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Devices for Assessing Oxygenation and Ventilation


Spirometry

For patients with acute exacerbations of asthma and chronic obstructive pulmonary disease (COPD), accurately estimating the severity of airflow obstruction is a critical component of their care. A focused history plus physical examination is the cornerstone of this assessment in the practice of emergency medicine. Wide variation exists in the ability to accurately diagnose airway obstruction, and a significant proportion of patients with marked airflow obstruction present without dyspnea. This blunted perception of disease severity may be a contributor to fatal and near-fatal asthma attacks. Similarly, patients presenting with acute exacerbations of asthma may experience subjective resolution of their symptoms after therapy, even when severe airflow obstruction is still present. Given these difficulties in recognizing airflow obstruction, objective measurement provides valuable information.

Spirometry is measurement of the volume of air exhaled during forced expiration. It can be interpreted as a function of time to determine the flow rate. Spirometry gives the most complete picture of lung mechanics and is the centerpiece of pulmonary function testing. Many parameters can be derived from a spirogram, the most useful of which are forced vital capacity (FVC), which is the total volume exhaled during a forced expiratory maneuver, and forced expiratory volume in 1 second (FEV 1 ), which is the average flow rate during the first second of the forced expiratory maneuver ( Fig. 2.1 ).

Figure 2.1, Diagrammatic representation of spirometry values. ERV, Expiratory reserve volume; FEV 1 , forced expiratory volume in 1 second; FVC, forced vital capacity; IRV, inspiratory reserve volume; RV, residual volume; TV, tidal volume.

The advent of small handheld devices allows convenient spirometric evaluation in the emergency department (ED). The most common objective measurement of respiratory mechanics used in the ED is peak expiratory flow rate (PEFR). PEFR is the maximum flow of gas achieved during a forced expiratory maneuver. It correlates well with standard spirometry and has been studied extensively in the ED and outpatient setting.

Indications

Evaluation of Acute Asthma Attacks

Currently, no standards exist for the measurement of pulmonary function parameters in ED patients, and practices vary widely. Most patients with asthma exacerbations can be evaluated, treated, and given a disposition with no further pulmonary function testing other than PEFR if quantitative assessment is deemed prudent. Several consensus guidelines recommend obtaining an objective measure of airflow obstruction in all patients seen in the ED with an acute exacerbation of asthma. Other guidelines have proposed that the decision to measure PEFR in patients with acute asthma should be individualized. It is reasonable that mild and easily reversible disease be evaluated and treated according to clinical judgment. If any pulmonary function parameters are to be used, their use should be optimized, including measurements at arrival, after initial treatment, and periodically thereafter.

Evaluation of Exacerbations of COPD

PEFR and spirometry testing can yield objective data on airflow obstruction during the ED evaluation of COPD exacerbations. Though used by some ED practitioners, consensus guidelines do not recommend routine use of these tests in the acute setting.

Differentiating Causes of Dyspnea

PEFR has been studied for its ability to differentiate between COPD and congestive heart failure (CHF). Insufficient data exist to recommend its routine use for this purpose in the ED.

Evaluation of Neuromuscular and Chest Wall Disease

Diseases of the chest wall and neuromuscular system can cause respiratory compromise. Though not commonly done in the ED, pulmonary function testing and assessment of negative inspiratory force can quantify the degree of impairment and help determine the level of admission needed.

Contraindications

Need for Immediate Intervention

Patients with severe respiratory compromise should receive aggressive therapy without delaying care for pulmonary function testing. Although portable pulmonary function testing may guide management, providing immediate intervention for those in distress should be the priority. Formal pulmonary function testing has limited value for acute exacerbations, and such assessments are most predictive when patients are at their baseline functional status.

Conditions That May Be Worsened by Increased Intrathoracic Pressure

Significant elevations in intrathoracic pressure will develop in patients performing a forced expiratory maneuver. Pneumothorax and pneumomediastinum may be exacerbated by the forced expiratory maneuver and aneurysms of the aorta or the cerebral vasculature could theoretically rupture with increased pressures. The presence of these conditions should be considered a relative contraindication to pulmonary function testing.

