Anthony M. Napoli and Baruch Krauss

Perspective

Monitoring is a routine part of emergency department (ED) care. From vital sign measurement to the provision of critical care, monitoring physiologic function provides clinicians with potentially relevant clinical information as a “snapshot” in time or in continuous fashion. In the former case, it allows for screening and triage, and in the latter, it allows for ongoing evaluation of response to treatment or changes in clinical condition.

This chapter focuses on the following monitoring modalities: noninvasive blood pressure measurement, pulse oximetry, and ventilation monitoring and the use of end-tidal carbon dioxide (ETCO₂) measurement. Fetal monitoring after trauma and cerebral function monitoring are also briefly discussed.

Noninvasive Blood Pressure Measurement

Blood pressure measurement by sphygmomanometer is the most common method of noninvasively measuring arterial pressure by indirectly measuring systolic, diastolic, mean arterial pressure, and pulse pressure. The pulse pressure closely resembles stroke volume when arterial compliance and resistance are accounted for. Therefore blood pressure closely represents the interplay of cardiac output and overall systemic vascular resistance, making it an important screening and monitoring tool in ED patients.

Traditional blood pressure measurement by sphygmomanometer can be obtained by Doppler, palpation, auscultation, and oscillometric methods. Palpation allows for only systolic pressure determination, is time and resource intensive, and generally is used only in noisy environments (such as emergency medical services [EMS] transport) where measurement by oscillometric means may be difficult. Doppler is particularly helpful in identifying the presence of a systolic pulse in hypotensive individuals or in internally (i.e., atherosclerosis) or externally (i.e., compartment syndrome) flow-limited extremities. Auscultation, otherwise known as “manual” blood pressure measurement, remains a common, reliable, and reproducible method of blood pressure measurement 100 years after the Korotkoff technique was discovered. This method relies on placing the stethoscope at the antecubital fossa, inflating the blood pressure cuff, and recording the pressure at which auscultation of the pulse returns (the systolic blood pressure) and when it disappears (the diastolic blood pressure). Auscultation is generally considered to underestimate the systolic blood pressure and overestimate the diastolic pressure.¹ Although accuracy has improved with standardization of the technology, this technique is occasionally prone to significant inaccuracy.² The oscillometric technique measures oscillations in the pressure during deflation of the cuff; the maximal point of oscillation represents the mean arterial pressure. This is the most common noninvasive method of measuring blood pressure in the ED, commonly called the “automatic” blood pressure. This method has several advantages over auscultation, including less susceptibility to noise, the lack of necessity for specific placement of a transducer over the artery, less resource usage, regular measurement intervals, and the ability to embed readings into other clinical monitors. Comparisons of auscultation and oscillometric measurement with intra-arterial measurement have found good agreement (mean percent error ≤3%).³ Cuff-based blood pressure measurement is limited by technique and clinical circumstances. The quality of the device, cuff size, body positioning, and arm positioning are all components of the measuring technique that can affect the accuracy. Clinical circumstances such as obesity, dysrhythmias, and extremes of age (young children and elderly) may also affect the accuracy.⁴ The ease of use of oscillometric blood pressure measurement makes this modality ideal for ED noninvasive monitoring.

Other methods of providing continuous, noninvasive blood pressure monitoring often rely on partial radial artery compression or arterial loading counterpressure and offer results comparable to those of an arterial catheter with the benefit of added mobility.5,6 Newer continuous, noninvasive blood pressure monitors have the advantage of providing additional advanced hemodynamic measures, such as stroke volume and cardiac output,⁷ which may be beneficial in critically ill patients.

The gold standard, and the most accurate method of measuring blood pressure, remains the intra-arterial catheter. The combination of the invasiveness of this method and the low but real risk of arterial injury or thrombosis makes its use infrequent. However, such monitoring is becoming more frequent in EDs when there are no available intensive care unit beds and prolonged resuscitation is carried out in the ED. Arterial monitoring is also increasingly a part of aggressive resuscitation management protocols in the ED. Common situations in which invasive arterial monitoring may be of benefit include (1) anticipated hemodynamic instability, (2) monitoring of conditions or treatments that result in large fluid or blood pressure shifts, (3) frequent arterial blood sampling, and (4) expected inaccuracies in noninvasive blood pressure management (e.g., because of obesity or dysrhythmias).

