End-tidal carbon dioxide (ETCO₂) monitoring or capnography displays the patient’s exhaled carbon dioxide concentration continuously during exhalation with the ETCO₂ being the peak value before the next breath is initiated. Capnometers estimate ETCO₂ through the use of infrared technology such that the concentration of CO₂ in the exhaled gas is measured by the absorption of infrared light by CO₂. Following its introduction into anesthesia practice, it has become the standard of care to continuously monitor ETCO₂ whenever general anesthesia is provided. Additionally, the measurement of ETCO₂ via infrared or other technology is standard of care whenever endotracheal intubation occurs. When used continuously in the ICU or operating room setting, the technology provides a continuous estimation of the partial pressure of CO₂ (PaCO₂) in the blood and serves as a disconnect alarm during mechanical ventilation, monitoring respiratory rate, and providing information regarding pulmonary function via the shape of the capnogram. Abrupt changes in the ETCO₂ such as decreases related to increased dead space may alert the clinician to decreased cardiac output or alterations in pulmonary perfusion related to gas or pulmonary embolism. Acute increases in ETCO₂ may be the initial sign of malignant hyperthermia or other hypermetabolic states.

The normal capnogram has 3 phases of exhalation and one of inspiration, generally labeled as phases 1–4. Phase 1 is the beginning of exhalation, representing dead space ventilation and therefore having no ETCO₂ present. If there is ETCO₂ present during phase 1, this is indicative of the rebreathing of exhaled gas, which may be due to inadequate fresh gas flows. Phase 2 is the rapid and steep upslope of the capnogram representing the emptying of alveolar gas with dead space gas. Phase 3 is the plateau phase, which in the normal state should be relatively horizontal. Upsloping of phase 3 of the capnogram indicates obstructive lung disease (asthma, bronchospasm) with differential emptying of alveoli with varying time constants. With the initiation of inspiration, there is an abrupt decrease of the ETCO2 (phase 4) which should return to 0 mmHg.

The ETCO₂ generally correlates in a clinically useful fashion with the PaCO₂ with the ETCO₂ being 2–4 mmHg lower than the PaCO₂. This correlation can be used clinically to adjust ventilation techniques both in the operating room and the ICU setting. However, this correlation is dependent on effective matching of ventilation with perfusion, and various technology and patient-related factors may interfere with this accuracy [27–31]. Such issues may be particularly relevant in the practice of pediatric anesthesia where smaller tidal volumes, type of ventilation (continuous versus intermittent flow), and sampling issues can affect the correlation of ET and PaCO₂.

Although initially introduced for intraoperative care, capnography has been brought out of the operating room to various clinical arenas. The concern that changes in pulse oximetry may take 60–90 s after the development of apnea in patients receiving supplemental oxygen has led some authorities to recommend the use of continuous ETCO₂ monitoring during procedural sedation. While not mandated by all of the current guidelines for procedural sedation, ETCO₂ monitoring has been shown to be a beneficial adjunct to monitoring during procedural sedation. While there may be some discrepancy between the ETCO₂ value and the PaCO₂ especially when monitoring from a nasal cannula, in many cases a direct correlation can be demonstrated [32, 33]. The capnograph provides a continuous and real-time demonstration that air exchange is occurring. If there is upper airway obstruction or central apnea, this is immediately demonstrated as the capnogram extinguishes [34, 35]. Several clinical studies have demonstrated the early identification of respiratory depression using this technology and have clearly indicated its superiority over pulse oximetry in many clinical scenarios in both pediatric and adult patients [36–40]. These data have also been supported by a recent meta-analysis, concluding that episodes of respiratory depression were 17.6 times more likely to be detected by capnography compared with standard monitoring [41]. Owing to the growing evidence, the American Society of Anesthesiologists has amended its standards for basic anesthetic monitoring effective from 2011 to include mandatory ETCO₂ monitoring during moderate and deep sedation.

