Monitoring the Patient in the Intensive Care Unit

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Monitoring the Patient in the Intensive Care Unit

Alexander B. Adams

Chapter Objectives

After reading this chapter you will be able to:

image Discuss the principles of monitoring the respiratory system, cardiovascular system, neurologic status, renal function, liver function, and nutritional status of patients in intensive care.

image Identify the risks and benefits of intensive care unit (ICU) monitoring techniques.

image Explain why the caregiver is the most important monitor in the ICU.

image Describe how to evaluate measures of patient oxygenation in the ICU.

image Explain why PaCO2 is the best index of ventilation for critically ill patients.

image Describe the approach used to evaluate changes in respiratory rate, tidal volume, minute ventilation, PaCO2, and end-tidal PCO2 values for monitoring purposes.

image Identify monitoring techniques used in the ICU to evaluate lung and chest wall mechanics and work of breathing.

image Discuss the importance of monitoring peak and plateau pressures in patients receiving mechanical ventilatory support.

image Identify monitoring techniques that have become available more recently, such as lung stress and strain, functional residual capacity, stress index, electrical impedance tomography, and acoustic respiratory monitoring.

image Describe the approach used to interpret the results of ventilator graphics monitoring.

image Describe the cardiovascular monitoring techniques used in the care of critically ill patients and how to interpret the results of hemodynamic monitoring.

image Discuss the importance of monitoring neurologic status in the ICU and the variables that should be monitored.

image Discuss evaluation of renal function, liver function, and nutritional status in the ICU.

image List and discuss the use of composite and global scores to measure patient status in the ICU, such as the Murray lung injury score and the APACHE severity of illness scoring system.

image Discuss monitoring and troubleshooting of the patient-ventilator system in the ICU.

Key Terms

The concept and purposes of monitoring have evolved over the past 50 years. The importance of monitoring was established with the advent of the intensive care unit (ICU) during the polio epidemic in the 1950s.1 Enhanced monitoring represents the main difference between a general hospital ward and the ICU. The purpose of monitoring is simple and clear: to measure in “real time” physiologic values that can change rapidly. The values can be analyzed and interpreted with the expectation that interventions such as fluid resuscitation, medication administration, or changes in ventilator settings can be made in time to prevent adverse consequences.

For the purposes of this chapter, monitoring emphasizes real-time measurements in the ICU and not intermittent diagnostic procedures such as radiographic imaging, electroencephalography, or angiography. The distinction between monitoring and diagnostic testing is not always well defined. Pulmonary function values are “monitored” over years in outpatient clinics with a concern for chronic disease management. In the ICU, data are monitored continuously or periodically over the course of the ICU admission. This chapter describes monitoring of the respiratory system, cardiovascular system, neurologic status, renal function, liver function, and nutritional status in the ICU.

Principles of Monitoring

Each monitoring test, procedure, or instrument carries certain risks and provides information that has value. There is an important balance between the risks and benefits of monitoring, especially when evaluating new monitoring techniques. Every test or monitoring technique should be continually judged for usefulness. Figure 46-1 depicts a continuum for judging the risk and benefit of tests, diagnostic procedures, and monitoring techniques. The most useful tests have little or no risk and high potential value. Less useful tests carry higher risk with little potential value. Use of a pulse oximeter probe carries little physical risk and provides valuable information about blood oxygenation (low risk-benefit ratio). However, there is a small but known risk of obtaining incorrect values with the instrument. Hemodynamic monitoring requires placement of a highly invasive Swan-Ganz catheter (pulmonary artery catheter) in the pulmonary artery to provide data that must be correctly interpreted. This type of monitoring should be undertaken only with an expectation of collecting important (high value) information. The risk-benefit ratio for pulmonary artery catheterization is high. For this reason and with fluid status data available from central venous pressure (CVP) monitoring, the widespread use of pulmonary arterial catheterization has markedly decreased.2

image
FIGURE 46-1 Balancing the value and risks of monitoring. A test or monitoring technique provides valuable data, but acquisition of the information may carry some risks. Generally, more invasive techniques have an increasing risk of complications; at the same time, the value of the data should be greater. The best monitoring test would be noninvasive, have little risk, and provide valuable data. In contrast, a poor test would be invasive with high risk and low value.

