CME ARTICLE |
https://doi.org/10.5005/jp-journals-11010-04208 |
Capnography: Principles and application in critical care medicine
Email: drbudania@gmail.com
Assistant Professor
Senior Resident, Department of Anaesthesiology, Kasturba Medical College, Manipal
Abstract
Capnography has become an essential monitoring tool in perioperative and critical care setting. It helps monitor dynamic physiological processes more precisely and promptly, compared to other monitoring modalities. Methods to measure and display carbon dioxide concentrations in respired gases include infrared spectrography, mass spectrography, Raman spectrography, photoacoustic analysers or colorimetry. It is used to confirm correct endotracheal tube placement, ensure effective and consistent chest compression, detect return of spontaneous circulation and diagnose alterations in haemodynamics and ventilation of critically ill patient. It helps to optimise fluid resuscitation and monitor shock progression by providing insight into adequacy of tissue perfusion. Thus, a detailed study and interpretation of capnographic waveform is essential.
Keywords: Capnogram, capnography, cardiac output, cardiopulmonary resuscitation, respiratory dead space.
How to cite this article: Budania L, Goyal K. Capnography: Principles and application in critical care medicine. Ind J Resp Care 2015; 4(2): 637-45.
Introduction
Capnography is a continuous, real time, noninvasive and rapid monitoring technique to measure and display carbon dioxide (CO2) in the gases breathed in and out during respiration. This can be used reliably to monitor changes in ventilation, circulation and metabolism.1–3 Since its application was first described in anaesthesiology over three decades ago, it has evolved as a mandatory monitoring equipment in the perioperative period and as an essential tool for monitoring ventilation and haemodynamics, in a critically ill mechanically ventilated patient.4–6 It provides useful physiological information, helps to keep track of effects of physiological interventions and assess prognosis in a critically ill patient.7,8 It prevents life-threatening events, by alerting health care providers allowing early institution of treatment.1,4
The CO2 concentration can be plotted against time (when it is called timed capnography) or volume (when it is termed volume capnography).1 The equipment used to generate the graph is called a capnograph. The waveform is called a capnogram.1 Capnometry is the numerical measurement and display of maximum inspiratory and expiratory CO2 concentration.1 Capnometer is the equipment used to measure and display only the reading (no waveform).1
Principle
The capnographs usually used in the operation theatre use infrared absorption spectroscopy to measure carbon dioxide concentrations. Infrared light is passed through a sample of respired gas. Molecules having dissimilar atoms absorb infrared radiations at a specific wavelength producing absorption bands. According to Beer-Lambert law, the amount of light absorbed depends upon the concentration of that substance. Energy generated by the absorption of infrared radiation causes molecules to vibrate, frequency of which depends upon molecular mass and atomic bonding within the molecule.1,3,9,10
Raman spectrography, photoacoustic (PAS) gas measurement and mass spectrography are other methods to measure carbon dioxide in the respired gases.
The colorimetric method consists of chemically treated foam indicator (Fenem indicator), in a plastic housing, which changes colour on exposure to CO2.1 It lacks waveform and gives false positive results when device is contaminated with epinephrine, surfactant or atropine.11,12 It cannot be used in infants less than 1 kg. Increased resistance to airflow makes it unsuited for patients breathing spontaneously. Excessive humidity in expired gas will render it inoperative in 15-20 min. It can be damaged by mucous, gastric contents and intratracheal epinephrine.11
Methods of sampling
There are two ways of sampling of gas for analysis: mainstream and sidestream sampling.
Sidestream sampling
A tiny aspirator samples gases from the patient's end of breathing system through a sampling tube and the gas is delivered in to a sample chamber (Figure 1). The sampling tube is made up of Teflon, impermeable to CO2 and does not react with anaesthetic agents. The optimal gas flow rate is 50- 200 mL/min, which ensures reliable capnograms in both adults and children. If sampling flow exceeds the expired gas flow, fresh gas will be entrained into sampling chamber. The sampling gas pump, tubing, connector to breathing circuit and water trap, all constitutes the site for gas leak. Depending upon the size and length of sampling tube, there is a delay in gas detection (CO2 flight time), which significantly increases when sampling gas flow is low and sampling dead space (length of tubing) is more. Gas from sampling chamber is diverted towards gas scavengers or reinjected in breathing circuit. It can be used for spontaneously breathing patients, along with O2 supplementation. Additionally, there is no problem with sterilisation, ease of connection and ease of use when patient is in unusual position.1,9,10
Mainstream sampling
In mainstream sampling analysers, sample chamber will be positioned within the patient's breathing circuit (Figure 2). The infrared rays traverse the gas within the sample chamber to an infrared detector within the chamber. It does not remove gas from breathing circuit. Hence, it has a faster response with no delay, minimal leakage and no sampling suction pumps are required. To prevent water condensation from humidity of expired gas, sensors are heated above body temperature. Hence, it must be kept away from patient's skin as it can cause burns. The device is relatively heavy and can cause endotracheal tube kinking or accidental extubation.1,3,9–11 Lighter mainstream capnographs are now available that do not add weight to the connections.