The recent literature and clinical experience clearly demonstrate that capnography is not only effective for confirming ETT placement but may also have an expanded role: the continuous monitoring of tube position in the OR and during transport and monitoring during procedural sedation as a means for providing the immediate identification of apnea or upper airway obstruction. It clearly remains the standard of care for intraoperative monitoring and to document correct endotracheal tube placement wherever endotracheal intubation occurs. Beyond this, it is rapidly becoming the standard of care during procedural sedation especially with the recent revisions of the ASA guidelines. It is also a key monitor in the ICU as a means of rapidly adjusting minute ventilation during mechanical ventilation and for assessing patients with respiratory illnesses. It may provide useful information following endotracheal intubation and during transport as a means of avoiding inadvertent hyperventilation which may be detrimental in patients with traumatic brain injury [42]. More recently, it has been suggested that it can be used to judge the adequacy of resuscitation during cardiac arrest. The depth of chest compression has been shown to correlate with the ETCO₂ values [43]. Additionally, higher ETCO₂ values during resuscitation have been shown to correlate with a greater chance of return of spontaneous circulation (ROSC). Although initial investigations suggested that an ETCO₂ greater than 10 mmHg predicted ROSC, this value has recently been questioned suggesting that the goal ETCO₂ during resuscitation should be considered higher, perhaps approaching 25 mmHg [44]. Although ETCO₂ was initially applied only in the presence of an ETT, design modifications and the development of new devices allow its application in patients with a native airway during spontaneous ventilation. Given its ability to immediately detect hypercarbia and perhaps respiratory depression, it has been suggested that its use during postoperative period to monitor patients receiving patient-controlled analgesia may lead to the early identification of respiratory depression prior to the development of respiratory arrest thereby improving patient safety [45].

Transcutaneous CO₂ (TC-CO₂) devices can also be used to provide a continuous estimate of PaCO₂ in various clinical scenarios. Although introduced for use in the neonatal population where both transcutaneous CO₂ and O₂ monitoring have been used, these devices have also found their way out of the NICU and into the ICU and OR setting as a means of continuously monitoring ventilation. Their use has been driven by concerns of clinical scenarios that affect the accuracy of ETCO₂ or limit their use (high-frequency ventilation techniques). In general, these devices apply heat to the skin at 43–45 °C leading to capillary vasodilatation, increasing the transit time of blood through the capillary, resulting in a close approximation of capillary and arterial PaCO₂. The vasodilatation of the capillary bed also allows CO₂ to diffuse from the arterial capillary lumen to the membrane of the transcutaneous monitor. Alterations in temperature affect the solubility of CO₂ in blood such that an increase in the temperature increases the partial pressure of CO₂ in the blood and a higher PaCO₂ value or a larger gradient between the actual PaCO₂ and the TC-CO₂. Additionally, the higher temperature increases the metabolic rate of the tissues, thereby further increasing the PaCO₂. These factors are corrected by an internal calibration factor that is used to calculate the TC-CO₂ and thereby makes the TC value an appropriate estimate of the PaCO₂. Although used less commonly than ETCO₂ devices, these devices have seen increased used in various clinical scenarios in both the OR and the ICU setting. These include conventional and high-frequency mechanical ventilation, in spontaneously breathing patients, during the transport of critically ill patients, and in other clinical scenarios including apnea testing during brain death examination and in the assessment of patients with diabetic ketoacidosis (DKA).

In a cohort of pediatric ICU patients with respiratory failure of various etiologies, ranging in age from 1 to 40 months, 100 simultaneously obtained sets of arterial, transcutaneous, and end-tidal CO₂ values were analyzed [46]. The end-tidal to arterial CO₂ difference was 6.8 ± 5.1 mmHg, while the transcutaneous to arterial CO₂ difference was 2.3 ± 1.3 mmHg, p < 0.0001. The absolute difference between the end-tidal and arterial CO₂ was ≤4 mmHg in 38 of 100 values, while the absolute difference between the transcutaneous and arterial CO₂ value was ≤4 mmHg in 96 of 100 values, p < 0.0001. The same investigators demonstrated the improved accuracy and potential application of TC-CO₂ monitoring in an older cohort of patients with respiratory failure who ranged in age from 4 to 16 years of age [47]. A final study evaluated the accuracy of transcutaneous CO₂ monitoring following cardiothoracic surgery in infants and children [48]. Given the potential for various physiologic factors including residual shunt and ventilation-perfusion mismatch which may exist following cardiopulmonary bypass (CPB) and surgery for infants with congenital heart disease, the authors speculated that ETCO₂ would be inaccurate in this patient population and of limited benefit for continuous monitoring in the pediatric ICU setting. The study population included 33 consecutive patients following surgery for congenital heart disease. Transcutaneous CO₂ monitoring was initiated if the initial ABG following CPB demonstrated an arterial to end-tidal gradient of 5 mmHg or more. Although the study validated the utility of TC-CO₂ monitoring and its improved accuracy over ETCO₂ in this unique patient population, problems with TC-CO₂ monitoring were noted in 3 patients who demonstrated cardiovascular instability and were requiring dopamine at 20 μg/kg/min and epinephrine at 0.3–0.5 μg/kg/min. The cutaneous vasoconstriction induced by these vasopressors impacted the accuracy of TC-CO₂ monitoring. Other reported ICU and OR applications of TC-CO₂ monitoring have included continuous CO2 monitoring during high-frequency ventilation, one-lung ventilation, laparoscopic surgery, apnea testing, and metabolic disturbances where changes in PaCO₂ may correlate with acidosis resolution [49–54]. In such settings, ETCO₂ may not be feasible due to the ventilation technique or inaccurate due to alterations in pulmonary ventilation-perfusion matching.

