485CHAPTER 43 Noninvasive Respiratory Monitoring and Assessment of Gas Exchange studied,32 and shielding of the probe from ambient light is often used clinically to improve performance Intravenous dye[.]
CHAPTER 43 Noninvasive Respiratory Monitoring and Assessment of Gas Exchange HEMOGLOBIN-OXYGEN DISSOCIATION CURVE Saturation (%) 20 80 15 60 Mixed venous (PvO2) 40 10 Arterial (PaO2) 20 CaO2 minus CvO2 Oxygen content (mL/100 mL) 100 0 20 40 60 80 100 Partial pressure of oxygen (PO2 [mm Hg]) 600 Total oxygen Hemoglobin-oxygen Dissolved • Fig 43.3 Hemoglobin-oxygen (Hb-O2) dissociation curve shows the percentage saturation of hemoglobin at each Po2 When the hemoglobin concentration is known, the content of oxygen can be calculated The total content includes the small additional content of oxygen in solution, which becomes significant at high levels of Po2 The saturation scale on the left applies only to the Hb-O2 line The scale on the right shows content values for a normal hemoglobin level of 15 g/100 mL blood (Modified from Albert RK, Spiro SG, Jett R, eds Clinical Respiratory Medicine, 2nd ed St Louis: Elsevier; 2004.) portion of the oxyhemoglobin dissociation curve—occur with little change in saturation Saturations measured by pulse oximetry overestimate Sao2 in the range of 76% to 90%, on average,23 and the accuracy of pulse oximetry falls with arterial oxygen saturations less than 70%.2,6 At arterial oxygen saturations below 70%, pulse oximetry may be more appropriate for showing trends.6 Of particular note to ICU providers, low peripheral perfusion and motion artifact are the most common causes of inaccurate pulse oximetry readings.5,15,24 Pulse oximetry sensors may be unable to distinguish a true signal from background in low perfusion states that result in diminished pulsations (e.g., vasoconstriction, low cardiac output, and hypothermia).25 This situation is usually displayed as an inadequate pulse message.2 Newer designs of pulse oximeters with signal-progressing algorithms that detect and ignore motion and pulse rate interferences help overcome these limitations.15,24 Recently, pulse oximeter probes that use reflectance technology have been developed for use on the forehead.7,26 These pulse oximeters are designed so that both light-emitting diodes and detectors are on the same surface, decreasing scattering of light through the tissue as compared to traditional probes.27,28 In studies of critically ill patients with poor perfusion, forehead reflectance probe oximetry measurement was found to be more accurate than finger probe.26,29,30 While limited, data suggest that forehead reflectance sensors are comparable in accuracy to digit sensors in pediatric patients.31 Other sources of error include interfering dyes, other pigments in the blood and ambient light.25 The extent of ambient light interference has been questioned for some of the pulse oximeters 485 studied,32 and shielding of the probe from ambient light is often used clinically to improve performance Intravenous dyes and certain colors of nail polish may falsely lower pulse oximetry readings The presence of dyshemoglobinopathies is an infrequent clinical problem but can result in erroneous pulse oximetry readings.25 Abnormal hemoglobin levels (carboxyhemoglobin, methemoglobin) that have similar absorbance spectra can lead to overestimation of the true Sao2.6 In methemoglobinemia, the iron in the heme groups in hemoglobin becomes oxidized from the ferrous (Fe21) state to the ferric (Fe31) state, which results in hemoglobin being unable to bind oxygen The presence of significant quantities of methemoglobin leads to tissue hypoxia because these molecules no longer participate in oxygen transport However, light absorbance by methemoglobin more closely resembles oxyhemoglobin than deoxyhemoglobin at the measured wavelengths, erroneously leading the pulse oximeter to indicate a higher percentage of oxygen saturation than expected.33,34 Similarly, the presence of carboxyhemoglobin may result in an erroneous reading in pulse oximetry because carbon monoxide–bound hemoglobin also does not participate in oxygen transport.35 To address this source of error, multiwavelength oximeters that can specifically detect carboxyhemoglobin and methemoglobin have been developed Masimo has developed a pulse oximeter, known as the Masimo Rainbow SET Rad 57, that uses eight wavelengths of light and is capable of determining levels of oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin.36 Because these other species of hemoglobin are recognized, it is also possible to have a continuous readout of total hemoglobin Since its development, multiple studies have been performed to determine the accuracy of the Rad 57 and