Capnography: An Indispensable Noninvasive Tool

Bhavani Shankar Kodali; Arunthevaraja  Karuppiah; Shobana  Bharadwaj

Capnography: An Indispensable Noninvasive ToolCHAPTER 1

Bhavani Shankar Kodali,

Arunthevaraja Karuppiah,

Shobana Bharadwaj

INTRODUCTION

Capnography is the measurement of the partial pressure of exhaled carbon dioxide (CO₂) and display of CO₂ waveform. The graphic representation is expressed as expired CO₂ concentration over time, or volume and is known as time, or volume capnogram. Capnography became a standard of monitoring during anesthesia practice in Europe in the late 1970s and the United States in mid-1980s. Since then, it has evolved as an essential component of standard anesthesia monitoring armamentarium.¹ Capnography has also expanded beyond the operating room into the emergency department, radiology, gastroenterology procedures, adult, and pediatric sedation sites. Furthermore, capnography has also made its impact in intensive care units (ICUs) across the world for monitoring ventilation and assuring correct location of endotracheal tube (ETT) continuously. Capnometry is a numeric measurement and display of the level of CO₂ concentration without waveform. The waveform of capnography is more valuable and reliable than capnometry for clinical interpretation.^2,3

PHYSICS

There are two types of capnography devices (Figs. 1, 2 and Table 1): (1)“sidestream” and (2) “mainstream”⁴ depending upon where the CO₂ analyzer is located in relation to the ventilator circuit. In a “mainstream” analyzer, sampling window is in the ventilator circuit (near the hub of ETT) and measures CO₂, while in a “sidestream”, the gas is located out of the ventilator circuit and gas sample is aspirated via 6 feet sampling tube. In both types, the gas analyzers work mostly using the principle of infra-red (IR) absorption spectroscopy (Fig. 3). An IR diode emits the light that traverses the chamber containing airway sample. The CO₂ molecules absorb IR light and unabsorbed IR light is detected by an IR detector. Molecules of CO₂ absorb IR radiation at a very specific wavelength (4.26 μm), with the amount of radiation absorbed having a nearly exponential relation to the CO₂ concentration (Beer-Lambert Law) present in the breath sample. The difference between the source light and absorbed light is transformed into CO₂ concentration and displayed as a capnography waveform.2

Fig. 1: Mainstream capnography: Infra-red (IR) light sensor is positioned in the airway circuit.

Fig. 2: Sidestream capnography: Infra-red (IR) light sensor is positioned outside the airway circuit. A T-piece adapter with a 6 feet disposable tubing is inserted between the breathing tube and airway circuit. (ET: endotracheal tube)

Water vapor, nitrous oxide, and anesthetic agents absorb IR light, but at a different wavelength. There are other techniques of measuring CO₂ using photoacoustic spectroscopy, Raman scattering, and mass spectrometry, which are not commonly used.

In the majority of capnographs, the exhaled concentration is plotted against the time (time capnography) and is the common method in practice. In volume capnography, the exhaled CO₂ is serially plotted against expired lung volume. Volume capnography is not routinely used. However, this may be a preferred technique in near future, as it enables indirect estimation of physiological dead space.⁵3

TABLE 1 Types of capnography and their characteristics

Sidestream capnography	Mainstream capnography
Used both for intubated and nonintubated patients	Predominantly used for intubated patients
Most widely used method	The sensor contains an infra-red (IR) measurement device with an electrical cord. The newer versions are extremely light
The gas sampling tube is inserted in between the breathing circuit and endotracheal tube. Only manufacturer recommended tubing material and length must be used	Adaptor containing IR sensor is interposed between the patient's breathing circuit and endotracheal tube. Sensor emits IR light to a photodetector on the other side of the adapter
Slight delay in the display of the CO₂ waveform	CO₂ waveform is measured immediately
Main advantage of sidestream capnography is that, it can be used in nonintubated patients, for instance, patients with oxygen facemask	Need special smaller lighter weight adapters for use in the nonintubated patients
A drawback is that the tubing may become blocked from water vapor or secretions	The mainstream sensor is heated above body temperature, which prevents water vapor condensation and allows the sensor to function in high moisture environments

Fig. 3: Measurement of carbon dioxide (CO₂) by infra-red (IR) technology.