Figure 1: Sidestream analyser
Figure 2: Mainstream analyser
Basic physiology
Carbon dioxide (CO2) generated as a result of cellular metabolism is transported to the heart by systemic circulation, from where it is pumped to pulmonary circulation. CO2 diffuses into alveoli and equilibrates with end-capillary blood (PACO2 = PcCO2).1,13 The actual concentration of CO2 in the alveoli depends upon ventilation and perfusion into the alveoli (V/Q ratio). The alveoli with higher V/Q ratio have lower CO2 compared to alveoli with low V/Q ratio that would have higher CO2.
As expiration begins, exhaled gas from breathing circuit and anatomical dead space (no perfusion, no gas exchange) containing no CO2 is exhaled (Phase I) (Figure 3). As expiration continues, there is a rapid rise in carbon dioxide concentration suggesting mixing of dead space and alveolar gas producing a S-shaped upstroke on capnograph (Phase II). This is followed by a plateau phase (Phase III) represents the CO2 gas from the alveoli. Although expiratory flow is higher initially and it tapers off at end of expiration, alveolar plateau has a positive slope. Positive slope indicates that the alveolar gas sampled at the end of exhalation is having higher CO2 concentration, as the diffusion of CO2 from pulmonary capillaries continues at same rate, but the dilution of CO2 into lung volumes reduces progressively as exhalation continues.1 The under ventilated alveoli (low V/Q, high PCO2) empty slowly and contribute to rising slope of phase III. Slope and height of phase III depends upon alveolar CO2 concentration and their emptying pattern. In turn, alveolar concentration depends upon ventilation and perfusion.
Figure 3: Normal time capnogram
The height of phase III depends upon cardiac output and the slope of phase II and III is dependent on emptying patterns of alveoli as well as V/Q ratio.7,8,14 At the end of exhalation, phase III reaches a peak, and is called endtidal partial pressure of CO2 (PETCO2). Hence, slow exhalation, as in acute asthmatic attack or COPD, will produce a steeper alveolar plateau and a higher PETCO2 (Figure 4).1 At the end of phase III, with the beginning of inspiration, capnograph takes an almost right angle turn and descends to baseline (Phase IV).
Concentration of CO2 in the inspiratory gas depends upon breathing circuit, inspiratory flow and fresh gas flow rate (Figure 5).1 Cardiac oscillations appear as small, regular, saw tooth like positive waves in Phase IV at the end of the expiratory phase. These are due to contraction and relaxation of the heart and intrathoracic great vessels on the lungs, forcing air in and out (Figure 6).2
The angle between phase II and III is referred to as the alpha angle (Figure 3). Normally, it is 100-110°. It increases with increase in slope of phase III and indirectly indicates V/Q status of lung.1 The angle between phase III and IV is nearly 90°, referred to as the beta angle (Figure 3). It increases during rebreathing and prolonged response time of the capnograph.1
Figure 4: Steeper alveolar plateau due to obstruction to expiratory flow
Figure 5: Rebreathing (elevated baseline) due to incompetent expiratory valve, low fresh gas flow rate, exhausted CO2 absorbent
Figure 6: Cardiac oscillations
Volume capnograph
CO2 concentration when plotted against expired volume is termed as ‘volume capnograph’ (Figure 7). It can be used in mainstream capnographs, where cuvette containing CO2 sensor is inserted between endotracheal tube and breathing circuit. Unlike timed capnography, inspiration is not monitored, and presence of endotracheal tube is mandatory for use of volume capnography.1,7,8
Phase I – Dead space gas (no CO2)
Phase II – Progressive alveolar emptying (midportion of phase II marks the mean airway-alveolar interface that separate the airway from alveoli)7,8
Phase III - Alveolar plateau (small positive slope reflects the different time constants of emptying alveoli and continuous influx of CO2 from pulmonary capillaries)
Volumetric capnography simultaneously measures expired CO2 and tidal volume (VT). Analysis of expired CO2 as function of exhaled volumes along with PaCO2 will give a precise idea about ratio of Vd (dead space) and VT.7,8
Figure 7: Volume capnogram
Clinical applications of capnography
The CO2 value as measured at the mouth reflects the following: tissue production of CO2, its transport from venous system to pulmonary artery (pulmonary perfusion), its elimination by lungs (alveolar ventilation) and gas delivery apparatus.15,16 Five characteristics should be inspected: height, frequency, rhythm, baseline and shape.1 Any leak or malfunction in gas sampling system from the airway to the sampling chamber, will result in an abnormal capnograph.