potential for clinical application In healthy volunteers, the Rad 57 was found to measure carboxyhemoglobin levels within the range of 0% to 15% with an uncertainty of 62% and measure methemoglobin levels within the range of 0% to 12% with an uncertainty of 0.5%.7,36 Data from patients suspected of carbon monoxide poisoning have demonstrated varied degrees of difference between carboxyhemoglobin levels measured by Rad 57 pulse oximetry and laboratory carboxyhemoglobin levels, ranging from 1.4% to 4.2%.7,37–39 For patients seen in the emergency department, large limits of agreement between measurement were noted (211.6% to 14.14%).39 Based on these findings, some authors have suggested that carboxyhemoglobin level measurement by CO-oximetry may not be interchangeable with laboratory evaluation and that further studies are required.7,37–39 Because CO-oximeters account for the presence of both carboxyhemoglobin and methemoglobin, blood gas samples sent for CO-oximetry should correctly measure hemoglobin saturation in cases in which measurable levels of either methemoglobin or carboxyhemoglobin are present or suspected Probe Placement Pulse oximetry probes typically are placed on fingers or toes, with the light-emitting diodes placed across the digit, opposite from the detector For premature and small infants, the probe often is placed around the entire palm or foot with good results In larger pediatric patients, the nasal ala may also be used.40 Transesophageal probes have been designed and are used for care of operative or critically ill patients with potentially poor peripheral perfusion.41–43 The complications of pulse oximetry are rare They include skin burns and pressure necrosis in newborns.2,6 Limited 486 S E C T I O N V Pediatric Critical Care: Pulmonary understanding of pulse oximetry by healthcare providers may be an underrecognized problem, along with time spent determining whether alarms are false.5,44 Capnometry and Capnography Another monitoring technology routinely used in the ICU is the measurement of CO2 Capnometry is the measurement of the partial pressure (or concentration) of CO2 in the patient’s airway during the entire ventilatory cycle A capnometer provides a numeric measurement of inspired and expired, proximal end-tidal CO2 (Petco2) Capnography is the graphic display of the partial pressure or concentration of CO2 as a waveform (capnogram), usually plotted as Pco2 versus time (Fig 43.4) When the waveform display is calibrated, capnography includes capnometry Tissue Oximetry In theory, measurement of tissue oxygenation will allow the clinician to fully assess oxygen delivery and utilization as opposed to solely oxygen content of the blood Because oxygen delivery is dependent on cardiac function, a primary benefit of tissue oximetry measurements may be for assessment of perfusion and cardiovascular function A number of technologies have been developed in order to measure the oxygenation of tissues themselves, including near-infrared spectroscopy (NIRS), direct tissue partial pressure of oxygen (Po2) measurement with an oxygen electrode, transcutaneous tissue electrodes, microdialysis, and electron paramagnetic resonance.45 Within pediatric ICU medicine, the use of NIRS is center and provider dependent, with many variations in practice Operating Principles of Capnometry Sampling of exhaled CO2 can be at the patient-ventilator interface (mainstream), diverted to a monitor (sidestream), or an intermediate connection.48 In the most common sampling method, gas is diverted from the airway and aspirated through a tube (sidestream) to the CO2 monitor A low dead space sidestream CO2 monitor is optimal for use in patients weighing less than 10 kg An alternative to the diverting instrument is the nondiverting— or “mainstream”—capnometer, in which a special flow-through adapter and CO2 monitor are placed in the patient’s airway The exhaled gas sample is exposed to various wavelengths of infrared light The relative amount of light absorbed by the exhaled sample is compared with the amount of light absorbed by a sample that does not contain CO2 By comparing the difference in absorption between the two samples, the capnometer determines the amount of CO2 in the sample gas, which it then displays as the CO2 concentration CO2 also can be measured semiquantitatively using a pH-sensitive indicator that changes from purple to yellow when exposed to CO2.49 The Easy Cap Petco2 detector is placed between the endotracheal tube and resuscitation bag after intubation; a color change from purple to yellow should be observed Up to six breaths may be necessary to allow for washout of retained CO2 in the event of esophageal intubation A constant purple color indicates