Equipment

Spirometers can be divided into two categories. Volume spirometers measure the amount of gas exhaled as a function of time. These devices tend to be cumbersome and are not ideally suited to the ED. Flow spirometers measure the flow of gas past a certain point and use that information to extrapolate volume and time data. These machines are smaller, simpler to use, and more portable. Flow spirometers determine gas flow by measuring the difference in pressure between two points in a tube (pneumotachograph), cooling of a heated wire (hot wire anemometer), or revolutions of a rotating vane. Most handheld spirometers also measure PEFR.

The most commonly used device to measure PEFR is the “mini-Wright” peak-flow flow meter ( Fig. 2.2 ). These meters provide accurate and reproducible measurements of PEFR. Variation exists between types and brands of peak-flow flow meters, so comparative measurements should ideally be recorded with the same brand of peak-flow flow meter when possible.

Figure 2.2, The “mini-Wright” peak flow meter.

Procedure

Calibrate the spirometer in accordance with the manufacturer's directions and examine the peak-flow flow meter to ensure that the measurement bar is resting at the zero line before beginning the procedure. For multi-patient devices, attach a disposable mouthpiece to the input orifice.

Before starting the test, explain the procedure and allow unfamiliar patients to practice a few times. Ideally, the patient should be in the standing position or, if not feasible, be seated upright in bed. Ask the patient to elevate the chin and hold the neck in a slightly extended position. A nose clip is not required for PEFR measurements, but may be useful when performing formal spirometry testing.

After a period of normal breathing, ask the patient to take a maximal inspiration with the lips sealed around the mouthpiece while taking care to keep the tongue from partially obstructing the mouthpiece. Because airflow is greatest when the lung volumes are highest and the airways are larger, the test is accurate only if performed after maximal inspiration. Request the patient to initiate a rapid, forceful expiration as soon as possible after reaching maximal inspiration ( Fig. 2.3 ). Coach the patient throughout the procedure and remind the patient to continue to make a forceful and complete exhalation. The PEFR usually occurs during the first 100 msec of expiration. In contrast, when performing spirometry, it is essential that the patient exhale fully. With both tests it is important to have a rapid, forceful exhalation rather than a slow, sustained one. Obtain three separate measurements for both spirometry and PEFR.

Figure 2.3, Measurement of the peak expiratory flow rate (PEFR) with a portable disposable peak flow meter. Ask the patient to take a maximal inspiration with the lips sealed around the mouthpiece and then initiate a rapid, forceful expiration immediately afterward. Three separate measurements should be obtained. Be sure to zero the device before each test. PEFR is the easiest and most common pulmonary function test used to evaluate asthma in the emergency department. Trends are more important than actual values because individual baselines vary widely. PEFR alone cannot be used to make accurate clinical decisions on admission or discharge.

PEFR measurements are very sensitive to technique and patient effort. Even a small decrease in effort can lead to considerable degradation of results. In addition, any deviation from technique can lead to inaccurate results and a faulty estimation of the degree of airway obstruction. Therefore emergency physicians should directly observe and guide their patients when performing this testing.

Interpretation

Obstructive diseases are characterized by a disproportionate decrease in airflow (FEV 1 ) in relation to the volume of gas exhaled (FVC). A decreased FEV 1 /FVC ratio with preservation of FVC indicates the presence of airflow obstruction. Restrictive diseases decrease total lung capacity and therefore decrease FVC to a greater degree than FEV 1 . Decreased FVC with a normal or increased FEV 1 /FVC ratio is indicative of restriction. It is useful to consider the FEV 1 /FVC ratio when attempting to determine whether a patient has airflow obstruction. In patients with an established diagnosis of obstructive disease, FEV 1 is the test that best reflects changes in lung function. Typical values are shown in Table 2.1 . These values are dependent on age, gender, ethnicity, and height and can be predicted from mathematical equations.