The measurements made during monitoring must be evaluated in the context of a patient’s overall clinical presentation. All monitored variables have defined boundaries for acceptable values and indicators of abnormality that may require immediate attention. The fidelity of a measurement—the degree to which the instrument is actually correct in measuring the value—must be known. There often is nearly complete trust in a digital value displayed, but the monitor may be displaying an incorrect value. When something looks wrong, the patient or the monitor may be the source of the abnormal value. Signals or values are susceptible to variability owing to artifacts, factitious events, physiologic variation, and instrument drift. Instrument software filtering may obscure the problem. Artifacts or nonphysiologic signals are frequently seen, such as when the patient or monitoring lines are moved. The artifacts usually are spikes or major shifts in values that are inconsistent with the patient’s clinical status, or there are nonphysiologic changes. Factitious events are values that are real and “out of range” but often are temporary, such as the elevation in airway pressure during a cough. Factitious events, being real, may require attention, whereas artifacts are usually self-resolving. In addition, the signal itself can exhibit a random variability related to the inherent imprecision of the signal or because of normal physiologic variability in the patient. Blood pressure changes within a certain range for many reasons.

The measuring instrument can produce values that are shifted in a systematic way—consistently high or low or in error in relation to the magnitude of the signal. These shifts are referred to as either parallel or slope shifts (Figure 46-2). Generally, all values must be interpreted with a background of training and experience so that one can understand when a value is normal and when it should be considered abnormal.

image
FIGURE 46-2 Errors in data obtained with a monitoring instrument. Under ideal circumstances, the measured values obtained with a monitoring instrument are correct, and the instrument displays the true values. In reality, there are two forms of shifting from the true values—a parallel shift and a slope shift. Parallel shifts have a constant difference from the true value; a 1-point calibration brings all values into line. A slope shift occurs when measured values have an increasing (or decreasing) difference from the true values. Slope shifts require 2-point calibration.

Monitored values should be accurate or unbiased; that is, the measured values should correctly reflect the real values. Values should be precise; measurements should not vary widely when repeated (the standard deviation of repeated measurements should be low). Systematic errors can be parallel (constant) or slope (change with value) errors that should be correctable by calibrating the instrument (although many monitors now have regular autocalibration algorithms).

image Rule of Thumb

The caregiver has the ability to interpret numerous monitored values and to intervene. Monitors can run continuously and be used to measure values that caregivers cannot “see.” The best monitor is the caregiver who understands the monitoring equipment, alarms, and resulting data.

Several types of instruments and monitors are used for ICU monitoring. The equipment either is at the bedside (point of care) or is brought to the bedside, or the specimens are transported from the patient to instruments in the clinical laboratory. The monitors can be noninvasive—connected to the surface of the body—or invasive—connected to lines or catheters that penetrate the body.

With the responsibility of detecting variations in physiologic data comes the responsibility of setting alarms appropriately. The alarms should be set to detect monitored values that require attention. Monitors also detect the artifacts, factitious events, and random variation often seen in the ICU. The ICU is a noisy environment with many alarms sounding that may not require immediate attention. Ventilators respond to coughs, and electrocardiographs (ECGs), oximeters, and vascular pressure monitors sound alarms with physical movements of the patient. Practitioners must develop mental filtering skills to evaluate true and false alarms and to know when to correct the alarm settings, when to wait for multiple alarms, and when to be concerned about an alarm signal (Box 46-1).