Metabolic causes of increased PETCO2 includes increased body temperature, shivering, convulsions, excessive production of catecholamines, administration of sodium bicarbonate, release of an arterial tourniquet with reperfusion of ischemic areas, intravenous glucose therapy, parenteral hyperalimentation and CO2 absorbed from peritoneal cavity after capnoperitoneum, pleural cavity after thoracoscopy or a joint after arthroscopy (Figure 8).1,7,8
Figure 8: Abrupt increase in PETCO2, occlusion or release of arterial tourniquet unclamping arterial
Utility of capnography in various clinical conditions
The actual value of end-tidal carbon dioxide, whether it is zero, low, or high, abruptly changing or gradually changing and the shape of the capnogram can provide valuable information about the clinical condition.
Verification of correct placement of endotracheal tube: The single most important utility of end-tidal carbon dioxide in the exhaled gas is the immediate and definitive evidence of correct endotracheal tube placement.1,3,4,10,11,17–20 Other devices such as colorimetric CO2 detectors, self-inflating oesophageal detectors, and no-waveform capnography do not match up the accuracy of waveform capnography for confirmation of endotracheal tube placement. Effective ventilation through supraglottic airway device should result into waveform capnography during cardiopulmonary resuscitation (CPR) and after return of spontaneous circulation (ROSC).14,33 With oesophageal intubation, small waveform capnogram may be visible transiently, as a result of gas that has entered stomach during bag and mask ventilation, bystander mouth-to-mouth ventilation, or if patient consumed carbonated beverages or medication. However, this diminishes rapidly in concentration, after six to eight complexes, and abnormal waveform will usually differentiate oesophageal from tracheal intubation (Figure 9).10,18,19,21,22
Figure 9: Oesophgeal intubation
Capnography can aid in performing blind oral or nasal intubation, to identify correct needle placement during transtracheal cricothyrotomy, and to verify tracheal intubation during awake fibreoptic bronchoscopy, especially when fibreoptic vision is obscured.10,22
Detection of apnoea, accidental extubation or ventilator disconnection: In a spontaneously breathing patient, it is used to detect apnoea whereas it may indicate ventilator disconnection in a patient on controlled ventilation.
Rebreathing: If fresh gas flow (FGF) is inadequate, rebreathing of exhaled gas occurs and can be detected by rising baseline and higher inspired CO2 concentration (Figure 5). FGF rate can be increased till there is no rebreathing of gas available to the patient.1
Other uses: Gradually rising or decreasing end- tidal carbon dioxide will indicate hypo and hyperventilation respectively. Other uses of capnography include monitoring of patients during transport, to ensure integrity of airway, to confirm and monitor endotracheal tube placement, and to monitor ventilation and circulation by ensuring continuous waveform capnography. Life-threatening airway disasters related to misplacement or displacement of endotracheal tube and ventilator disconnection can be averted by early detection.18,21
Capnography can detect significant acute respiratory events, even before changes in oxygen saturation or observed apnoea or hypoventilation, during emergency department procedural sedation and analgesia.21,23,24 It can detect inadvertent placement of nasogastric tube into trachea.18,21
In healthy individuals, there is no alveolar dead space which means there are no nonperfused alveoli. In them, P(a-A)CO equals to P(a-ET)CO , which makes end-tidal carbon dioxide concentration (PETCO2) a good estimate of arterial carbon dioxide (PaCO2). In normal individuals, P(a-ET)CO2 may vary from 2-5 mm Hg, which is an index of gas exchange reflecting resistance to diffusion across alveolar- capillary membrane of CO2.1,7,8,25 It increases with age, pulmonary disorders, pulmonary embolism, decreasing cardiac output (CO), hypovolaemia and anaesthesia.