TABLE 2.1
Approximate Values
Modified from Hankinson JL, Odencrantz JR, Fedan KB: Spirometric reference values from a sample of the general US population, Am J Respir Crit Care Med 159:179, 1999.
FEV 1 (L) FVC (L) FEV 1 /FVC Ratio (%)
Male 3.0–5.0 3.5–6.0 75–85
Female 2–3.5 2.5–4.0 75–85
FEV 1 , Forced expiratory volume in 1 second; FVC, forced vital capacity.

Isolated measurements of PEFR are not reliable in making the diagnosis of asthma because of significant variation between individuals. It is appropriate to use PEFR to monitor the degree of airflow obstruction in known asthmatics.

Although measures of airflow obstruction are not standalone tests, when considered along with other clinical factors, they can guide decisions regarding the disposition of patients with acute asthma exacerbations. The highest of three PEFR or FEV 1 measurements should be used and, whenever possible, compared with the patient's personal best. In circumstances in which previous best values are unknown or thought to be inaccurate, comparison with predicted values is appropriate. References are available for predicted PEFR values for adults by race and ethnicity. Values for children are presented in Tables 2.2 and 2.3 . The National Asthma Education and Prevention Program has used the results of FEV 1 and PEFR testing to classify the severity of asthma exacerbations ( Table 2.4 ).

TABLE 2.4
Severity of Asthma Exacerbations According to Objective Measures of Airflow Obstruction
% OF PERSONAL BEST OR PREDICTED (FEV 1 OR PEFR) SEVERITY OF EXACERBATION
≤30 Life-threatening
31–50 Severe
51–80 Moderate
>80 Mild
FEV 1 , Forced expiratory volume in 1 second; PEFR, peak expiratory flow rate.

TABLE 2.2
Predicted Peak Expiratory Flow Rate in Males 8-20 Years of Age a
Height (Inches)
AGE (YEAR) ETHNICITY 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74
8 Caucasian 160 178 197 217 — — — — — — — — — — —
African American 143 163 184 205 — — — — — — — — — — —
Mexican American 152 174 197 221 — — — — — — — — — — —
9 Caucasian — 184 203 223 243 264 286 — — — — — — — —
African American — 165 186 207 230 253 277 — — — — — — — —
Mexican American — 177 200 224 249 275 301 — — — — — — — —
10 Caucasian — — 210 230 251 272 294 317 — — — — — — —
African American — — 190 211 234 257 281 306 — — — — — — —
Mexican American — — 205 229 254 279 306 334 — — — — — — —
11 Caucasian — — — 239 260 281 303 326 349 — — — — — —
African American — — — 217 240 263 287 312 338 — — — — — —
Mexican American — — — 235 260 286 312 340 369 — — — — — —
12 Caucasian — — — — 271 292 314 337 360 385 — — — — —
African American — — — — 248 271 295 320 346 373 — — — — —
Mexican American — — — — 268 294 321 348 377 406 — — — — —
13 Caucasian — — — — — 304 326 349 372 397 422 448 — — —
African American — — — — — 282 306 331 357 383 411 439 — — —
Mexican American — — — — — 304 331 358 387 416 447 478 — — —
14 Caucasian — — — — — — 340 363 386 411 436 462 488 — —
African American — — — — — — 319 344 369 396 424 452 481 — —
Mexican American — — — — — — 342 370 399 428 459 490 522 — —
15 Caucasian — — — — — — — 378 402 426 451 477 504 531 —
African American — — — — — — — 358 384 411 438 467 496 526 —
Mexican American — — — — — — — 384 412 442 472 503 536 569 —
16 Caucasian — — — — — — — — 419 443 468 494 521 548 576
African American — — — — — — — — 401 428 455 484 513 543 574
Mexican American — — — — — — — — 427 457 487 519 551 584 618
17 Caucasian — — — — — — — — — 462 487 513 539 567 595
African American — — — — — — — — — 447 475 503 532 562 593
Mexican American — — — — — — — — — 474 504 536 568 601 635
18 Caucasian — — — — — — — — — 482 507 533 560 587 615
African American — — — — — — — — — 469 496 524 554 584 615
Mexican American — — — — —- — — — — 492 523 554 587 620 654
19 Caucasian — — — — — — — — — 504 529 555 581 609 637
African American — — — — — — — — — 492 520 548 577 607 638
Mexican American — — — — — — — — — 513 543 575 607 640 674
20 Caucasian — — — — — — — — — 527 552 578 605 632 660
African American — — — — — — — — — 518 545 574 603 633 664
Mexican American — — — — — — — — — 535 565 597 629 662 697