that the tube is not in the trachea or the presence of poor pulmonary perfusion A tan color may indicate tracheal intubation with poor pulmonary perfusion or esophageal intubation with retained CO2 Near-Infrared Spectroscopy NIRS is a noninvasive method used to measure the Hgb-oxygen saturation of a local region of interest This is achieved through the use of multiple (2 to 4) wavelengths of near-infrared light directed via a cutaneous probe into the underlying tissue, which are absorbed by pigments such as myoglobin, hemoglobin, and cytochrome.46 In contrast to pulse oximetry, NIRS sampling is not specific to pulsatile blood flow and captures measurements from arterial, capillary, and venous blood compartments.45 Cerebral NIRS oximeters are calibrated by manufacturers with the assumption that the contribution of venous blood to oximetry readings is approximately 70% to 75% and the displayed SaO2 is determined through an algorithm.45,46 Accordingly, this may be a source of error Studies have reported varying degrees of accuracy for cerebral NIRS oximeters and have also found large variation in reading errors.45–47 Authors of these studies have postulated that this is possibly due to variations in the ratios of cerebral venous and arterial blood volume in response to alterations of blood oxygen and carbon dioxide (CO2) levels, violating the assumption of constant ratios of 70:30 or 75:25.45,46 Extracranial tissue changes due to superficial vasoconstriction secondary to vasoactive medication (e.g., norepinephrine, phenylephrine) or sympathetic response to pain or hypothermia can be additional sources of error with cerebral NIRS oximetry.45 Based on these findings, many authors recommend the use of cerebral oximetry as trend monitors and not absolute tissue oxygenation measures or injury threshold determinants.45,46 CO2 (mm Hg) Both physiologic anomalies and technical factors can result in Petco2 values that not approximate partial pressure of arterial carbon dioxide (Paco2) For Petco2 to approximate Paco2, two assumptions must be met: (1) the lung units must empty Real-time 50 Trend D 37 Clinical and Technical Issues C A B E • Fig 43.4 Normal capnogram Points A and B: Baseline during inspiration or dead space Points B and C: Expiratory upstroke (mix of dead space and alveolar gas) Points C and D: Expiratory or alveolar plateau Point D: Proximal end-tidal carbon dioxide, or Petco2 Points D and E: Inspiratory downstroke (Modified from Zwerneman K End-tidal carbon dioxide monitoring: a VITAL sign worth watching Crit Care Nurs Clin North Am 2006;18:217–225.) CHAPTER 43 Noninvasive Respiratory Monitoring and Assessment of Gas Exchange synchronously with uniform time constants and (2) ventilation and perfusion must be well matched in the lung units Additionally, technical variables can produce Petco2 values that not approximate Paco2 These include the design of the gas sampling system, distance the gas must be transported, and the instrument’s calibration methods Gas Sampling Issues The gas sampling method used by a capnometer affects the accuracy of the capnogram and Petco2 measurements Relevant factors include the location of the ventilatory circuit from which the gas is sampled, the distance over which the gas is transported before analysis, and the sample flow rate of the instrument With a nondiverting or mainstream device, the CO2 monitor is placed on the airway so that there is no need to divert gas from the airway This sampling configuration typically is available only in infrared capnometers because only infrared CO2 monitors can be designed small enough to fit on the airway A study comparing mainstream Petco2 to a novel method that sampled distal end-tidal CO2 via a special double-lumen tube found that the distal end-tidal samples had the best correlation with Paco2 and remained reliable even when severe lung disease was present.50 Another sampling configuration is seen in the proximal-diverting device A lightweight, low-profile airway adapter is placed in the patient’s airway, and gas is sampled from the airway and transported to the sensor, which is placed near the patient but not in the airway itself A third sampling configuration is found in the distal-diverting device, the classic sidestream capnometer In a distal-diverting system, gas is sampled from the airway and transported to the CO2 monitor, which is located in the display unit distal to the patient The accuracy of the capnogram, Petco2 measurements, and displayed values depends on the sampling site (Fig 43.5) In continuous gas flow circuits, sampling in or at the endotracheal tube results in the most accurate values because there is little contamination with fresh gas from the breathing circuit (point