricle at the end of this phase. It is clinically acceptable to measure the pressure required to fill the ventricles as an indirect assessment of ventricular preload. Left atrial filling pressure or pulmonary artery wedge pressure is used to assess left ventricular preload. Right atrial pressure is used to assess right ventricular preload. Volumetric parameters provide a closer measurement to ventricular preload for the right ventricle.

Afterload

Afterload refers to the tension developed by the myocardium during ventricular systolic ejection. More commonly, afterload is described as the resistance, impedance, or pressure that the ventricles must overcome to eject their blood volumes. Afterload is dependent on a number of factors, including volume and mass of blood ejected, the size and wall thickness of the ventricles, and the impedance of the vasculature. In the clinical setting, the most sensitive measure of afterload is systemic vascular resistance (SVR) for the left ventricle and pulmonary vascular pressure (PVR) for the right ventricle.

Afterload has an inverse relationship to ventricular function. As resistance to ejection increases, the force of contraction decreases, resulting in a decreased stroke volume. As resistance to ejection increases, an increase in myocardial oxygen consumption occurs.

ECG monitoring

Continuous ECG monitoring allows the registration of beating frequency, cardiac rhythm and ischemic episodes (depression of ST-segment). One must, however, be aware that ischemia detection is incomplete when monitoring only either the anterior (lead V₄) or the posterior/inferior region (lead II) of the left ventricle. Furthermore, ECG does tell us nothing about electromechanical coupling of the heart.

Central venous pressure

The central venous pressure (CVP) reflects the pressure in the central veins - usually measured in the thoracic cavity - when they enter the right atrium. CVP fluctuates with respiration, so the time of measurement can be important. Normally CVP should be measured in the endexspiratory state. CVP (right atrial pressure) indicates pressure and not volume.

Indications for CVP measurements include:

• Diagnostic measurements.

• Monitoring and guiding fluid management.

• Monitoring and guiding pharmacological interventions.

The relationship between the right heart side pressure and the intravascular volume is unpredictable and is affected by many factors (e.g. the tone of the systemic circulation).

The problems in interpreting CVP as an volume indicator can be summarized as following: Shocked patients: low intravascular volume with compensatory vasoconstriction: CVP low. Rapid resuscitation: fluid poured into a constricted patient will increase blood pressure and push CVP up rapidly, but the vasculature may still be constricted. Redistribution: normally the patient's vasculature will dilate a little and the fluid will redistribute slowly. CVP falls to zero but the patient remains constricted. Anesthesia: the constricted patient is given general anesthesia. Acute vasodilation occurs and the fluid redistributes instantly, exposing a large volume deficit. Blood pressure and CVP plummet.

Therefore careful interpretation of the CVP is important! The CVP should always be considered in conjunction with other cardiovascular parameters. Under normal circ*mstances the right heart sided pressures should indirectly reflect left sided pressures, and the left sided filling pressure may be an indicator of left ventricular function.

Kidney function

Diuresis depends strongly on heart function. Oliguria due to prerenal failure is an early indicator of pump failure of the heart.

Pulse oximetry

Pulse oximetry monitors oxygenation. Simultaneously, beating frequency is recorded, as well as - in combination with the ECG - an eventual pulse deficit. A strong variation in the amplitude may indicate volume deficiency.

Arterial pressure monitoring

Peripheral arterial lines (A. radialis) offer several advantages in comparison of monitoring blood pressure with a cuff. The line provides continuous measurement of blood pressure and can be used for sampling of blood gases. A strong variation in the amplitude may indicate volume deficiency.

In the setting of marked vasoconstriction or hypotension, the arterial line gives more accurate pressure values than a blood pressure cuff; however, in case of strong centralization of circulation, blood pressure measurements done with peripheral arterial lines may considerably differ from the core hemodynamics.

Indications include

• Rapidly changing clinical circ*mstances in critically ill patients (e.g. hemorrhage, sepsis).