COLORIMETRIC DEVICE

The colorimetric CO₂ detector (Figs. 4A and B) can be used for verification of correct placement of an ETT in the trachea. It has especially treated litmus paper, which changes color from purple to yellow. The device is not very accurate, particularly when CO₂ output is low during CPR. 4Waveform capnography is the preferred approach currently. Nonetheless, the colorimetric device is a reasonable option to check placement of an ETT for emergency intubations.

Figs. 4A and B: Calorimetric capnometer. The pH sensitive paper changes from purple to yellow when CO₂ is detected.

PHYSIOLOGICAL OVERVIEW

A good understanding of time capnogram (Fig. 5) and volume capnogram (Fig. 6) will help clinicians to avail of full benefit of capnography in clinical practice.

Time Capnography

Time capnogram (Fig. 5) is usually interpreted as two segments—(1) an inspiratory segment and (2) an expiratory segment, and two angles— alpha and beta.⁶ Capnograms may be evaluated breath by breath, or trends may be assessed as valuable clues to a patient's physiologic status. Expiratory segment is divided into three phases, I to III. Inspiratory segment is designated as phase 0 (Table 2).

The angle between phase II and III is called alpha angle, which increases as slope of phase III increases. Normally, it is about 100–110°. Airway obstruction increases the angle due to increase in the slope phase II and phase III. The response time of the capnograph, sweep speed, and the respiratory cycle time also affect the angle. On the other hand, the angle between phase III and phase 0 is called beta angle, which is normally about 90°. During rebreathing, this angle increases. Occasionally, an upward blip or spike, known as phase IV, can occur towards the end of phase III. This terminal elevation represents emptying of alveoli with long time constants containing higher CO₂ concentration. The maximal CO₂ level, the best reflection of alveolar CO₂, is known as end-tidal CO₂ (EtCO₂). This is generally depicted in concentration as EtCO₂ or if in partial pressure of CO₂, PEtCO₂. This is the number that is displayed on the monitor. PEtCO₂ is the best reflection of alveolar CO₂ (P_ACO₂) and, normally the arterial CO₂ (PaCO₂)−PEtCO₂ difference is about 5 mm Hg due to the alveolar dead space. 5There are several clinical possibilities where abnormal capnograms can occur; the most common to anesthesia practice are depicted in Figures 7A to F.

Fig. 5: Normal time capnogram: Phase I: End of inhalation and beginning of exhalation (dead space ventilation). Phase II: Rapid rise of CO₂ due to mixing of the dead space gas with alveolar CO₂. Phase III: Alveolar plateau. The exhalation of the CO₂ from the alveoli. It reaches a peak where the partial pressure of CO₂ is the highest. Phase 0: Partial pressure of CO₂ decreases rapidly at the beginning of the inspiration.

Fig. 6: Volume capnogram.

Applications of Time Capnography

In the operating rooms, capnography plays a vital role in detecting breathing circuit disconnection instantly when the capnograph no longer detects the EtCO₂.6

TABLE 2 Segments of time capnography

Segment	Phase
Expiratory segment	Phase I	Initial stage of exhalation, in which the gas sampled is anatomical dead space and apparatus dead space gas, free of CO₂ (dead space ventilation)
	Phase II	Expiratory upstroke—combination of dead space and alveolar gas
	Phase III	Alveolar plateau phase—exhalation of mostly alveolar gas. It is important to note that the expiratory plateau is not an isocapnic trace but rather it progresses with a very slight and steady increase in the _PCO₂ as the alveolar fraction is expelled from the lungs
Inspiratory segment	Phase 0	Inspiratory downstroke—inhalation of CO₂ free gas; patient inspiring fresh gas