A in Fig 43.5) The Y-connector of the breathing circuit is the next best sampling site (point B in Fig 43.5) However, if the fresh gas flow is large compared with the expiratory flow rate of the patient (as may be the case in neonates and small children), the capnogram and Petco2 values may be distorted as a result of dilution with the fresh gas flowing through the Y-connector If gas is sampled “downstream” from the patient, the waveform and Petco2 are increasingly diluted by fresh gas from the circuit (points C and D in Fig 43.5) If gas is sampled “upstream” from the patient in the fresh gas supply, none of the exhaled CO2 is detected and the measured Petco2 is zero (point E in Fig 43.5) Therefore, the best sampling site is within the patient’s endotracheal tube or at the tube connector, as far as possible from the Y-connector of the breathing circuit.51 In breathing circuits with intermittent flow (demand-valve ventilators) and in larger children with large exhaled tidal volumes, the capnogram and Petco2 values are usually unaffected by minor changes in sampling location If the tidal volume is small (e.g., as in infants and children) and the sample flow rate is large (i.e., 150 mL/min), the capnogram and Petco2 measurements may be significantly diluted by the entrainment of fresh gas Using a capnometer system with a low sample flow rate, typically less than 75 mL/min, restores the waveform and Petco2 readings to more accurate values Physiologic Basis Petco2 monitoring is the noninvasive measurement of exhaled CO2 at the plateau of the CO2 waveform Petco2 concentration reflects Paco2, cardiac output, percentage of dead space, and airway time constants In healthy subjects, the Petco2 concentration is to mm Hg less than the Paco2.52 Petco2 concentration represents the Pco2 of all ventilated alveoli, whether they are perfused or not Therefore, any condition that reduces pulmonary perfusion of ventilated alveoli increases the difference between Paco2 and Petco2 When ventilation and perfusion are well 40 A 33 B E A B C D 15 10 C D 487 E • Fig 43.5 Accuracy of end-tidal carbon dioxide (CO2) measurements is highly dependent on obtaining good samples of expiratory gas from the patient If sampled gas is contaminated with fresh gas from the breathing circuit, the measured values will not be accurate The best samples are obtained from a site nearest to the source of CO2, the patient (point A) 488 S E C T I O N V Pediatric Critical Care: Pulmonary matched throughout the lung, the Paco2 and partial pressure of Petco2 are nearly equal, normally 40 mm Hg If a discrepancy between ventilation and perfusion exists, a difference between the Paco2 and Petco2, also known as (a-et)DPco2, occurs Comparison between Petco2 concentration and Paco2 helps to differentiate between change in alveolar ventilation, CO2 production, or pulmonary perfusion as a cause of the change in Petco2 concentration Dead Space Ventilation Dead space is the volume of gas in the airways and lung that participates in tidal breathing but does not participate in gas exchange Obvious examples are the volume of the endotracheal tube and ventilator circuit (apparatus dead space) and the volume of the tracheal lumen and central airways (anatomic dead space) A less obvious, but still important, source of error in critically ill patients is alveolar or physiologic dead space This dead space is attributable to lung units in which ventilation greatly exceeds perfusion Gas exchange in these overventilated, relatively underperfused lung units is less efficient than normal The fraction of tidal volume that is delivered to all dead spaces taken together can be calculated using the Bohr equation A sample of mixed expired air collected over numerous breaths is analyzed for mixed Pco2 (Peco2) The total fraction of dead space per tidal volume is given by Vd/Vt (Paco2 Peco2)/Paco2 Physiologic dead space (Vd/Vt) can be determined from a variant of the Bohr equation: Vd/Vt (Paco2 Petco2)/Paco2 or Vd/Vt (Petco2/Paco2) where Petco2 is used in place of a sample of mixed expired air In many clinical situations, dead space ventilation is an appreciable fraction of tidal breathing, including severe respiratory dysfunction,53 pulmonary hypoperfusion, pulmonary thromboembolism, and cardiac arrest In these conditions, the clinician using a capnometer may see a large arterial to end-tidal Pco2 gradient (typically 10 mm Hg) This gradient can be used as an indicator of severity of disease, and Petco2 can be used to evaluate trends rather than as a specific measure of alveolar Pco2 Differential Diagnosis of Abnormal Capnograms The capnogram probably is the single most reliable and effective monitor of pulmonary ventilation The integrity and