As with all noninvasive monitors, attention to detail regarding specific aspects of TC-CO₂ monitoring is required to ensure the accuracy of the technique. When compared with ET-CO₂ monitoring, TC-CO₂ monitoring requires a longer preparation time including a 5-min calibration period prior to placement and then an additional 5–10 min equilibration period after placement on the patient to allow for an equilibration between the transcutaneous and arterial CO₂ values. The electrode should be recalibrated and placed at another site every 4 h to avoid burns or blistering of the skin when the device is warmed to 45 °C, while newer devices that attach to the ear lobe and are heated to 43 °C have a lower risk of thermal injury and may be left in place for a longer period of time. While various sites (chest or abdomen) have been used in neonates and infants, we prefer the volar aspect of the forearm in adults especially those with obesity. Accuracy may be affected by technical factors related to the monitor itself including trapped air bubbles, improper placement technique, damaged membranes, and inappropriate calibration techniques. Patient-related issues that affect accuracy include variations in skin thickness; the presence of edema, tissue hypoperfusion, and hypothermia; or the administration of medications that result in cutaneous vasoconstriction. When considering these noninvasive monitors of PaCO₂, there are advantages and disadvantages to both (Table 21.2). Given their different clinical utilities, perhaps they should be considered as complimentary rather than competing devices.

The latest addition to the respiratory armamentarium is the rainbow acoustic monitoring (RAM, Masimo Corporation, Irvine, California). RAM technology provides continuous, noninvasive monitoring of respiration rate using an innovative adhesive sensor with an integrated acoustic transducer that is externally applied to the patient’s neck. The patented technology, known as Signal Extraction Technology (SET®), separates and processes the respiratory signal to display continuous acoustic respiration rate. Preliminary studies have suggested that this technology is responsive to changes in respiratory rate and is superior to capnography in detecting respiratory pauses. Its use of noninvasive adhesive sensors that are applied externally to the lateral aspect of the patient’s neck may be more easily tolerated than ETCO₂ capturing devices. In a study comparing acoustic monitoring with capnography in 33 adults in the post-anesthesia care unit following general anesthesia, the acoustic monitor and capnometer both reliably monitored ventilatory rate, while the acoustic monitor more sensitively identified pauses in ventilation [55]. Although the acoustic monitor was statistically more accurate and more precise than capnography, the clinical differences were modest without proven clinical significance. The authors concluded that acoustic monitoring may provide an effective and convenient means of monitoring ventilatory rate in postsurgical patients. Other studies have demonstrated that respiratory rate monitoring with the acoustic monitor correlates well with that monitored by capnography and that the device is better tolerated than other devices such as a facemask for capnography (Capnomask) [56, 57]. Its preliminary validation and success in these areas have led to its use in the pediatric population for perioperative monitoring and in the adult population outside of the operating room [58–60]. While all of these preliminary studies have suggested that it is better tolerated than capnography by mask or nasal cannula, other advantages over capnography have yet to be determined. Further work is needed to delineate its role in perioperative and postoperative respiratory monitoring.

Along with respiratory monitoring (oxygen saturation and ETCO₂), the monitoring of cardiovascular parameters and hemodynamic function remains an integral component of standard perioperative monitoring. Although blood pressure monitoring remains a required component of perioperative care, it fails to provide us with even more essential information regarding cardiovascular performance including cardiac output, systemic tissue oxygen delivery, and tissue oxygenation. While these monitors are not considered essential for perioperative care, there remains great interest in the technology that would allow us to noninvasively monitor these parameters.