Figs. 7A to F: Examples of several abnormal capnograms. (A) Capnogram of a patient with severe chronic obstructive pulmonary disease (COPD), or other causes leading to the increased airway resistance, such as asthma, endobronchial intubation, endotracheal tube kinking. No plateau is reached before the next inspiration. The gradient between PEtCO₂ and arterial CO₂ is increased; (B) Downward wave during plateau phase indicates spontaneous respiratory effort; (C) Cardiogenic oscillations appear as small, regular, tooth like humps at the latter part of the expiratory phase. The rate of the “humps” is identical to the patient's heart rate; (D) A leak in the sampling line during positive pressure ventilation; (E) Failure of inspired CO₂ to return to zero due to an incompetent expiratory valve or exhausted CO₂ absorbent; (F) Bifid waveform of expired CO₂ in patient with emphysema undergoing elective surgery after unilateral lung transplantation. The initial upstroke represents gas from the normal (transplanted) lung, which is followed by gas exhaled from the remaining (emphysematous) lung.

This is followed by drop-in oxygen saturation. EtCO₂ is probably the best method to detect disconnections; a decrease or absence of EtCO₂ is highly sensitive but not specific for anesthesia circuit disconnections. Beyond this important and vital application, capnography serves to provide the clinician with enormous data information about the patient's overall physiology that can help clinicians to identify cardiac, ventilatory, and metabolic abnormalities and they are outlined below:

Confirmation of tracheal intubation: 2015 American Heart Association Guidelines on Advanced Adult Cardiac Life Support endorsed continuous waveform capnography in addition to clinical assessment for confirming the ETT placement.⁷ To note, a normal waveform can occur when the tube has been placed in the right mainstem bronchus too. A flatline waveform immediately after ETT placement usually indicates esophageal placement. Other common scenarios with a flatline waveform include:
- Anesthesia circuit disconnections.
- Technical malfunction of the monitor or sampling tube.
- Endotracheal tube complete obstruction (e.g., clotted blood).
- Prolonged cardiac arrest with cellular death (no CO₂ production at a cellular level).
Unrecognized misplaced endotracheal intubation by healthcare providers was widespread in the past^8,9 and has been significantly reduced by the use of EtCO₂ monitoring. Confirmation of correct tube placement using clinical signs has shown to be unreliable. The “Fourth National Audit Project” by Royal College of Anesthetists (RCoA) and Difficult Airway Society stressed the importance of capnography in confirming tracheal intubation in all clinical settings through “No Trace = Wrong Place” campaign. Capnography also provides reliable information on the correct placement of supraglottic airway devices.
Assessing tracheal tube and tracheostomy patency and position: Capnography for the duration the patient remains on artificial airway was cited as the most effective way of reducing morbidity and mortality.¹⁰ Capnography monitoring gives real-time information on the patient's airway patency. A change in waveform is usually seen before a decrease in oxygen saturation and should be immediately assessed.¹¹
Guide adequacy of ventilation: Capnography is commonly used to detect hypercapnia due to hypoventilation and hypocapnia due to hyperventilation.¹¹ In general, the PaCO₂ levels are higher by about 5 mm Hg. In patients with essentially normal lungs and cardiac output, EtCO₂ can be used to noninvasively monitor PaCO₂. An initial arterial blood gas can be performed to determine the gradient (Arterial-EtCO₂), further ventilatory adjustments can be made just using EtCO₂ as a guide. If the patient has chronic obstructive pulmonary disease (COPD) or unstable cardiac output, EtCO₂ may not be a perfect guide. It can also be used for weaning patients from mechanical ventilation. Arterial to EtCO₂ difference gives a fair idea about physiological dead space. If the gradient stabilizes or it decreases over time from initially a large gradient, this demonstrates indirectly that the patient's V/Q status is improving.¹²8
Procedural sedation: Capnography has been widely used as a respiratory monitor in addition to pulse oximeter during surgical procedures requiring sedation. Capnography will identify hypoventilation long before the pulse oximeter detects hypoxemia, this is especially true in patients receiving supplemental oxygen during sedation (Figs. 8A and B). A comprehensive meta-analysis of 13 randomized clinical trials provides clear evidence that capnography monitoring is more sensitive to identify respiration related adverse events used during procedural sedation. In addition, there is a statistically significant and clinically meaningful reduction in episodes of desaturation as well as requirement of assisted ventilation.¹³ During sedation, expired gases are diluted by oxygen administration, and the CO₂ waveforms may not appear in normal shape. Under these circumstances, a change from baseline is an important criterion of excessive sedation resulting in hypoventilation or airway obstruction. A systematic approach to changes in CO₂ waveforms can troubleshoot respiratory events during sedation (Flowchart 1).