function of CO2 (mm Hg) 50 Real-time the patient’s cardiopulmonary system and the breathing circuit both affect the capnogram, and malfunctions often can be detected by changes in the capnogram.50,54 The capnogram displays the CO2 concentration in the patient’s airway over time (see Fig 43.4) The essentials of a normal capnogram are (1) zero baseline during early exhalation, which reflects gas exhaled from the anatomic dead space; (2) sharp upstroke during mid-exhalation, which reflects the transition to alveolar gas; (3) relatively horizontal alveolar plateau (prolonged exhalation caused by obstructive lung disease causes a steeper plateau); and (4) sharp downstroke and return to a zero baseline at the start of inhalation A capnogram without these normal attributes suggests an anomaly in the patient’s cardiopulmonary system, a malfunction in the airway, a malfunction in the gas delivery system, or an error in the measurement system.55 Gradually Decreasing End-Tidal Carbon Dioxide Concentration When the capnogram retains its normal morphology but there is a slow, progressive drop in Petco2 (Fig 43.6), the possible causes include falling body temperature, slowly decreasing systemic or pulmonary perfusion, and hyperventilation Sedation and neuromuscular blockade attenuate the normal body mechanisms for generating heat to preserve body temperature As body temperature falls, the patient’s rate of metabolism and CO2 production also fall If ventilation is controlled and kept constant as body temperature decreases, alveolar CO2 concentration and arterial Pco2 decrease This decrease is reflected in the capnogram as a slow decrease in Petco2 over many minutes Another cause of decreasing Petco2 is a fall in total body perfusion associated with blood loss or cardiovascular depression As systemic and pulmonary perfusion decrease, alveolar dead space increases, with a resultant fall in Petco2 Sustained Low End-Tidal Carbon Dioxide Concentrations Without Plateaus Occasionally, with no apparent malfunction in the breathing circuit or in the patient’s cardiopulmonary status, the capnogram shows sustained low Petco2 values without a good alveolar plateau (Fig 47.7) In this situation, Petco2 is not a good estimate of alveolar Pco2 The absence of a good alveolar plateau suggests that either full exhalation is not occurring before the beginning of the next breath or the patient’s tidal volume is being diluted with fresh gas because of a small tidal volume, high aspirating sample rate, or high fresh gas dilution from the circuit Several maneuvers are available to distinguish between these possibilities Incomplete emptying of the lungs may be suggested by adventitial sounds, such as wheezing or large airway rhonchi with compromise of small airway patency caused by bronchospasm or Trend 37 • Fig 43.6 Decreasing proximal end-tidal carbon dioxide (CO2) level (Modified from Zwerneman K End-tidal carbon dioxide monitoring: a VITAL sign worth watching Crit Care Nurs Clin North Am 2006;18:217–225.) CHAPTER 43 Noninvasive Respiratory Monitoring and Assessment of Gas Exchange CO2 (mm Hg) 50 Real-time 489 Trend 37 • Fig 43.7 Airway obstruction or obstruction in breathing circuit (Modified from Zwerneman K End-tidal carbon dioxide monitoring: a VITAL sign worth watching Crit Care Nurs Clin North Am 2006;18:217–225.) secretions If rhonchi are present, tracheal suctioning often corrects the partial obstruction and restores full exhalation Bronchospasm may be treated with a variety of bronchodilators An endotracheal tube that is kinked or partially obstructed by secretions may prevent full exhalation Passing a suction catheter down the endotracheal tube usually confirms or eliminates this possibility Gently squeezing the child’s chest to assist with a forced exhalation often produces a waveform in which the CO2 concentration continues to rise toward an alveolar plateau If the plateau is present, the “squeeze Petco2” value may be taken as a good estimate of alveolar CO2 concentration When no signs of partial airway obstruction are present, another explanation for this type of capnographic waveform should be considered In infants and other patients who have small tidal volumes, the aspirating sample rate may exceed the expiratory flow rate near the end of exhalation When this occurs, the aspirating sample is diluted with fresh gas from the breathing circuit, resulting in a drop-off of the plateau and a fall in Petco2 as a result of dilution Reducing the flow rate of fresh gas or moving the sampling site closer to the endotracheal tube connector usually corrects the problem In very small newborns, a sample rate of 100 to 250 mL/min may be too high to result in good plateaus despite instituting the