Practical Issues of Hemodynamic Monitoring at the Bedside

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Rationale for hemodynamic monitoring

The arguments to defend the use of specific types of monitoring techniques can be roughly grouped into three levels based on their level of validation [1]. At the basic level, the specific monitoring technique can be defended based on historical controls. At this level, prior experience using similar monitoring was traditionally used and presumed to be beneficial. The mechanism by which the benefit is achieved need not be understood. The second level of defense comes through an understanding of

The physiologic basis for hemodynamic monitoring

On a philosophical level, one may consider the monitoring of critically ill patients as serving a dual function. First, it can be used to document hemodynamic stability and the lack of need for acute interventions, and second, it can be used to monitor when measured variables vary from their defined baseline values. Accordingly, knowing the limits to which such monitoring reflects actual physiological values is an essential aspect of its utility.

On the physical side, hemodynamic monitoring can

Arterial pressure monitoring

After pulse rate, arterial pressure is the most common hemodynamic variable monitored and recorded. Blood pressure is usually measured noninvasively using a sphygmomanometer and the auscultation technique. Importantly, very large and obese subjects in whom the upper arm circumference exceeds the width limitations of a normal blood pressure cuff will record pressures that are higher than they actually are. In such patients, using the large thigh blood pressure cuff usually resolves this problem.

Indications for arterial catheterization

The arterial catheter is frequently inserted as a “routine” at the admission of patients to the ICU for continuous monitoring of blood pressure and repetitive measurements of blood gases. There is no evidence to support this exaggerated clinical practice. Although probably the only proven indication for arterial catheterization is to synchronize the intra-aortic balloon of counterpulsation, there are some others indications whereby the information obtained is valuable in the assessment and

Methods of measuring central venous pressure

CVP is the pressure in the large central veins proximal to the right atrium relative to atmosphere. In the ICUs, the CVP is usually measured using a fluid-filled catheter (central venous line or Swan-Ganz catheter) with the distal tip located in the superior vena cava connected to a manometer, or more often to a pressure transducer of a monitor, displaying the waveform in a continuous fashion. CVP can also be measured noninvasively as jugular venous pressure, the height of the column of blood

Pulmonary artery catheterization and its associated monitored variables

Pulmonary arterial catheterization allows the measurement of many clinically relevant hemodynamic variables (see Box 1). One can measure the intrapulmonary vascular pressures including CVP, Ppa, and by intermittent balloon occlusion of the pulmonary artery, Ppao and pulmonary capillary pressure (Ppc). Furthermore, by using the thermodilution technique and the Stewart-Hamilton equation, one can estimate cardiac output and EFrv, global cardiac volume, and intrathoracic blood volume. Finally, one

Pulmonary artery pressure

The determinants of Ppa are the volume of blood ejected into the pulmonary artery during systole, the resistance of the pulmonary vascular bed, and the downstream left atrial (LA) pressure. The pulmonary vascular bed is a low-resistance circuit with a large reserve that allows increases of cardiac output with minor changes in the Ppa. On the other hand, increases in the downstream venous pressure (eg, left ventricular failure) or in the flow resistance (eg, lung diseases) raise the Ppa.

Methods of measuring pulmonary artery occlusion pressure

Numerous studies by physicians have demonstrated that the ability to accurately measure Ppao from a strip chart recording or a freeze-frame snapshot of the monitor screen is poor. Many initiatives have been put into place to educate physicians and nurses, but the reality is that because the pressure measured also reports changes in intrathoracic pressure, a value which is always changing, the accuracy of Ppao measures is likely to remain poor.

The Ppao value is thought to reflect the LV filling

Measuring cardiac output

Cardiac output can be estimated by many techniques, including invasive hemodynamic monitoring. Pulmonary blood flow using a balloon floatation PAC equipped with a distal thermistor, and transpulmonary blood flow using an arterial thermistor, both with a central venous cold volume injection, can be used. Similarly, minimally invasive echo Doppler techniques can measure blood flow at the aortic value and descending aortic flow using esophageal Doppler monitoring. Cardiac output can be measured

Measuring venous oxygen saturation

SvO2 reflects the pooled venous O2 saturation, and is an important parameter in the assessment of the adequacy of DO2 and its relation with VO2. A decrease of SvO2 could be explained by a decrease in DO2 or any of the parameters that determine this, such as SaO2, cardiac output, and hemoglobin concentration, and also by an increase in VO2. A decrease of DO2 will be followed by stable VO2, with a consequent decrease of the SvO2 until a critical value of DO2 is reached where the tissues are no

Summary

All surgical patients require monitoring to assess cardiovascular stability, and sometimes may benefit from optimization of their hemodynamic status. Therefore, all surgeons require a basic understanding of physiological underpinnings of hemodynamic monitoring. The physiological rationale is still the primary level of defense for monitoring critically ill patients.

Arterial catheterization to monitor arterial pressure is a safe procedure with a low complication rate; however, it should be used

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    This work was supported by Grant federal funding HL67181 and HL0761570.

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