Figs. 8A and B: A drop in respiratory rate or height noticed in capnography during procedural sedation.

Flowchart 1: Algorithm for differential diagnosis of respiratory adverse events during sedation with capnography.
9
During percutaneous tracheostomy placement: Percutaneous tracheostomy is an invasive procedure and may rarely result in significant patient harm due to incorrect placement. Capnography improves safety by confirming that the tracheostomy tube is correctly placed in the trachea at the end of the procedure. Capnography can be also used as an alternative to fiberoptic bronchoscopy, if unavailable or contraindicated, to detect correct tracheal needle placement before performing percutaneous tracheostomy.¹⁴
Monitoring patients with raised intracranial pressure: Capnography is an essential tool for management of patients with raised intracranial pressure (ICP), both in the operating room and critical care setting. PaCO₂ has a profound and reversible effect on cerebral blood flow. Cerebral blood flow is linearly responsive to PaCO₂ level.¹⁵ Capnography can be used as a continuous monitor of EtCO₂ which is the best reflective of PaCO₂ level during ICP management.
Monitoring response to treatment of bronchospasm: A patient with bronchospasm usually has a capnogram with an exaggerated up-sloping plateau phase, with a prolonged phase II representing slow expiration of respiratory gases. In addition to conventional methods, capnography can be used to assess response to bronchodilators.¹¹ Capnography is advantageous when compared to peak flow meter because it is independent of efforts and provides continuous monitoring.
Estimation of cardiac output: An increase in cardiac output and pulmonary blood flow results in better perfusion of the alveoli and a rise in EtCO₂. Provided the ventilation remains constant, the EtCO₂ provides a continuous trend of pulmonary blood flow and therefore, an estimation of cardiac output. During constant minute ventilation and tissue CO₂ production, an abrupt reduction in blood flow reduces EtCO₂ via two mechanisms. First, a reduction in venous return causes a decrease in delivered CO₂ to the alveoli, resulting in a decrease in alveolar _PCO₂ (PaCO₂) and, consequently, EtCO₂. Second, reduced pulmonary vascular flow will result in an increase in alveolar dead space, which will dilute the CO₂ from normally perfused alveolar spaces, thus decreasing EtCO₂ below PaCO₂.¹⁶ A number of studies have shown that EtCO₂ can be useful in estimating a change in cardiac output in a variety of clinical scenarios like cardiac arrest, circulatory shock, and major surgeries.^17,18
Assessment of efficacy of cardiopulmonary resuscitation (CPR) and prediction of survivability: Utilization of capnography in the following ways during CPR improves the survival outcomes in cardiac arrest.^19,20
- Confirms the correct placement and patency of the airway device.
- Monitor ventilation, avoiding hyper and hypoventilation.
- Assess the adequacy of chest compression during CPR.
- Early assessment of return of spontaneous circulation (ROSC) during CPR.
- Used in decision making for prognostication during CPR. In intubated patients, failure to achieve an EtCO₂ of >10 mm Hg by waveform capnography after 20 minutes of CPR may be considered as one component of a multimodal approach to decide when to terminate resuscitative efforts but should not be as a sole criterion⁷(Fig. 9).10
  