Box 46-1   Monitors and Monitoring Data

The best monitor continues to be the caregiver: the respiratory therapist (RT), nurse, and physician. Changes in the patient’s condition are detected and monitored most directly by means of patient assessment in the ICU. Chapter 15 describes the important details and bedside assessment skills required of caregivers in the ICU. This assessment is valuable because the information is obtained by a decision maker—the caregiver. The art of the physical examination is being lost with the use of monitoring instruments. A common observation is a group of caregivers staring intently at monitors while a patient is waving for attention. Nevertheless, monitors continue to improve, and they are often relied on to substitute for skills of observation. Monitors are needed for two main reasons: (1) continuous assessment (humans need breaks) and (2) measurement of values that caregivers cannot detect, such as ECG findings or airway pressure.

Pathophysiology and Monitoring

Monitoring produces numbers from measurements. Fixing the numbers rather than managing the dysfunction often becomes the goal of treatment. A commonly occurring example is increasing the fractional inspired oxygen concentration (FiO2) in response to reductions in pulse oximeter oxygen saturation (SpO2). Whenever possible, the goals of monitoring and treatment should be based on managing and treating pathophysiology. For each organ or organ system, there is a conceptual framework for the disease process, and treatment should be based on correcting or adapting to the pathophysiologic condition.

The lungs contain 300 million alveoli, but a basic model of lung injury reduces a lung disorder to three types of injured lung units (Figure 46-3): (1) alveoli that are ventilated but not perfused (dead space units), (2) alveoli that are perfused but not ventilated (shunt units), and (3) alveoli that are receiving either partial ventilation or partial perfusion (image mismatch). In terms of treatment options, image mismatching is more responsive to oxygen (O2) therapy than shunt or dead space units. However, all three forms of disordered lung units display reduced O2 saturation. Within the context of this model, treatment should be aimed at the source of the disorder and not an attempt to “fix” the arterial partial pressure of oxygen (PaO2) or SpO2.

image
FIGURE 46-3 A model for the causes of uneven matching of gas and blood in the lung.

Respiratory Monitoring

Gas Exchange

The most important function of the lungs is uptake of O2 from air into the arterial blood and disposal of carbon dioxide (CO2) from mixed venous or pulmonary artery blood into the environment. Arterial blood gas (ABG) values contain this gas exchange information. A typical ABG report includes PaO2, PaCO2, pH, a calculated HCO3−, and an estimated base excess or deficit. ABG samples can be obtained quickly and analyzed rapidly. The typical absolute values, predicted values, derangements including compensation, and basic interpretation of ABGs are described in Chapter 18. However, ABGs do not tell the complete story; other values assessing gas exchange that are actively obtained or calculated are discussed in this section.

Monitoring Oxygenation

Tissue oxygenation depends on FiO2, inspired partial pressure of oxygen (PiO2), alveolar oxygen tension (PAO2), arterial oxygenation (PaO2, SaO2, oxygen content of arterial blood [CaO2]), oxygen delivery, tissue perfusion, and O2 uptake.

Arterial Pulse Oximetry

The goal of breathing and circulation is adequate tissue oxygenation. All organs require O2 delivery that meets O2 use demands; the brain and kidneys have particularly high requirements. An important innovation in monitoring is the use of oximetry—a color spectrum measurement of pulsing arterial blood is used for continuous assessment of arterial oxygenation (SpO2). The human eye is not good at detecting or quantifying arterial hypoxemia. Frank cyanosis does not develop until there is at least 5 g/dl of deoxyhemoglobin in the blood.3 The threshold at which cyanosis becomes apparent is affected by skin perfusion, skin pigmentation, and hemoglobin concentration. ABG analysis has been the accepted method of detecting hypoxemia in critically ill patients, but obtaining arterial blood can be painful and cause complications, and ABG analysis does not provide immediate or continuous data. For these reasons, SpO2 has become the standard for a continuous, noninvasive assessment of SaO2. SpO2 does not measure PaCO2, and patients breathing an elevated FiO2 can build up CO2 (increased PaCO2), although SpO2 values are acceptable. Ventilatory failure may go unnoticed unless ABGs are measured.