a Modified from equations from Hankinson JL, Odencrantz JR, Fedan KB: Spirometric reference values from a sample of the general US population, Am J Respir Crit Care Med 159:179, 1999.

TABLE 2.3
Predicted Peak Expiratory Flow Rate in Females 8-18 Years of Age a
Height (Inches)
AGE (YEAR) ETHNICITY 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74
8 Caucasian 162 175 190 204 220 — — — — — — — — — —
African American 166 180 195 211 227 — — — — — — — — — —
Mexican American 164 180 197 214 233 — — — — — — — — — —
9 Caucasian — 195 209 223 239 255 271 — — — — — — — —
African American — 190 205 221 237 254 271 — — — — — — — —
Mexican American — 195 212 229 248 267 286 — — — — — — — —
10 Caucasian — — 226 241 256 272 288 305 — — — — — — —
African American — — 215 231 247 264 281 299 — — — — — — —
Mexican American — — 225 243 261 280 300 320 — — — — — — —
11 Caucasian — — — 256 271 287 303 320 338 — — — — — —
African American — — — 240 257 273 291 309 328 — — — — — —
Mexican American — — — 255 273 292 312 332 353 — — — — — —
12 Caucasian — — — — 284 300 317 334 351 369 388 — — — —
African American — — — — 266 283 301 319 337 357 376 — — — —
Mexican American — — — — 283 302 322 342 363 385 407 — — — —
13 Caucasian — — — — — 311 328 345 362 380 399 418 — — —
African American — — — — — 293 310 329 347 366 386 407 — — —
Mexican American — — — — — 311 331 351 372 393 416 439 — — —
14 Caucasian — — — — — — 337 354 371 389 408 428 — — —
African American — — — — — — 320 338 357 376 396 417 — — —
Mexican American — — — — — — 338 358 379 401 423 446 — — —
15 Caucasian — — — — — — 344 361 378 397 415 435 — — —
African American — — — — — — 330 348 367 386 406 426 — — —
Mexican American — — — — — — 343 363 384 406 428 451 — — —
16 Caucasian — — — — — — — 366 383 402 420 440 460 — —
African American — — — — — — — 358 377 396 416 436 457 — —
Mexican American — — — — — — — 367 388 410 432 455 479 — —
17 Caucasian — — — — — — — 369 386 405 423 443 463 — —
African American — — — — — — — 368 386 406 426 446 467 — —
Mexican American — — — — — — — 370 391 412 435 458 482 — —
18 Caucasian — — — — — — — 370 387 406 424 444 464 — —
African American — — — — — — — 378 396 416 435 456 477 — —
Mexican American — — — — — — — 371 392 413 436 459 482 — —

a Modified from equations from Hankinson JL, Odencrantz JR, Fedan KB: Spirometric reference values from a sample of the general US population, Am J Respir Crit Care Med 159:179, 1999.

Multiple guidelines and articles have advocated specific cutoff values for PEFR and FEV 1 to guide decisions on disposition. There is variation across these guidelines and no consensus that absolute cutoffs should exist. Spirometric values including PEFR and FEV1 should be viewed as additional data points to be considered, along with other clinical variables such as response to inhaled bronchodilators, in determining the disposition of asthmatics seen in the ED. Poor response to treatment or failure to recover PEFR or FEV 1 to greater than 70% of personal best are examples of factors that might favor admission rather than outpatient management of acute asthma exacerbations.

