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Monitoring and Testing the Critical Care Patient


Cherisse Berry

, MD, New York University School of Medicine

Reviewed/Revised Dec 2022
Topic Resources

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 intracranial pressure and/or 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 Central Venous Catheterization A number of procedures are used to gain vascular access. Most patients’ needs for IV fluid and drugs can be met with a percutaneous peripheral venous catheter. If blind percutaneous placement... read more Central Venous Catheterization or arterial catheter Arterial Catheterization A number of procedures are used to gain vascular access. Most patients’ needs for IV fluid and drugs can be met with a percutaneous peripheral venous catheter. If blind percutaneous placement... read more Arterial Catheterization 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 ionized calcium levels measured. Patients receiving total parenteral nutrition need weekly liver tests and coagulation profiles. Other tests (eg, blood culture for fever, serial CBCs for possible active blood loss]) 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 Pulmonary artery occlusion pressure (pulmonary artery wedge pressure) 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... read more Pulmonary artery occlusion pressure (pulmonary artery wedge 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 Potential Indications for Pulmonary Artery Catheterization Potential Indications for Pulmonary Artery Catheterization .


Potential Indications for Pulmonary Artery Catheterization

Cardiac disorders

Acute valvular regurgitation

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

* Particularly if inotropic drugs are required.


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 Normal Pressures in the Heart and Great Vessels 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.

Pulmonary Artery Catheter Insertion

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 Idiopathic Pulmonary Fibrosis Idiopathic pulmonary fibrosis (IPF), the most common form of idiopathic interstitial pneumonia, causes progressive pulmonary fibrosis. Symptoms and signs develop over months to years and include... read more Idiopathic Pulmonary Fibrosis , pulmonary hypertension Pulmonary Hypertension Pulmonary hypertension is increased pressure in the pulmonary circulation. It has many secondary causes; some cases are idiopathic. In pulmonary hypertension, pulmonary vessels may become constricted... read more ).

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 Mitral Stenosis Mitral stenosis is narrowing of the mitral orifice that impedes blood flow from the left atrium to the left ventricle. The usual cause is rheumatic fever. Common complications are pulmonary... read more , mitral regurgitation, high levels of positive end-expiratory pressure (> 10 cm H2O), and changes in left ventricular compliance (eg, due to myocardial infarction Acute Myocardial Infarction (MI) Acute myocardial infarction is myocardial necrosis resulting from acute obstruction of a coronary artery. Symptoms include chest discomfort with or without dyspnea, nausea, and/or diaphoresis... read more Acute Myocardial Infarction (MI) , 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 Pulmonary Hypertension Pulmonary hypertension is increased pressure in the pulmonary circulation. It has many secondary causes; some cases are idiopathic. In pulmonary hypertension, pulmonary vessels may become constricted... read more (which may make the balloon difficult to wedge).

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)

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).



Pulmonary artery catheters (PACs) may be difficult to insert. Cardiac arrhythmias Overview of Arrhythmias The normal heart beats in a regular, coordinated way because electrical impulses generated and spread by myocytes with unique electrical properties trigger a sequence of organized myocardial... read more Overview of 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 Infective Endocarditis Infective endocarditis is infection of the endocardium, usually with bacteria (commonly, streptococci or staphylococci) or fungi. It may cause fever, heart murmurs, petechiae, anemia, embolic... read more Infective 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, can be used 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 Testing Injury is the number one cause of death for people aged 1 to 44. In the US, there were 278,345 trauma deaths in 2020, about 70% of which were accidental. Of intentional injury deaths, more than... read more (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 evaluates 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 Noninvasive cardiac output assessment reference 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... read more Noninvasive cardiac output assessment reference ). 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, loss of lung sliding in an area spanning over 3 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. (See How To Do E-FAST Examination. Point-of-care ultrasonography Point-of-care ultrasonography )

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

Several types of intracranial pressure monitors are available. Extraventricular drain (EVD) is the most useful method; a catheter is placed 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 EVD is also the most invasive method, has the highest infection rate, and is the most difficult to place. Occasionally, the EVD becomes occluded due to severe brain edema.

Other types of intracranial devices include an intraparenchymal monitor, subarachnoid bolt, subdural bolt, 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.

Near Infrared Spectroscopy (NIRS)

NIRS is a noninvasive method of continuously monitoring end organ oxygenation and perfusion. NIRS sensors are usually placed on the skin above the target tissue to monitor mitochondrial cytochrome redox states, which reflect tissue perfusion. NIRS 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. NIRS monitoring of small-bowel pH may be used to gauge the adequacy of resuscitation.

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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