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Monitoring and Testing the Critical Care Patient

By

Cherisse Berry

, MD, New York University School of Medicine

Last full review/revision Nov 2020| Content last modified Nov 2020
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Some monitoring of critical care patients depends on direct observation and physical examination and is intermittent, with the frequency depending on the patient’s illness. Other monitoring is ongoing and continuous, provided by complex devices that require special training and experience to operate. Most such devices generate an alarm if certain physiologic parameters are exceeded. Every intensive care unit (ICU) should strictly follow protocols for investigating alarms.

Monitoring usually includes measurement of vital signs (temperature, blood pressure, pulse, and respiration rate), quantification of all fluid intake and output, and often daily weight. Blood pressure may be recorded by an automated sphygmomanometer, or an arterial catheter can be used for continuous blood pressure monitoring. A transcutaneous sensor for pulse oximetry is used as well.

Blood Tests

Although frequent blood draws can destroy veins, cause pain, and lead to anemia, ICU patients typically have routine daily blood tests to help detect problems early. Placement of a central venous catheter or arterial catheter can facilitate easy blood sampling without the need for repeated peripheral needle sticks, but the risk of complications must be considered. Generally, patients need a daily set of electrolytes and a complete blood count (CBC). Patients should also have magnesium, phosphate, and calcium levels measured. Patients receiving total parenteral nutrition need weekly liver enzymes and coagulation profiles. Other tests (eg, blood culture for fever, CBC after a bleeding episode) are done as needed.

Point-of-care testing uses miniaturized, highly automated devices to do certain blood tests at the patient’s bedside or unit (particularly ICU, emergency department, and operating room). Commonly available tests include blood chemistries, glucose, arterial blood gases (ABGs), CBC, cardiac markers, and coagulation tests. Many are done in < 2 minutes and require < 0.5 mL blood.

Cardiac Monitoring

Most critical care patients have cardiac activity monitored by a 3-lead system; signals are usually sent to a central monitoring station by a small radio transmitter worn by the patient. Automated systems generate alarms for abnormal rates and rhythms and store abnormal tracings for subsequent review.

Some specialized cardiac monitors track advanced parameters associated with coronary ischemia, although their clinical benefit is unclear. These parameters include continuous ST segment monitoring and heart rate variability. Loss of normal beat-to-beat variability signals a reduction in autonomic activity and possibly coronary ischemia and increased risk of death.

Pulmonary Artery Catheter (PAC) Monitoring

Use of a pulmonary artery catheter (PAC, or Swan-Ganz catheter) is becoming less common in ICU patients. This balloon-tipped, flow-directed catheter is inserted via central veins through the right side of the heart into the pulmonary artery. The catheter typically contains several ports that can monitor pressure or inject fluids. Some PACs also include a sensor to measure central (mixed) venous oxygen saturation. Data from PACs are used mainly to determine cardiac output and preload. Preload is most commonly estimated by the pulmonary artery occlusion pressure. However, preload may be more accurately determined by right ventricular end-diastolic volume, which is measured using fast-response thermistors gated to heart rate.

Despite longstanding use, PACs have not been shown to reduce morbidity and mortality. Rather, PAC use has been associated with excess mortality. This finding may be explained by complications of PAC use and misinterpretation of the data obtained. Nevertheless, some physicians believe PACs, when combined with other objective and clinical data, aid in the management of certain critically ill patients. As with many physiologic measurements, a changing trend is typically more significant than a single abnormal value. Possible indications for PACs are listed in the table Potential Indications for Pulmonary Artery Catheterization.

Table
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Potential Indications for Pulmonary Artery Catheterization

Cardiac disorders

Acute valvular regurgitation

Complicated heart failure

Complicated myocardial infarction

Ventricular septal rupture

Hemodynamic instability*

Assessment of volume status

Hemodynamic monitoring

Cardiac surgery

Postoperative care in critically ill patients

Surgery and postoperative care in patients with significant heart disease

Pulmonary disorders

Complicated pulmonary embolism

* Particularly if inotropic drugs are required.