  Fig. 9: Waveform capnography showing changes in the end-tidal carbon dioxide during cardiopulmonary resuscitation (CPR) and after ROSC.
Detection of inadvertently placed nasogastric tube: In a meta-analysis of nine clinical trials of mechanically ventilated patients, the use of capnography for inadvertent tracheal placement of the nasogastric tube had a sensitivity ranging from 0.88 to 1.00, specificity 0.95–1.00, positive likelihood ratio 15.22–283.35, negative likelihood ratio 0.01–0.25.²¹
Monitoring during patient transport: Monitoring capnography waveforms with pulse oximetry during transport assures the integrity of airway and prevents mishaps during interhospital or intrahospital transfers. If ventilation is kept constant, an abrupt decrease in EtCO₂ values must be immediately investigated; it may be due to decreased cardiac output.²²
Sepsis: Capnography has potential prognostic value in sepsis to gauge lactate levels. EtCO₂ demonstrates an inverse relationship with lactate levels.^23,24 EtCO₂ levels <25 mm Hg demonstrate a 0.93 sensitivity for mortality in these studies. In another study, a significant correlation was noted between EtCO₂ and sequential organ failure assessment (SOFA) score (r = −0.35, p < 0.01), and EtCO₂ and lactate level (r = −0.35, p < 0.01). A receiver operator curve for EtCO₂ and SOFA > 2 had an area under curve (AUC) of 0.69. EtCO₂ of <35 has a sensitivity of 0.73 (95% CI 0.56–0.85) and specificity 0.50 (0.38–0.62) in predicting SOFA scores >2. EtCO₂ < 35 mm Hg had a sensitivity of 0.60 (0.22–0.88) and specificity 0.42 (0.32–0.52) in predicting lactate >4 with an AUC of 0.62. There was a statistically significant correlation between EtCO₂ and SOFA scores, but association was not strong enough for clinical decision making.²⁵
Trauma resuscitation: In blunt trauma prehospital patients who underwent intubation, EtCO₂ were greater in survivors (30.8 mm Hg in survivors and 26.3 mm Hg in nonsurvivors). EtCO₂ also demonstrates a strong inverse relationship with lactate in patients with penetrating trauma.²⁶ A recent 11study released in injury finds capnography levels <35 mm Hg had an association with indicators of shock and need for blood transfusion in the first 6 hours of admission.²⁷
Fluid responsiveness: In a recent study, capnography outperformed other indices such as, pulse pressure variation, systolic blood pressure, heart rate, and mean blood pressure during fluid challenge in patients undergoing mechanical ventilation.²⁸ Further studies are required to assess the efficacy of EtCO₂ for fluid management.
Seizures: EtCO₂ can provide an assessment of the ventilatory status of a patient who is actively seizing or in a postictal state.²⁶
Postoperative care: Capnography use in the postoperative period can identify early respiratory depression before oxygen desaturation occurs, especially in patients receiving supplemental oxygen.²⁹ This can significantly improve patient outcomes and avoid the need for costly interventions.³⁰

Volume Capnography

Volume capnography is the graphical representation of partial pressure of CO₂ versus exhaled volume. This measurement is made noninvasively at every breath by a combination of flow and CO₂ sensors, which are positioned together at the Y piece of ventilator circuit. Under normal circumstances there is a difference of 2–5 mm Hg between arterial and alveolar CO_2, (PaCO₂-PEtCO₂ gradient) but this can differ considerably with ventilation perfusion (V/Q) mismatch in the lungs. Volume capnography can provide much more information than time capnography in situations where there is an alteration in V/Q ratio. A volume capnogram can be divided into various components as defined here (Fig. 6). A horizontal line representing PaCO₂ (arterial blood sampled during the PEtCO₂ recordings) is drawn on the CO₂ trace. The area under the curve (green area), is the volume of CO₂ in the breath and represents effective alveolar ventilation. The remaining area represents wasted ventilation (physiological dead space). A vertical line is drawn through phase II so that the two areas p and q are equal. Area under light blue represents anatomical dead space and area under dark blue represents alveolar dead space.²²