Pulse Oximetry

Pulse oximetry is widely used for ED and out-of-hospital patient monitoring. It provides a real-time assessment of the effectiveness of interventions and disease progression as related to oxygenation. Its use has become so widespread as both a screening and monitoring tool that it is frequently regarded as the “fifth vital sign.”

Pulse oximetry is based on the essential premise of the Beer-Lambert law that the concentration of an absorbing substance can be determined if the characteristic wavelength of that substance, the intensity of the light transmitted through the substance, and the distance of transmission are known. Device design accounts for the intensity of light and the transmission distance. Two light-emitting diodes (LEDs) give off light at wavelengths characteristic of oxyhemoglobin and deoxyhemoglobin (660 nm and 940 nm). Absorption characteristics of tissue, arterial blood, and venous blood are generally static over a cardiac cycle. Therefore through measurement of transmitted light several hundred times per second, the beat-to-beat alterations that occur in pulsatile blood can be isolated and the static components filtered out. The fractional difference between the two wavelengths is measured and reflected in the pulse oximetry.

Pulse oximetry measures the percentage of arterial hemoglobin that is in the oxyhemoglobin state. It reflects the amount of oxygen that hemoglobin is carrying as a percent of the maximum it can carry; this is commonly referred to as the percent arterial oxygen saturation (SaO₂). Pulse oximetry does not measure the partial pressure of oxygen in the blood (PaO₂); however, SaO₂ represents by far the larger reservoir of blood. Nonetheless, an awareness of the relationship between the two measures is important, as one main advantage of pulse oximetry is to obviate the need for multiple samples for arterial blood gas measurement. The relationship between SaO₂ and PaO₂ is described by the oxyhemoglobin dissociation curve. Although the two measures correlate well, their relationship is nonlinear. Users should be aware that large changes in the SaO₂ occur with small changes in the PaO₂ in hypoxic patients, but this is generally not of concern because pulse oximeters are generally accurate between 80% and 100%. Below this range, the role is reversed, and large changes in PaO₂ may occur with small changes in SaO₂. Pulse oximetry retains its usefulness because clinically important hypoxia exists regardless of whether the pulse oximeter is reading 85% or 65%. Measurements can be made on the ears or fingers, although the fingers are generally better in patients with poor perfusion.

Early ED use of pulse oximetry came through extrapolation from anesthesiology literature, which demonstrated less frequent and shorter episodes of desaturation and earlier recognition of hypoxia during or after anesthesia.8,9 In the ED, pulse oximetry is largely used in initial triage of patients, particularly those with cardiopulmonary complaints, or in the monitoring and management of patients who are sedated or critically ill, particularly with respiratory distress or during endotracheal intubation. In the latter case, pulse oximetry has been shown to reduce the frequency of hypoxic episodes. SaO₂ values at or below 96% have been shown to be 100% sensitive for detection of hypoxia (PaO₂ < 70 torr), but such a cutoff can be different in patients with obstructive lung disease.^10,11

Pulse oximetry has several important clinical limitations. By design, pulse oximetry measures only two wavelengths. At these wavelengths, traditional pulse oximeters are unable to distinguish oxyhemoglobin and deoxyhemoglobin from two other important dyshemoglobinemias: methemoglobin (MetHb) and carboxyhemoglobin (COHb). Multiwavelength CO-oximeters are now available that can distinguish the four wavelengths.¹² Oxygen saturation by pulse oximetry will be equivalent to actual saturation, presuming insignificant amounts of MetHb and COHb exist in the blood. In the setting of MetHb or COHb exposure, the pulse oximeter will read falsely elevated levels of SaO₂ even at relatively high exposures. In heavy smokers, COHb levels of 3 to 15% can commonly be found, but this has little impact on the accuracy of the SaO₂. Although signal artifact is probably the most common technical limitation to adequate measurement, other limitations include low perfusion and therefore low pulsatile component, ambient light, deep skin pigmentation, methylene blue, and nail polish. Averaging saturation measurements over several seconds limits the impact of motion artifact and improves the accuracy.