Procedure

The pulmonary artery catheter (PAC) is inserted through a special catheter in the subclavian (usually left), internal jugular (usually right), or, less often, a femoral vein with the balloon (at the tip of the catheter) deflated. Once the catheter tip reaches the superior vena cava, inflation of the balloon permits blood flow to guide the catheter. The position of the catheter tip is usually determined by pressure monitoring (see table Normal Pressures in the Heart and Great Vessels for intracardiac and great vessel pressures) or occasionally by fluoroscopy when available. Entry into the right ventricle is indicated by a sudden increase in systolic pressure to about 30 mm Hg; diastolic pressure remains unchanged from right atrial or vena caval pressure. When the catheter enters the pulmonary artery, systolic pressure does not change, but diastolic pressure rises above right ventricular end-diastolic pressure or central venous pressure (CVP); ie, the pulse pressure (the difference between the systolic and diastolic pressures) narrows. Further movement of the catheter wedges the balloon in a distal pulmonary artery. Once in place in the pulmonary artery, the balloon should be deflated. A chest x-ray confirms proper placement.

Table
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Normal Pressures in the Heart and Great Vessels

Type of Pressure

Average (mm Hg)

Range (mm Hg)

Right atrium

3

0–8

Right ventricle

Peak-systolic

25

15–30

End-diastolic

4

0–8

Pulmonary artery

Mean

15

9–16

Peak-systolic

25

15–30

End-diastolic

9

4–14

Pulmonary artery occlusion (pulmonary artery wedge)

Mean

9

2–12

Left atrium

Mean

8

2–12

A wave

10

4–16

V wave

13

6–12

Left ventricle

Peak-systolic

130

90–140

End-diastolic

9

5–12

Brachial artery

Mean

85

70–150

Peak-systolic

130

90–140

End-diastolic

70

60–90

Adapted from Fowler NO: Cardiac Diagnosis and Treatment, ed 3. Philadelphia, JB Lippincott, 1980, p. 11.

The systolic pressure (normal, 15 to 30 mm Hg) and diastolic pressure (normal, 5 to 13 mm Hg) are recorded with the catheter balloon deflated. The diastolic pressure corresponds well to the occlusion pressure, although diastolic pressure can exceed occlusion pressure when pulmonary vascular resistance is elevated secondary to primary pulmonary disease (eg, pulmonary fibrosis, pulmonary hypertension).

Pulmonary artery occlusion pressure (pulmonary artery wedge pressure)

With the balloon inflated, pressure at the tip of the catheter reflects the static back pressure of the pulmonary veins. The balloon must not remain inflated for > 30 seconds to prevent pulmonary infarction. Normally, pulmonary artery occlusion pressure (PAOP) approximates mean left atrial pressure, which in turn approximates left ventricular end-diastolic pressure (LVEDP). LVEDP reflects left ventricular end-diastolic volume (LVEDV). The LVEDV represents preload, which is the actual target parameter. Many factors cause PAOP to reflect LVEDV inaccurately. These factors include mitral stenosis, high levels of positive end-expiratory pressure (> 10 cm H2O), and changes in left ventricular compliance (eg, due to myocardial infarction, pericardial effusion, or increased afterload). Technical difficulties result from excessive balloon inflation, improper catheter position, alveolar pressure exceeding pulmonary venous pressure, or severe pulmonary hypertension (which may make the balloon difficult to wedge).

Elevated PAOP occurs in left-sided heart failure. Decreased PAOP occurs in hypovolemia or decreased preload.

Mixed venous oxygenation

Mixed venous blood comprises blood from the superior and inferior vena cava that has passed through the right heart to the pulmonary artery. The blood may be sampled from the distal port of the PAC, but some catheters have embedded fiberoptic sensors that directly measure oxygen saturation.

Causes of low mixed venous oxygen content (SmvO2) include anemia, pulmonary disease, carboxyhemoglobin, low cardiac output, and increased tissue metabolic needs. The ratio of arterial oxygen saturation (SaO2) to (SaO2 − SmvO2) determines the adequacy of oxygen delivery. The ideal ratio is 4:1, whereas 2:1 is the minimum acceptable ratio to maintain aerobic metabolic needs.

Cardiac output (CO)

Cardiac output is measured by intermittent bolus injection of ice water or, in newer catheters, continuous warm thermodilution (see Measurement of cardiac output and flow). The cardiac index divides the cardiac output by body surface area to correct for patient size (see table Normal Values for Cardiac Index and Related Measures).