Changes in PETCO2 is usually indicative of change in PaCO2, and thus measuring ETCO2 reduces the need for repeated, difficult and painful, arterial blood gas analysis.2,3,16,26 This correlation is insignificant in case of abnormal cardiac output and ventilation perfusion ratio.11
Lung heterogeneity creates regional differences in CO2 concentration and sequential emptying causes rise in alveolar plateau. Alveolar dead space represents alveoli with high V/Q ratio, ventilated but not perfused. In pulmonary thromboembolism, sudden vascular occlusion leads to increase in dead space indicated by drop in PETCO2.7,8,11,18,25 Similarly, in acute lung injury, there is increase in alveolar dead space. PEEP and recruitment open up some of these collapsed alveoli improving ventilation. This is indicated by an increase in PETCO2.7,8,19 However, if high PEEP causes overdistention of alveoli, alveolar dead space increases, leading to drop in PETCO2.7,8,19 PaCO2 represents mean alveolar concentration of CO2, whereas, PETCO2 represents peak alveolar concentration of CO2 eliminated from lung areas with low V/Q ratio. Difference between PaCO2 and PETCO2 is a reflection of total dead space of lungs. This helps to assess severity of pulmonary disease and evaluate the response to therapy, especially therapies intended to improve dead space to tidal volume ratio.3,18,19,21
Bronchospasm and kinking of ETT (at least 50% occlusion of the ETT) causing partial airway obstruction (expiratory obstruction) leads to prolonged phase II, steeper alveolar plateau (phase lll), increased alpha angle (angle between Phase II and Phase III) and increased height of CO2 waveform (higher PETCO2) (Figure 4).7,8 Severe bronchospasm causing total airway obstruction, can result in failure to detect PETCO2.7,8,10,18,21 Therefore, breath- by-breath analysis of exhaled CO2 concentration monitors alveolar ventilation, pulmonary perfusion and gas exchange, in critically ill mechanically ventilated patient.
In obese patients, capnography demonstrates biphasic emptying and higher PETCO2 than PaCO2. Increases in expiratory flow resistance may reflect a slower expiratory flow with slow accumulation of alveolar CO2 and alveoli that empty last may have more time for CO2 diffusion (Figure 10).13
Figure 10: Concave volume capnograph associated with obstructive expiratory flow
In patients with constant ventilation, there is a significant drop in PETCO2 along with decrease in cardiac output. Reduction in CO leads to decreased delivery of CO2 to the lungs and, consequently, increase in alveolar dead space, resulting in high V/Q ratio.10,11 Reduced blood flow to lungs can also result from surgical manipulation of heart or thoracic vessels, wedging of pulmonary artery catheter and pulmonary thromboembolism (amniotic fluid, thrombus, fat, gas, or tumour) (Figure 11).1 A rapid decrease in PETCO in absence of changes in blood pressure or central venous pressure of heart rate indicates an air embolism without hemodynamic fluctuations, which can further enlarge in size and lead to further decrease in PETCO2.1
PETCO2 is a good indicator of early stages of haemorrhage due to early detection of changes in cardiac output. During early phase of haemorrhage blood pressure can still be normal with substantial amount of blood loss. Capnometry alerts health care provider about tissue hypoperfusion, pulmonary hypoperfusion causing an increase in physiological dead space (alveolar ventilation is same but perfusion reduced drastically) and provides a very useful warning of impending haemorrhagic shock.25,27,28 Capnometry guided fluid resuscitation helps enhance tissue perfusion without detrimental effects of aggressive resuscitation.
Figure 11: Abrupt decrease in PETCO2, sudden hypotension, pulmonary thromboembolism or circulatory arrest
Sublingual capnometry shows great promise for early recognition of patient at risk of tissue hypoperfusion, hypoxia and shock progression because it avoids disturbance of measurement caused by hyperventilation.25,29 High gradient between PslCO2 and PaCO2 is associated with impaired microcirculation and a worse prognosis in critically ill patient.29 Transcutaneous capnography differentiates between hypoxaemia related to V/Q mismatch and hypoventilation, documents correction of nocturnal hypoventilation, and may detect ventilator induced hyperventilation, a possible cause for central apnoea/hypopnoea and glottis closure.30
With cardiac arrest, CO2 produced at tissue level is no longer transported to lungs, where elimination of CO2 takes place (Figure 11).11,18,25 During cardiopulmonary resuscitation (CPR), presence of exhaled CO2 is a better guide to presence of circulation as compared to electrocardiogram, blood pressure or pulse. Consistency of CPR is an important determinant of successful resuscitation, and a continuous waveform capnography helps maintaining consistent chest compressions. If PETCO2 is greater than 10 mm Hg during chest compression, chances of survival and return of spontaneous circulation (ROSC) increases significantly.4,10,11,14,18,20,31 A sudden increase in PETCO2 during CPR may indicate ROSC (Figure 12).4,10,11,18,20,31 Capnography can thus predict and improve outcome following CPR.