Noninvasive Oxygenation Monitoring: Pulse Oximetry

Pulse oximetry, or noninvasive measurement of the percentage of hemoglobin bound to O 2 , provides real-time estimates of arterial saturation in the range of 80% to 100% and gives early warning of diminished capillary perfusion while avoiding the discomfort and risks associated with arterial puncture. Pulse oximetry has become the standard of care in a wide variety of clinical settings.

Technology

Oximetry is based on the Beer-Lambert law, that states the concentration of an unknown solute dissolved in a solvent can be determined by light absorption. Pulse oximetry combines the principles of optical plethysmography and spectrophotometry. The probe, set into a reusable clip or a disposable patch, is made up of two photodiodes, which produce red light at 660 nm and infrared (IR) light at 900 to 940 nm, and a photodetector, which is placed across a pulsatile vascular bed. Pulse oximeter sensors are typically placed on the finger, with the light centered over the nailbed, not the fat pad. However, other locations such as the toes and earlobes are used in some settings such as pediatrics. These particular wavelengths are used because the absorption characteristics of oxyhemoglobin and reduced hemoglobin are quite different at the two wavelengths. The majority of the light is absorbed by connective tissue, skin, bones, and venous blood. The amount of light absorbed by these substances is constant with time and does not vary during the cardiac cycle. A small increase in arterial blood occurs with each heartbeat, thereby resulting in an increase in light absorption ( Fig. 2.4 ). By comparing the ratio of pulsatile and baseline absorption at these two wavelengths, the ratio of oxyhemoglobin to reduced hemoglobin is calculated.

Figure 2.4, Factors influencing light absorption through a pulsatile vascular bed.

Because the pulse oximeter uses only two wavelengths of light, it can distinguish only two substances. Pulse oximeters measure “functional saturation”: the concentration of oxyhemoglobin divided by the concentrations of oxyhemoglobin plus reduced hemoglobin. The disadvantage of functional saturation is that the denominator does not include other hemoglobin species that may be present, such as carboxyhemoglobin and methemoglobin . The advantage of using only two wavelengths in the oximeter is that the cost, size, and weight of the device are reduced. Alternative devices, such as the CO-oximeter, use four or more wavelengths to measure “fractional saturation,” and are able to quantify additional hemoglobin species.

Physiology

Arterial O 2 saturation (Sa o 2 ) measures the large reservoir of O 2 carried by hemoglobin, 20 mL of O 2 /100 mL of blood, and arterial O 2 partial pressure (Pa o 2 ) measures only the relatively small amount of O 2 dissolved in plasma, approximately 0.3 mL of O 2 /100 mL of blood. Sa o 2 correlates well with Pa o 2 , but the relationship is nonlinear and is described by the oxyhemoglobin dissociation curve ( Fig. 2.5 ). In hypoxemic patients, small changes in Sa o 2 represent large changes in Pa o 2 because these Sa o 2 values fall on the steep portion of the curve. Conversely, measurements of Sa o 2 are relatively insensitive in detecting significant changes in Pa o 2 at high levels of oxygenation because these Sa o 2 values fall on the plateau portion of the curve.

Figure 2.5, Oxyhemoglobin dissociation curve. Measurements of Sa o 2 are relatively insensitive in detecting significant changes in Pa o 2 at high levels of oxygenation because these Sa o 2 values fall on the plateau portion of the curve (labeled). Hence, O 2 saturation is an insensitive way of detecting early compensation in patients with asthma.

Currently available pulse oximeters are accurate and precise when saturation ranges from 70% to 100%. This range is satisfactory because for most patients an O 2 saturation of 80% is as much an urgent warning as is one lower than 70%. Testing of pulse oximeters has shown that at 75% saturation, bias is scattered uniformly between underestimation and overestimation.