Other variables can be calculated from cardiac output. They include systemic and pulmonary vascular resistance and right ventricular stroke work (RVSW) and left ventricular stroke work (LVSW).

Table
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Normal Values for Cardiac Index and Related Measurements

Measurement

Units ± SD

Oxygen uptake

143 ± 14.3 mL/minute/m2

Arteriovenous oxygen difference

4.1 ± 0.6 dL

Cardiac index

3.5 ± 0.7 L/minute/m2

Stroke index

46 ± 8.1 mL/beat/m2

Total systemic resistance

1130 ± 178 dynes-second-cm-5

Total pulmonary resistance

205 ± 51 dynes-second-cm-5

Pulmonary arteriolar resistance

67 ± 23 dynes-second-cm-5

SD = standard deviation.

Adapted from Barratt-Boyes BG, Wood EH: Cardiac output and related measurements and pressure values in the right heart and associated vessels, together with an analysis of the hemodynamic response to the inhalation of high oxygen mixtures in healthy subjects. Journal of Laboratory and Clinical Medicine 51:72–90, 1958.

Complications

Pulmonary artery catheters (PACs) may be difficult to insert. Cardiac arrhythmias, particularly ventricular arrhythmias, are the most common complication. Pulmonary infarction secondary to overinflated or permanently wedged balloons, pulmonary artery perforation, intracardiac perforation, valvular injury, and endocarditis may occur. Rarely, the catheter may curl into a knot within the right ventricle (especially in patients with heart failure, cardiomyopathy, or increased pulmonary pressure).

Pulmonary artery rupture occurs in < 0.1% of PAC insertions. This catastrophic complication is often fatal and occurs immediately on wedging the catheter either initially or during a subsequent occlusion pressure check. Thus, many physicians prefer to monitor pulmonary artery diastolic pressures rather than occlusion pressures.

Noninvasive Cardiac Output Assessment

Other methods of determining cardiac output, such as point-of-care ultrasonography, esophageal Doppler monitoring, and thoracic bioimpedance, are being developed to avoid the complications of pulmonary artery catheters (PACs). Although these methods are potentially useful, none is yet as reliable as a PAC.

Point-of-care ultrasonography

Point-of-care ultrasonography has become indispensable in critical care for rapid diagnosis of functional as well as anatomic abnormalities. Hand-held ultrasound devices are portable and thus save time and obviate the need to move the patient. The quality of information obtained from point-of-care ultrasonography sometimes matches or exceeds that provided by more expensive and labor-intensive imaging techniques. Judicious use of ultrasonography decreases exposure to ionizing radiation. In acute care, point-of-care ultrasonography is particularly useful for evaluating the abdomen, thorax, and heart. It can sometimes be used to diagnose deep vein thrombosis.

Abdominal ultrasonography can be used to identify free (extravascular) fluid, typically as part of focused assessment with sonography for trauma (FAST—usually done during trauma evaluation and resuscitation). Free fluid in a hypotensive trauma patient is likely to be blood, an indication for surgical intervention. Other abdominal organs can also be evaluated.

Cardiac ultrasonography is essential for evaluating anatomy as well as hemodynamic status by assessing chamber size, wall motion, contractility, and ejection fraction. A focused rapid echocardiographic evaluation (FREE) is one example of a structured ultrasound assessment. FREE is done using the 4 standard echocardiographic windows: the parasternal long axis, parasternal short axis, apical, and subxyphoid windows. FREE measures left ventricular ejection fraction (EF), stroke volume (SV), cardiac output (CO), and cardiac index (CI) and allows for the evaluation of patients with hypotension (1). In evaluating patients with hypotension, ultrasonography is indispensable in confirming the following:

  • Hypovolemia: Even if the inferior vena cava looks full (as can occur in a hypovolemic ventilated patient), hypovolemia is suggested by a hyperdynamic left ventricle with almost no blood at the end of systole and little at the end of diastole.

  • Left ventricular dysfunction: Left ventricular dysfunction is suggested by wall motion abnormalities and decreased ejection fraction that is either measured or estimated (by an experienced operator who assesses the overall size and apparent contractility and inward movement and thickening of the various segments of the left ventricular wall).

