Invasive Hemodynamic Monitoring

Systemic Arterial Blood Pressure

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  The continuous measurement of arterial pressure is essential in hemodynamic monitoring of critically ill patients.

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  Arterial pressure is the input pressure for organ perfusion.

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  In the CICU, insertion of an indwelling arterial catheter into either the arm (brachial or radial sites) or groin (femoral arterial site) is often used to provide more precise monitoring.

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  The advantages of arterial catheterization over noninvasive techniques are continuous monitoring of arterial pressure and its waveform and providing a site for repetitive blood sampling.

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  Arterial pressure is a function of both vasomotor tone and cardiac output (CO).

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  Local metabolic demands determine local vasomotor tone that, in turn, determines blood flow distribution.

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  Perfusion pressure and local vascular resistance determine organ perfusion of all capillary beds.

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  Flow is proportional to local metabolic demand if there is no hemodynamic instability to cause increased sympathetic tone.

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  CO primarily determines arterial pressure in the setting of varying degrees of local blood flow and, because it is proportional to local metabolic demand, there is no normal value in an unstable, metabolically active patient.

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  The literature currently suggests maintaining patients who were previously nonhypertensive at a mean arterial blood pressure (MAP) of 65 mm Hg, consistent with the initial MAP target recommended by the Surviving Sepsis Guidelines.

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  In a clinical trial that examined the effects of resuscitative efforts with fluid and vasopressors in patients with circulatory shock to varying MAP targets, ranging from 60 to 90 mm Hg, no increased organ blood flow could be determined above a MAP of 65 mm Hg.

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  However, evidence in the septic shock literature indicates that a MAP of 75 to 85 mm Hg may reduce the development of acute kidney injury in patients with chronic arterial hypertension.

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  As a result, it has been suggested to consider more individualized targets for older patients with hypertension or atherosclerosis and in patients with septic shock.

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  The indications for arterial catheterization ( Table 23.1 ).

  Table 23.1

  Arterial Catheterization

  Modified from Polanco PM, Pinsky MR. Practical issues of hemodynamic monitoring at the bedside. Surg Clin North Am . 2006;86(6):1431–1456.

  Probable Indications for Arterial Catheterization

  Guide to management of potent vasodilator drug infusions to prevent systemic hypotension

  Guide to management of potent vasopressor drug infusions to maintain a target mean arterial pressure

  As a port for the rapid and repetitive sampling of arterial blood in patients in whom multiple arterial blood samples are indicated

  As a monitor of cardiovascular deterioration in patients at risk for cardiovascular instability

  Useful Applications of Arterial Pressure Monitoring in the Diagnosis of Cardiovascular Insufficiency

  Differentiating cardiac tamponade (pulsus paradoxus) from respiration-induced swings in systolic arterial pressure; tamponade reduces the pulse pressure but keeps diastolic pressure constant. Respiration reduces systolic and diastolic pressure equally, such that pulse pressure is constant.

  Differentiating hypovolemia from cardiac dysfunction as the cause of hemodynamic instability. Systolic arterial pressure decreases more following a positive pressure breath as compared with an apneic baseline during hypovolemia. Systolic arterial pressure increases more during positive pressure inspiration when left ventricular (LV) contractility is reduced.

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  In most cases, the choice of location for insertion of the catheter is the radial artery because femoral artery cannulation is more often associated with displacement during patient movement and hemorrhage that is more difficult to control.

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  Although arterial catheterization is an invasive procedure with inherent risks (temporary vascular occlusion 20% and hematoma 14%), most complications are not severe, with permanent ischemic damage, sepsis, and pseudoaneurysm occurring in less than 1% of cases.

Pulmonary Artery Catheterization

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  Pulmonary artery (PA) catheterization permits additional measurements of CO, right heart, and pulmonary pressures that make it possible to calculate other derived hemodynamic parameters, such as cardiac work indices and systemic and pulmonary vascular resistance.

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  These fundamental hemodynamic variables help further describe the disordered physiologic state with sufficient precision to enhance management decisions and aid in the care of critically ill patients.

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  The PA catheter allows for determination of various fundamental hemodynamic parameters, including measurement of CO by thermodilution (TD), right atrial (RA), right ventricular (RV), PA, and pulmonary capillary wedge pressures (PCWP), and sampling of blood from the PA, RV, and RA.

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  Pulmonary vascular and systemic resistance, as well as RV and left ventricular (LV) stroke work, can then be derived.

Pulmonary Artery Catheter

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  The most commonly used PA catheter is a 7.5 Fr thermodilution, triple-lumen, radiopaque catheter, 110 cm long made of polyvinylchloride.

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  Most catheters are heparin coated to reduce thrombogenicity.

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  The outside is marked with black rings every 10 cm from the tip that allow determination without fluoroscopy of the appropriate catheter length at which to inflate the distal balloon.

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  The distal lumen terminates at the tip of the catheter, whereas the RA lumen terminates 30 cm proximal to the tip. There is a venous infusion lumen 1 cm proximal to the RA lumen.

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  A thermistor bead located 3 to 5 cm from the tip is connected to an external thermistor connector by a wire.

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  The external thermistor is, in turn, linked to a computer that allows for determination of CO by the TD method.

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  A soft latex balloon with a maximum inflating capacity of 1.0 to 1.5 mL is affixed to the distal tip of the PA catheter.

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  Upon inflation, the balloon engulfs the catheter tip, cushioning the transmitted force, limiting injury of endocardial surfaces, and reducing the frequency of arrhythmias.

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  The inflated balloon facilitates flow-directed advancement of the catheter through the right heart into the PA.

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  Once inflated in a distal branch of the PA, the balloon occludes the vessel and allows for measurement of PCWP through the catheter tip.

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  The catheter can serve multiple functions, including measurement of CO by TD, PA temperature, and intracardiac pressures (RA, RV, PA, PCW).

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  Blood sampling can be done through the active lumens of the catheter.