• Monitoring and guiding the use of vasoactive drugs with rapid cardiovascular effects.

• Monitoring and guiding acute interventions (e.g. major surgery, resuscitation).

• Blood sampling.

Pulmonary artery catheter

The pulmonary artery catheter offers several advantages over central venous pressure monitoring. When the balloon tip of a Swan Ganz catheter is properly wedged in a branch of the pulmonary artery, the pressure sensed by the catheter tip represents that in the left atrium, taking aside a specific problem of pulmonary capillary wedge pressure monitoring in the septic patient. Left atrial pressure, which equals left ventricular filling pressure in the absence of mitral stenosis, is an excellent indicator of the adequacy of fluid resuscitation done. If the pressure is low (less than 12 mmHg), additional fluid resuscitation is indicated. If the pressure is high (greater than 20 mmHg), additional fluid is unlikely to improve cardiac performance further, and vasopressors are probably indicated for circulatory support. Although the catheter can yield a vast amount of information (see table III), the distinction between the need of fluids or vasopressors is its most useful application.

PA catheters may be used for both diagnosis and therapy. Clinical indications include:

• Postmyocardial infarction: to assess hemodynamic status and monitor and guide therapy.

• Cardiac surgery: to monitor cardiac function.

• Major surgery: in the presence of myocardial dysfunction or for preoperative optimization of hemodynamics.

• Resuscitation: in case of hemodynamic instability during fluid replacement: to assess left ventricular function.

• Septic shock: assessment of LV function and fluid status.

• Diagnosis of high and low pressure pulmonary edema.

• Measurement of oxygen transport, enabling optimization of ventilation and perfusion.

• Pre-eclampsia and eclampsia: to monitor fluid status and assess intravascular volume.

In the early 1990s, a new technology was introduced for continuous measurement of cardiac output using a specific thermodilution technology. The accuracy and reliability of the continuous thermodilution system has been confirmed by several studies.

A specific right ventricular catheter allows calculation of enddiastolic and endsystolic right ventricular volumes, of stroke volume and ejection fraction by analyzing beat by beat. This technology might be helpful in monitoring right heart failure states like right ventricular infarction.

Though the pulmonary artery catheter may be helpful in many circ*mstances, nevertheless its benefit has not yet been demonstrated in a prospective manner, and the results of a recent trial have put the basis for many controversial discussions about its benefit. On the other hand, these problems also hold for all new, even less validated alternative monitoring techniques like the transcardiopulmonary indicator dilution technique for measurement of cardiac putput as well as total and intrathoracic blood volume; the latter is postulated to represent a better indicator of preload than pulmonary capillary wedge pressure. At present, however, pulmonary artery catheter monitoring is recommended in the hemodynamically unstable, critically ill patient.

Transesophageal Echocardiography (TEE)

Echocardiography is making major inroads into the critical care units. In general, physical examination of critically ill cardiac patients is limited in its accuracy in predicting measured physiologic data. Available studies suggest that protocol-driven pulmonary artery catheter management will modify central venous pressure driven management some 40% of the time, with strong suggestions of improvement in outcome. Recent data suggest that in patients with pulmonary artery catheter-driven care, the use of TEE modifies management another 40% of the time. Diagnosis of many disease processes and pathophysiologic derangements are beyond the capabilities of routine invasive monitoring techniques and can only be made by bedside echocardiography.

Since its introduction in the early eighties, echocardiography has undergone a huge technological and clinical evolution. The indications of echocardiography as a diagnostic and monitoring tool in the peri-operative and critical care setting have increased exponentially because of its potential to accurately assess cardiovascular dynamics. TEE is able to assess global and regional left ventricular function and can reliable evaluate the different determinants of ventricular function such as preload, contractility and afterload. The short axis view of the left ventricle is a basic and readily available part of this imaging technique. Moreover, its adequate visualization of the great vessels leads to an appreciation of cardiovascular interaction and helps to differentiate between cardiac and vascular causes of hemodynamic disturbances.