  • Right ventricular failure: The right ventricle should be 60% the size of the left ventricle, triangular, and have a rough inside surface. Right ventricular failure may suggest pulmonary embolism.

  • Pericardial effusions and tamponade

Thoracic ultrasonography can be used to identify pleural fluid and pneumothorax with higher sensitivity and negative predictive value than with plain x-rays. For example, lung sliding in an area spanning over three intercostal spaces and A-lines (horizontal artifacts) are each nearly 100% sensitive, and, when combined, appear highly specific. The echogenicity of pleural fluid and changes in the pleura and adjacent lung parenchyma also help determine the etiology of pleural fluid.

Point-of-care ultrasonography is also useful for looking for deep venous thrombosis and evaluating intra-abdominal organs.

Esophageal Doppler monitor (EDM)

This device is a soft 6-mm catheter that is passed nasopharyngeally into the esophagus and positioned behind the heart. A Doppler flow probe at its tip allows continuous monitoring of cardiac output and stroke volume. Unlike the invasive PAC, the esophageal Doppler monitor (EDM) does not cause pneumothorax, arrhythmia, or infection. An EDM may actually be more accurate than a PAC in patients with cardiac valvular lesions, septal defects, arrhythmias, or pulmonary hypertension. However, the EDM may lose its waveform with only a slight positional change and produce dampened, inaccurate readings.

Thoracic bioimpedance

These systems use topical electrodes on the anterior chest and neck to measure electrical impedance of the thorax. This value varies with beat-to-beat changes in thoracic blood volume and hence can estimate cardiac output. The system is harmless and provides values quickly (within 2 to 5 minutes); however, the technique is very sensitive to alteration of the electrode contact with the patient. Thoracic bioimpedance is more valuable in recognizing changes in cardiac output in a given patient than in precisely measuring its absolute value.

Noninvasive cardiac output assessment reference

  • 1. Murthi SB, Hess JR, Hess A, et al: Focused rapid echocardiographic evaluation versus vascular catheter-based assessment of cardiac output and function in critically ill trauma patients. J Trauma Acute Care Surg 72 (5):1158–1164, 2012. doi: 10.1097/TA.0b013e31824d1112

Intracranial Pressure (ICP) Monitoring

Intracranial pressure monitoring is standard for patients with severe closed head injury and is occasionally used for some other brain disorders, such as in selected cases of hydrocephalus and idiopathic intracranial hypertension (pseudotumor cerebri) or in postoperative or postembolic management of arteriovenous malformations. These devices are used to monitor ICP (normally 5 to 15 mm Hg) and to optimize cerebral perfusion pressure (mean arterial pressure minus intracranial pressure). Typically, the cerebral perfusion pressure should be kept > 60 mm Hg.

Several types of intracranial pressure monitors are available. The most useful method places a catheter through the skull into a cerebral ventricle (ventriculostomy catheter). This device is preferred because the catheter can also drain cerebrospinal fluid (CSF) and hence decrease intracranial pressure. However, the ventriculostomy is also the most invasive method, has the highest infection rate, and is the most difficult to place. Occasionally, the ventriculostomy becomes occluded due to severe brain edema.

Other types of intracranial devices include an intraparenchymal monitor and an epidural bolt inserted between the skull and the dura through which a pressure sensor is passed. Of these, the intraparenchymal monitor is more commonly used. All intracranial pressure devices should usually be changed or removed after 5 to 7 days because infection is a risk.

Other Types of Monitoring

Sublingual capnometry uses a similar correlation between elevated sublingual PCO2 and systemic hypoperfusion to monitor shock states using a noninvasive sensor placed under the tongue. This device is easier to use than gastric tonometry and responds quickly to perfusion changes during resuscitation.

Tissue spectroscopy uses a noninvasive near infrared (NIR) sensor usually placed on the skin above the target tissue to monitor mitochondrial cytochrome redox states, which reflect tissue perfusion. NIR may help diagnose acute compartment syndromes (eg, in trauma) or ischemia after free tissue transfer and may be helpful in postoperative monitoring of lower-extremity vascular bypass grafts. NIR monitoring of small-bowel pH may be used to gauge the adequacy of resuscitation.

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