preceding corrective measures Then, either a capnographic system having a very low sampling rate (50 mL/min) can be used or the capnogram can be used as a gross monitor of the integrity of the ventilatory circuit and trends in cardiopulmonary function rather than as an accurate estimate of alveolar ventilation Sustained Low End-Tidal Carbon Dioxide Concentration With Good Plateaus In some circumstances, the capnogram demonstrates a low Petco2 with a widened arterial Pco2 – Petco2 [(a-et) DPco2]and preservation of a good alveolar plateau This discrepancy may indicate that the capnograph is malfunctioning or miscalibrated The clinician can evaluate this possibility by sampling one’s own exhaled co2 and verifying that the Petco2 concentration is between 5% and 6% (equivalent to 38–46 mm Hg) If the instrument is functioning properly and is well calibrated, a wide (a-et) DPco2 is an indication of excessive dead space ventilation in the patient Exponential Decrease in End-Tidal Carbon Dioxide An exponential drop in Petco2 that occurs within a short time (e.g., a dozen or so breaths) almost always signals a sudden and probably catastrophic event in the patient’s cardiopulmonary system (see Fig 43.6) The basis for this capnogram is a sudden and dramatic increase in alveolar dead space ventilation Possible causes include sudden hypovolemia, circulatory arrest with continued pulmonary ventilation, and pulmonary embolus with thrombus or air Only after ruling out these catastrophic events and determining that the patient is hemodynamically stable should more mundane explanations for the exponential decay in Petco2 be considered The most common noncatastrophic cause is an accidental increase in ventilation attributable to an incorrect ventilator adjustment, resulting in a gradual decrease in Petco2 However, it is important to note that even doubling the alveolar ventilation decreases the Petco2 to only half of the preadjustment value, not to the near-zero values that may accompany catastrophic cardiopulmonary events Gradual Increase in Both Baseline and End-Tidal Carbon Dioxide A gradual rise in both baseline and Petco2 value indicates that previously exhaled CO2 is being rebreathed from the circuit (Fig 43.8) In this situation, the inspiratory portion of the capnogram fails to reach the zero baseline, and there may actually be a premature rise in CO2 concentration during the inspiratory phase of ventilation Petco2 usually increases until a new equilibrium alveolar CO2 concentration is reached, when excretion once again equals production Clinical Applications The role of Petco2 monitoring has expanded in recent years to include prehospital and emergency department settings.52,56 It can be used to verify tracheal intubation, detect complete airway obstruction, and monitor ventilation during sedation.48,49,52,57 Capnometry and capnography are useful in verifying endotracheal tube position following intubation and serve as continuous monitors of endotracheal tube location This is based on the observation that CO2 is exhaled through the trachea and not from the esophagus, which allows differentiation between endotracheal and esophageal intubation.58,59 Confirmation of endotracheal intubation with a colorimetric detector is highly accurate in pediatric patients and is more sensitive and specific when compared with pulse oximetry and auscultation on physical examination.58 Capnography can also be employed during transport of patients for early detection of endotracheal tube dislodgement, allowing for faster correction and avoidance of life-threatening consequences related to hypoxic arrest.60 This is particularly significant in the pediatric population, in whom the distance between the vocal cords and carina is small Petco2 monitoring can also be helpful in monitoring the respiratory status of patients placed on noninvasive positive-pressure ventilation or during procedural sedation in which a nonintubated patient’s respiratory efforts are difficult to visualize.61,62 Capnography may detect alveolar hypoventilation earlier than pulse oximetry in patients receiving moderate sedation or high ... the best sampling site is within the patient’s endotracheal tube or at the tube connector, as far as possible from the Y-connector of the breathing circuit.51 In breathing circuits with intermittent... because there is little contamination with fresh gas from the breathing circuit (point A in Fig 43.5) The Y-connector of the breathing circuit is the next best sampling site (point B in Fig 43.5)... malfunction in the breathing circuit or in the patient’s cardiopulmonary status, the capnogram shows sustained low Petco2 values without a good alveolar plateau (Fig 47.7) In this situation, Petco2