SpO₂ (oxygen saturation as measured by pulse oximetry) may obviate the need for repeated, invasive arterial sampling to determine oxygen saturation (and to some degree PaO₂), but it does not adequately provide insight into pH or partial pressure of carbon dioxide (PaCO₂). More important, SaO₂ does not aid in determining the adequacy of ventilation when significant hypercapnia may precede hypoxia and supplemental oxygen may mask hypoventilation. For this situation, one needs ventilation monitoring through ETCO₂ measurement.

End-Tidal Carbon Dioxide Monitoring

The concentration of carbon dioxide (CO₂) in an exhaled breath is intrinsically linked to tissue metabolism, systemic circulation, and ventilation.¹³ Capnography is the graphic record, represented as a waveform, or capnogram, of the instantaneous CO₂ concentrations in respired gases during a respiratory cycle.^14,15 Capnography provides continuous, real-time, breath-to-breath feedback on the clinical status of the patient, allowing the clinician to determine the baseline ventilatory status and to track changes over time.¹⁶ Capnography is also a diagnostic monitoring modality because certain disease conditions are associated with characteristic waveforms.^14–17 Although the concentrations of CO₂ can be displayed continuously throughout the respiratory cycle, by convention only the maximum CO₂ concentration at the end of each tidal breath, the ETCO₂, is ordinarily displayed. Capnometry is the quantitative measurement of ETCO₂ displayed as a number without a waveform. Colorimetric detectors use color scales to estimate ranges of ETCO₂ but are not sufficiently accurate to give quantitative measurements. Their use is therefore limited to confirmation of correct endotracheal tube (ETT) placement and its continuous location in the trachea. Although originally used during general anesthesia in the operating room, ETCO₂ monitoring has now become a standard monitoring modality in the ED and nonhospital medical setting.

CO₂ monitors are configured as either sidestream or mainstream, depending on the location of the photoelectric detector or sensor. Mainstream devices measure CO₂ directly from the airway, with the sensor attached directly to the ETT. Sidestream devices, more commonly used by EMS personnel and in the ED, aspirate a sample of gas through tubing into a sensor located inside the monitor and are used for both intubated and nonintubated patients. They are lightweight and may be integrated into nasal-oral cannulae that simultaneously sample CO₂ and deliver low-flow oxygen, allowing for continuous oxygen delivery during procedural sedation and analgesia.

Colorimetric CO₂ detectors use pH-sensitive filter paper impregnated with metacresol purple, which changes color from purple (<4 mm Hg CO₂) to tan (4-15 mm Hg CO₂) to yellow (>20 mm Hg CO₂) depending on the concentration of CO₂.13,15 (See Chapter 1.) The indicator, housed in a plastic casing, is inserted between the ETT and the ventilator bag and detects changes on a breath-by-breath basis.¹³ These detectors are also inexpensive and easy to use and should be available in every ED and on every EMS unit that performs intubation for confirmation of ETT placement if capnography or capnometry is not available.

In patients with normal cardiopulmonary function, there is a close correlation between alveolar CO₂ (PACO₂) and arterial CO₂ (PaCO₂). The ETCO₂ is usually 2 to 5 mm Hg less than the PaCO₂ because of the dilution of the end-tidal gases by physiologic dead space gas.18,19 Conditions that affect ventilation-perfusion ratios (including pulmonary embolism), cardiac arrest, hypovolemia, obstructive lung disease, and the lateral decubitus position can widen the Pa-ETCO₂ gradient.^18–22 Several recent studies, however, have shown high concordance between ETCO₂ and PaCO₂ in adult asthmatics and in children with moderate and severe respiratory distress from bronchiolitis, asthma, and pneumonia.²⁰ Although ETCO₂ may not accurately reflect the absolute PaCO₂ in critically ill patients, it is still valuable in detecting ventilatory trends and identifying sudden airway events.

Analysis of the shape of the capnogram can yield valuable diagnostic information.14–16 A normal capnogram has four phases (Fig. 5-1A). Phase 1-2 represents a CO₂-free portion of the respiratory cycle. Most often this is the inspiratory phase, although it may represent apnea or a disconnection of the device from the patient. An elevation of this baseline above zero implies rebreathing of CO₂, as in increased dead space in the circuit or contamination of the sensor.^14–16