Clinical Utility

Pulse oximetry peripheral oxygen saturation (Sp o 2 ) offers an advantage in assessing the adequacy of oxygenation over arterial blood gas analysis by providing continuous measurements. Direct measurement of Sa o 2 requires blood gas values coupled with knowledge of the actual hemoglobin levels in a patient's blood. Sa o 2 measurement is estimated with pulse oximetry. In this chapter we equate Sa o 2 and Sp o 2 .

There are limited data on the clinical efficacy of pulse oximetry monitoring in the ED. Anesthesia studies have demonstrated that continuous monitoring of saturation can improve detection of hypoxemia and related events, though this practice has not been shown to impact major clinical outcomes such as mortality or transfer to the intensive care unit (ICU). Nonetheless, there may be benefit to use in emergency medicine, particularly in those with primary cardiopulmonary disease or critically ill patients. Rapid recognition of adverse physiologic events should allow prompt initiation of therapeutic interventions.

Indications

Recommended uses for pulse oximetry fall into two broad categories: (1) as a real-time indicator of hypoxemia, continuous oximetry monitoring can be used as a warning system because many adverse patient events are associated with arterial desaturation, and (2) as an end point for titration of therapeutic interventions to avoid hypoxia ( Box 2.1 ).

Box 2.1
Clinical Applications of Pulse Oximetry

  • Assessing the adequacy of preoxygenation before ET intubation

  • Monitoring oxygenation during emergency airway management

  • Monitoring the ventilator and changes in Fi o 2

  • Providing an early indicator of ventilator dysfunction

  • Assisting in routine weaning from O 2 therapy

  • Monitoring patients in acute respiratory distress

  • Monitoring during procedural sedation and analgesia

  • Monitoring during interhospital and intrahospital transport

Fi o 2 , Fractional concentration of inspired O 2 .

Pulse oximetry can also be used to assess peripheral perfusion and evaluate for possible ischemia in the extremities. Although clinical experience validates its use, minimal data are available for such utilization in the ED. Vascular surgeons will use a pulse oximetry probe on a finger or toe to assess the results of vascular surgery on the arm or leg. Peripheral artery occlusion from peripheral artery disease may be suggested by comparison of pulse oximetry readings in the extremities. Decreased peripheral oxygenation may be detected in patients with compartment syndrome, traumatic arterial injury, and external compression of the proximal circulation ( Fig. 2.6 ).

Figure 2.6, Pulse oximetry can be used to assess the distal circulation after vascular surgery for trauma and to initially evaluate other causes of decreased peripheral perfusion. A, This patient had a markedly swollen and ischemic finger from a tight ring. B, Although the need for immediate removal of the ring is clinically obvious, a pulse oximetry probe confirmed ischemia with an O 2 saturation of 61%. The uninvolved fingers registered 99%. Following ring removal the saturation returned to normal, thus suggesting that fasciotomy need not be performed. C, When a discharged EpiPen caused a pale finger, injection of phentolamine was considered. D, When pulse oximetry demonstrated a saturation of 96% (97% to 98% in the other fingers), injection was not performed and the circulation spontaneously normalized over a period of 30 minutes.

Procedure

The location for the probe is determined by the clinical situation and the types of probes available ( Fig. 2.7 ). A reusable clip-on probe works well on digits that are easily accessible. Other sites include the earlobe, the nasal bridge, the septum, the forehead/temporal artery, and the foot or palm of an infant. More central locations may provide better readings in cold ambient temperature or during movement. Tape and splints can be used to secure oximetry probes and minimize motion.

Figure 2.7, Pulse oximeter sensors. A, Reusable adult sensor, attached to finger. B, Single-use adult sensor, attached to finger. C , Single-use pediatric sensor, attached to toe. Note that the light source is centered over the nail, not the fat pad. See text for various parameters that affect pulse oximetry readings.