Applications of Volume Capnography

Assessing adequacy of ventilation.
- Volume capnography can assess the adequacy of ventilation. However, in mechanically ventilated ICU patients, there is often a large difference between simultaneous measurements of PaCO₂ and PEtCO₂. Once the difference between PEtCO₂ and PaCO₂ is established volume capnography can be used as a guide of PaCO₂.
Estimation of optimal positive end-expiratory pressure (PEEP).
- Volume capnometry can help estimate optimal PEEP and response to PEEP can be used to guide therapy in patients with acute respiratory distress syndrome (ARDS). It can also be used to determine prognosis in patients with ARDS.12
Exclusion of pulmonary embolism.
- Pulmonary embolism can be excluded by volume capnography when D-dimers are positive and track the efficacy of thrombolysis in patients with major thromboembolism.
Noninvasive estimation of cardiac output and volume responsiveness.
- The accuracy of estimation with volume capnography is similar to esophageal Doppler ultrasound, pulse contour analysis, and thoracic bioimpedance.
Estimation of dead space.
- Volume capnography can be used to determine breath by breath physiological dead space (Bohr Dead space).⁵

LIMITATIONS

Even more than 40 years after the introduction of capnography, patients are still dying because of unrecognized esophageal intubation or tracheal tube displacement. These deaths are occurring either through failure to use this reliable technology, or failure to interpret an abnormal capnograph waveform.³¹ In some countries, capnography units are not used even when available due to lack of knowledge to interpret capnography.³¹ Despite valuable information provided by capnography, there are few limitations like any other monitoring modality and hence requires caution in the interpretation of the data. Capnography is more reliable in patients where there is isolated ventilation, perfusion, or metabolism issue. However, patients with mixed pathophysiology pose challenges for interpretation.

False-positive CO₂ detection could occur in esophageal intubations if the patient ingested carbonated beverages. Acidic solution exposure, such as stomach content fluid or vinegar, can result in qualitative color change in colorimetric method. Waveform capnography is much more reliable under these circumstances.

Mainstream devices are near ETT and face, and hence caution must be exercised to avoid facial burns due to IR assembly. Sidestream devices may result in time delay, but this is of less clinical significance in clinical practice. The sampling lines and IR window are prone to obstructions by secretions. The sampling tube must be positioned antigravity (vertically upward) and a small filter between the sampling line and the ETT minimizes the blockade with secretions.

It is common to use EtCO₂ monitoring during transfer of patients by aircraft and in high altitude locations. In order to provide safe patient transport care, a knowledge about ambient pressure effect on gas analysis is essential. Gas analyzers are calibrated to measure partial pressure of the gas at sea level. Reduction of atmospheric pressure at high altitude will affect capnography in the following ways:³²

Pumping of gas through the sample chamber—more powerful pump may be required to maintain flow rates.
Calibration inaccuracies may occur—this can be corrected by recalibration at high altitude.13
Fall in barometric pressure may be electronically sensed as a gas leak within the monitor. However, the clinical significance of these variations can be minimized, or must be considered while interpreting EtCO₂ values.

FUTURE TRENDS AND CONCLUSION

With growing understanding of capnography, and increasing litigations outside of the operating rooms, it is inevitable that many procedural sedation facilities, ICU, and emergency departments will use capnography in future. In the emergency medicine environment, capnography has expanded from CPR and ventilated patients, to use in spontaneously breathing patients in every clinical application, across the continuum of care. The use of capnography could soon become a standard of care in all critically ill on mechanical ventilation during interhospital or intrahospital transfer. The addition of algorithms and technology like integrated pulmonary index (IPI) has also helped clinicians to recognize and intervene in any respiratory compromise. Capnography can facilitate airway management in a neonate as an indicator of gas exchange, lung aeration, early identification of obstruction, early recognition of ROSC, and probably the quality of the administered chest compressions.³³ Volumetric capnography can make a profound impact on the respiratory weaning protocol and shorten the duration of respiratory life support on patients requiring mechanical ventilation. There is growing trend of utilization of this technology in the ICU to measure physiological dead space, oxygen consumption, and CO₂ production.

KEY POINTS

Capnography is a standard of care monitoring during anesthesia procedures.
Capnography has also become a standard of care to monitor ventilation during sedation procedures.
American Heart Association has endorsed capnography to be used during cardiopulmonary resuscitation (CPR).
Volume capnography can provide physiological dead space estimations breath by breath.
A knowledge of physics, physiology and clinical interpretations of capnography are essential to avail full benefit of capnography in clinical practice.

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Chapter Notes

Capnography: An Indispensable Noninvasive ToolCHAPTER 1

KEY POINTS