SpO2 measurements are based on spectrophotometric principles, which are based on the law of Beer-Lambert. Lightweight probes direct filtered light of specific wavelengths (usually two) through a digit, the bridge of the nose, or an earlobe or reflected from a forehead sensor. The relative absorption of these spectrophotometric beams after passing through the tissue (which differs for O2 saturated and desaturated blood) is converted into the appropriate saturation value with processor-stored algorithms.

Tissue O2 sensors designed to measure SaO2 of muscle or the brain were developed more recently.4 Brain oxygenation is crucial, and deep muscle tissue in compartment syndromes can become deoxygenated. This tissue oxygen sensing technique involves positioning of an emitter and detector (of usually four wavelengths) on the skin surface over the tissue or organ of interest. Light from the emitter reflects from tissue at a depth of one-third the distance between the emitter and detector. The light received by the detector is read, and algorithms determine tissue oxygenation, not SpO2.

image Rule of Thumb

SpO2 values have been considered a fifth vital sign. There are two very important problems with relying on SpO2 to monitor adequate respiratory function: (1) SpO2 values reflect oxygenation, not ventilation; and (2) SpO2 measurement is susceptible to numerous factors that can produce false values.

Although SpO2 has been universally adopted, it does have limitations (Box 46-2).5,6 Motion artifact is an important problem, resulting in inaccurate readings and false alarms. Motion artifacts are common because of shivering, seizure activity, pressure on the sensor, or transport of the patient. The choice of probe site may also affect accuracy. Finger probes appear to be more accurate than forehead, nose, or earlobe probes during low perfusion states. Intense daylight and fluorescent, incandescent, xenon, and infrared light sources have caused errors in SpO2 readings. Anemia and deeply pigmented skin can affect the accuracy of SpO2; however, the effect of anemia is not clinically significant until the hemoglobin level is markedly reduced. Carboxyhemoglobin and methemoglobin can produce falsely high SpO2 values, and some colors of nail polish, particularly blue, green, and black, interfere with light transmission and absorbency, as do some blood-borne dyes, such as indocyanine green and methylene blue, which tend to produce falsely low SpO2 values. Although rare, exposure to numerous toxins and drugs, including topical benzocaine, can elevate methemoglobin and produce falsely elevated SpO2 values.

Box 46-2   Values Affecting Pulse Oximetry

Mini Clini

Trusting the Pulse Oximeter

Oxygen Consumption

Oxygen consumption (image) is the volume of O2 consumed by the body in milliliters per minute. Normal resting image is approximately 250 ml/min, and image increases with activity, stress, and temperature. The acquisition of O2 from the lungs into the circulatory system is described by the Fick equations:

< ?xml:namespace prefix = "mml" />V˙O2=Q˙T(CaO2−Cv¯O2)orQ˙T=V˙O2/(CaO2−Cv¯O2)

image

in which image is cardiac output (CO), and CaO2 and image are arterial and mixed venous oxygen contents. If the values for image, CaO2, and image are normal, this becomes:

V˙O2in ml/min =Q˙T(CaO2−Cv¯O2)

image

=(5000 ml/min)(20 ml/100 ml−15 ml/100 ml)

image

=(5000 ml/min)(5 ml/100 ml)=250 ml/min

image

To an engineer or systems analyst, the function of the lungs is summarized by the Fick equation—O2 in the alveoli diffuses into mixed venous blood flowing by at the pace of CO. This process converts deoxygenated venous blood to oxygenated arterial blood.