In several cases of cardiac arrest with ongoing CPR, absence of detectable capnography during ventilation was misinterpreted as being due to cardiac arrest, rather than due to oesophageal intubation or ETT obstruction.32 Capnography is not susceptible to mechanical artefacts associated with chest compression. However, false high values of PETCO2 were noted when epinephrine or sodium bicarbonate was used.11,18,25
Any leak in breathing circuit will decrease the minute ventilation, as detected by increase in PETCO2 (Figure 13). Low fresh gas flow, exhausted soda lime and ventilator malfunction may also lead to increase in PETCO2 (Figure 13). However, a leak in sample tubing, circuit disconnection and ventilator malfunction can lead to decrease in PETCO2 (Figure 14).1,7,8 Contamination of sample expired gas with fresh gas or too high a sampling rate results in loss of alveolar plateau (phase ll) (Figure 15). Leak in sampling tube, during positive pressure ventilation, can lead to peaking of alveolar plateau (Figure 16). Spontaneous breathing attempt in a mechanically ventilated patient, due to wearing off of neuromuscular blockade, produces clefts in capnograph, also known as ‘curare cleft’ (Figure 17).
The shape of normal neonatal capnograph is different because the dead space is smaller and respiratory rate is higher. Thus, the baseline is shorter, there is a sharper rise in CO2 concentration and little if any, alveolar plateau.13 Capnography can also be used in paediatrics to detect leak around an uncuffed ETT and thereby aid appropriate ETT size selection, blockage or kinking of ETT, and endobronchial intubation.4
Figure 12: Abrupt increase in PETCO2, suggestive of ROSC
Figure 13: High PETCO2
Figure 14: Low PETCO2
Figure 15: Contamination of sample expired gas with fresh gas or too high a sampling rate
Figure 16: Leak in sampling line during positive pressure ventilation
Figure 17: Spontaneous breathing attempts to breath in mechanically ventilated patient
| CO2 production | Alveolar ventilation |
| Fever | Hypoventilation |
| Sodium bicarbonate | Endobronchial intubation |
| Venous embolism | Rebreathing |
| Tourniquet release | Partial airway obstruction |
| Hyperthermia | Apparatus malfunction |
| Exercise | Leak in breathing circuit |
| Malignant hyperthermia | Exhausted soda lime |
| Pulmonary perfusion | Less FGF |
| Increased cardiac output | Ventilator malfunction |
| Increased BP |
| CO2 production Hypothermia | Alveolar ventilation Hyperventilation |
| Extubation | |
| Pulmonary perfusion | Total airway obstruction |
| Decreased cardiac output | Apnoea/hypopneat |
| Hypotension | Apparatus malfunction |
| Hypovolemia | Leak in sample tubing |
| Pulmonary embolism | Circuit disconnection |
| Ventilator malfunction |
| Sudden increase in PETCO2 | Gradual increase in PETCO2 |
| Sudden increase in cardiac | Hypoventilation |
| output (ROSC) | Increased CO2 production |
| Sodium bicarbonate | |
| Sudden decrease in PETCO2 | Gradual decrease in PETCO2 |
| Sudden hypotension Sudden hyperventilation | Decreased CO2 production |
| Massive pulmonary embolism | Hyperventilationt |
| Occlusion of ETT | |
| Disconnection or leakage in artificial ventilator system |
Drawbacks
Capnographs, especially the mainstream type add dead space in breathing circuit.18 They also add to the weight of breathing circuit and can lead to accidental extubation or kinking of ETT. In sidestream capnography, gas sampling rate may sometimes be high enough to trigger a mechanical breath when flow-triggering is used in mechanically ventilated patient.18
Infrared spectrum of CO2 has some similarities to spectra of oxygen and nitrous oxide, hence, a correction factor has to be incorporated into the calibration of capnograph when nitrous oxide is being used.18 Contamination of the monitor by secretions, high sampling rate, too long a sampling tube, or obstruction of sampling chamber can lead to unreliable results.18 Reusable mainstream sensors should be subjected to high-level disinfection as per manufacturer's recommendations.18
Conclusions
Capnography is a safe, reliable, cost-effective, accurate and noninvasive modality of monitoring, which enhances the patient safety and may help to optimise metabolism, haemodynamics and ventilation in a mechanically ventilated patient. It displays information about CO2 production at tissue level, its transport to pulmonary circulation, diffusion into lungs and finally, its elimination by alveolar ventilation, in a single graphical representation.7 A better understanding of various CO2 curves and dead space is of utmost importance.
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