The computer analyzes the incoming data to identify the arteriolar pulsation and displays this parameter as beats per minute. Newer devices also display a pulse plethysmograph ( Fig. 2.8 ). Simultaneously, O 2 saturation is displayed on a beat-to-beat basis. Some machines have hard-copy capability and can provide paper documentation of the patient's status. Machines differ in their display when a pulsatile flow is not detected. Either the reading will not display at all, or the Sa o 2 value will be given along with a poor-signal quality warning. It is important to evaluate serial measurements and to verify that the measurements correlate with other clinical markers.

Figure 2.8, Patient monitor displaying a pulse plethysmograph (arrow). The patient's heart rate (84) and O 2 saturation (95) are also displayed (blue numbers).

Interpretation

Patients with normal physiologic gas exchange have an O 2 saturation between 97% and 100%. When Sa o 2 falls below 95%, hypoxemia may be present, which may be baseline for some patients with cardiac or lung disease. O 2 saturation below 90% represents hypoxemia. As with spirometry, an isolated, low early measurement of Sa o 2 does not mandate admission because of the potential for rapid response to therapy. Low Sa o 2 readings should be heeded as important clinical warning signs. Pulse oximetry may be affected by numerous extrinsic factors, and a decline in O 2 saturation with serial measurements should always prompt an evaluation of respiratory status and adequacy of circulation.

Although pulse oximetry represents a significant advance in noninvasive monitoring of oxygenation, clinicians must recognize and understand its limitations. Pulse oximetry measures only O 2 saturation. In contrast to arterial blood gas determination, pulse oximetry provides no direct information on pH or the arterial partial pressure of CO 2 (Pa co 2 ). Nonetheless a room-air Sa o 2 value of 97% or higher strongly rules against hypoxemia and moderate to severe hypercapnia. A room-air Sa o 2 value of 96% or less has been demonstrated to be 100% sensitive and 54% specific for detecting hypoxemia (Pa o 2 <70 mm Hg), and 100% sensitive and 31% specific for detecting hypercapnia (Pa co 2 >50 mm Hg). Using a cutoff value of 92% or less for room-air Sa o 2 is more accurate in identifying hypoxemia in patients with COPD.

Pulse oximetry is not a substitute for monitoring ventilation because of the variable lag time between the onset of hypoventilation or apnea and a change in O 2 saturation. During procedural sedation, monitoring of ventilation is a more desirable goal for prevention of hypoxemia and hypercapnia than simple pulse oximetry (see section on Procedural Sedation and Analgesia under Carbon Dioxide Monitoring later in this chapter). Hypoventilation and the resultant hypercapnia may precede a decrease in hemoglobin O 2 saturation by many minutes. Supplemental O 2 may mask hypoventilation by delaying the eventual O 2 desaturation that pulse oximetry is designed to monitor and recognize. Other limitations of pulse oximetry are summarized in Box 2.2 .

Box 2.2
Factors Affecting Pulse Oximetry Readings

  • Severe anemia: Satisfactory readings obtained down to a hemoglobin level of 5 mg/dL

  • Motion artifact: See text regarding probe sites

  • Dyes: Transient effect unless resulting in methemoglobinemia

  • Light artifact: Minimize by covering the probe with opaque material

  • Hypoperfusion: An inadequate pulse signal will be displayed

  • Electrocautery: Minimize by increasing the distance of the sensor from the surgical site

  • Deep pigmentation: Use the fifth finger, earlobe, or other area with lighter pigmentation

  • Dark nail polish: Remove with acetone or place a sensor sideways on the digit

  • Dyshemoglobinemias (e.g., carboxyhemoglobin and methemoglobin): Falsely elevate true oxygen saturation readings

  • Elevated bilirubin: Accurate up to a bilirubin level of 20 mg/dL in adults; no problem reported in jaundiced children

  • High saturation: Pulse oximetry not useful for monitoring hyperoxemia in neonates

  • Fetal hemoglobin: No effect on pulse oximetry; falsely reduced CO-oximetry readings

  • Venous pulsations: Artificially lower O 2 saturation; choose a probe site above the heart

  • Dialysis graft (arteriovenous fistula): No difference from the contralateral extremity unless the fistula is producing distal ischemia

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