An alternative calculation for image that does not require an arterial or mixed venous blood sample or measurement of cardiac output (CO) is:

V˙O2=[(FiO2)(V˙I)]−[(F E¯O2)(V˙E)]

image

where FiO2 and image are the mean inspired and expired fractional concentrations of oxygen, and image and image are the inspired and expired minute ventilations. If image equals image (normally image is slightly greater than image, but the difference is small), this becomes:

V˙O2=(FiO2−F E¯O2)V˙E

image

If the values for minute ventilation and inspired and expired gas concentrations are normal, this becomes:

V˙O2=(0.21−0.168)(6 L/min)=0.252 L/min,or approximately 250 ml/min

image

A normal resting image of 250 ml/min represents approximately 25% of normal O2 delivery (1000 ml/min). The blood carries a large reservoir of O2 that can diffuse into deoxygenated tissues under high O2 demand conditions.

image may be useful in determining nutritional requirements and adequacy of O2 delivery and may occasionally help determine the cause of a high ventilation requirement. If there is a stable tissue demand for O2, measurements of image may be used to follow the hemodynamic response to therapeutic interventions.

Alveolar-Arterial Oxygen Tension Difference

The alveolar and arterial oxygen tension difference (P(A − a)O2) is a useful measurement of the efficiency of gas exchange. A healthy person breathing room air has P(A − a)O2 of approximately 5 to 15 mm Hg. This value increases with age to approximately 10 to 20 mm Hg in elderly adults. P(A − a)O2 also increases normally with an increase in FiO2. A healthy person has P(A − a)O2 of 5 to 15 mm Hg while breathing room air; however, P(A − a)O2 increases to 100 to 150 mm Hg when the person is breathing 100% O2.

Most importantly, an abnormal increase in P(A − a)O2 is associated with gas exchange problems. If PaO2 is 80 mm Hg while the patient is breathing 100% O2, a significant gas exchange problem is present even though oxygenation seems acceptable.

If PaO2 is 80 mm Hg, PaCO2 is 40 mm, FiO2 is 1, and barometric pressure (PB) is 760 mm Hg, then:

PAO2=PiO2−(PaCO2) (1.25)

image

PiO2=[(PB−PH2O) (FiO2)]=713

image

PAO2=[(760−47) (1)]−(40) (1.25)=663 mm Hg

image

P(A−a)O2=PAO2−PaO2=663−80

image

P(A−a)O2=583 mm Hg

image

where PH2O is water vapor pressure. P(A − a)O2 is markedly elevated considering that normal (A − a)O2 is 100 to 150 mm Hg while the patient is breathing 100% O2. (A − a)O2 also can be used to give a rough estimate of percentage shunt, where:

Shunt≅(P(A−a)O2on 100% O2)20

image

In this example, percentage shunt would be approximately 29%: P(A − a)O2/20 = 583/20 = 29.2%.

P(A − a)O2 takes into account PaCO2 and eliminates hypoventilation and hypercapnia from consideration as the sole cause of hypoxemia. Although P(A − a)O2 is infrequently calculated, astute experienced clinicians consciously or subconsciously estimate P(A − a)O2 of every blood gas value.

PaO2/FiO2 Ratio

The PaO2/FiO2 ratio has become important for the determination of the extent of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) in multicenter collaborative studies.7,8 A normal PaO2/FiO2 ratio while breathing room air is about 400 to 500 mm Hg. The PaO2/FiO2 ratio provides an index for the effect of O2 on PaO2 when a range of FiO2 settings may be prescribed. The index allows comparisons of severity between patients or if FiO2 is changed in the same patient. ARDS has been defined by a PaO2/FiO2 ratio less than 200 mm Hg. The PaO2/FiO2 ratio is easy to calculate and a reliable index of gas exchange when FiO2 is greater than 0.5 and PaO2 is less than 100 mm Hg—values often observed in critically ill patients. The level of applied mean airway pressure (MAP), which is strongly affected by PEEP, must also be considered as an independent factor that influences the PaO2/FiO2 ratio.

image Rule of Thumb

ALI/ARDS definitions include:

Related posts:

Patient Education and Health Promotion Airway Management Review of Thoracic Imaging Lung Expansion Therapy Nutrition Assessment Gas Exchange and Transport