Close CAPNOGRAPHY: DO WE USE IT OPTIMALLY? Tinus Dippenaar Department of Anaesthesiology, Pretoria Academic Hospital. University of Pretoria. E-mail: firstname.lastname@example.org INTRODUCTION Capnography is part of everyday anesthetic practice. Anyone practicing anesthesia according to a safe standard of practice will shudder at the thought of anesthetizing a patient without the option of measuring end tidal carbon dioxide (EtCO2). There is a wealth of information to be gathered by detecting EtCO2. The key to interpreting CO2 status is in the analysis of the waveform a capnograph displays. PHYSIOLOGY Interpretation of the capnogram begins with an understanding of the normal elimination of CO2. Blood returning to the heart from the periphery carrying CO2, enters the lungs with a partial pressure of 46 mmHg. As it makes contact with the alveoli capillary carbon dioxide diffuses quickly over the capillary membrane into the alveoli. By the time the blood exits the lungs, the partial pressure of CO2 in the alveoli approximates the partial pressure of CO2 in the arterial blood. A CO2 detector takes advantage of this relationship to non-invasively estimate PaCO2. PHYSICS BEHIND A CAPNOGRAPH A CO2 detector measures exhaled CO2. Capnographs use infrared absorption spectrophotometry to derive a value for CO2 in a sample of exhaled gas. Infrared absorption spectrophotometry relies of the principle that a certain gas with a non- symmetrical molecular configuration (like CO2 or H2O vapour) will absorb infrared light at a specific wavelength, and that the absorption will be proportional to the concentration of that particular gas sample in the analyzer chamber. The pattern of the change in concentration during the phases of the respiratory cycle is represented on a screen as a graph. Where the infrared analyzer is placed in line as part of the breathing circuit it is known as a mainstream analyser. The problem with this kind of device is that it adds dead space to the breathing circuit, as well as that it is heavy and can displace the endotracheal tube. Further more, the Infrared analyzer warms up during use and can cause burns to the face of the patient should it be in contact with the skin of the cheek or neck. Side stream analyzers on the other hand aspirate gas samples from the airway at a rate of 50 – 100ml/min (depending on the model of analyzer) and divert it through a sample line to an infrared detector for analyses. These analyzers are susceptible to contamination by water and secretions, or occlusion of the sample line, or may have problems in functioning optimally in neonates where small tidal volumes and high respiratory rates are utilized. The Capnogram A normal capnogram has four phases (I-II-III-0) as shown in figure 1. The flat line A-B (phase I) represents early exhalation. The air in this part of the respiratory cycle comes from the anatomic and physiologic dead space (trachea, bronchi) and is relatively CO2 free. There fore no CO2 is measured. As exhalation continues, alveoli containing CO2 start emptying their content into the larger airways. This creates a near vertical, rapidly rising ascending limb (line B-C, phase II). The angle at which phase II and III meet is known as the α-angle. At the termination of normal exhalation, nearly all of the exhaled gas contains CO2, resulting in a slower increasing or Close flattened plateau (line C-D, phase III). At the end of the plateau and exhalation is point D, the point that measures the end tidal CO2 content of the sample. As inspiration occurs, near vertical downstroke phase (line D-E, phase IV) is observed. The angle at which phase III goes into phase 0 is known as the β-angle. Phase 0 represents inspiration of CO2 free gas mixture and thus a rapid decline in measured CO2 levels. Figure 1: The capnogram. Current annotation is on the left. INTERPRETATION OF THE CAPNOGRAM When interpreting a capnogram, it is done according to a systematic approach. The capnogram is analyzed in term of the frequency of the pattern, the rhythm, the height of the graph, the form and position of the base line and the overall shape of the graph. The frequency of the capnogram is usually displayed as the respiratory rate on the monitor. Abnormality in the rhythm may be due to the patient taking spontaneous breaths because the muscle relaxant has worn of, or because the patient is being ventilated in the intermitted mandatory ventilation (IMV) mode. Figure 2: Two spontaneous breaths precedes a mandatory ventilation breath. Baseline analysis must take into account the trend at which the baseline of the changes. These changes may occur gradually or suddenly. A gradual rise in the baseline is usually due to rebreathing (depleted soda lime, inadequate fresh gas flow). While a sudden rise in baseline value is most often due to contamination of the sample line and or chamber. Figure 3a: Gradual rise in baseline CO2. Close Figure 3b: Sudden shift in baseline CO2. A faulty inspiratory valve in the circle system has a distinct pattern. Figure 4: A stuck inspiratory valve in the circle system. Mechanical ventilation using the Bain co-axial circuit is another reason for an abnormal baseline. Figure 5: Mechanical ventilation using a Bain circuit. With esophageal intubation the capnogram will not display any reading and the graph will remain at zero. Should the stomach be inflated due to overzealous mask ventilation, some CO2 might be recorded but will disappear swiftly. CO2 containing drinks in the stomach will give bizarre graphs. Close Figure 6: Esophageal intubation may give no value at all, or show decreasing PetCO2. Prolongation of phase II and III may be seen in pregnancy. Figure 7: Wave form in a pregnant patient. The most well known example of prolongation of phase II and III is the patient with airway obstruction. Figure 8: The patient with obstructive airways disease. The patient with emphysema may present with a unique wave from in which the plateau phase slopes downward instead of up Close Figure 9: Note the down slope of phase III in the patient with emphysema. The single lung transplant patient shows a biphasic phase III. This is due to the different emptying time constants each lung now exhibits Figure 10: Single lung transplant patient waveform. The patient with kyphoscoliosis also has a unique graph, also due to the different rates at which each lung adds its fractional CO2 content to the total measured fraction. Figure 11: Kyphoscoliosis. Abnormalities of phase III include the curare cleft, signifying the return of diaphragm activity. Figure 12: Curare cleft. Close In patients on CPAP or PEEP using a rebreathing circuit, dilution of the sample by the fresh gas supply may be seen as a notch near the end of phase III Figure. 13: Dilution of sample gas. A faulty inspiratory valve may also present as a small notch just before the start of the descending limb of the graph Figure 14: Faulty inspiratory valve. Especially in pediatric patients, the respiratory rate may be so high and the sample collection rate to low to keep up with the changes in CO2 concentration over the duration of a breath. The abnormal pattern seen is due to the dispersion of gas in the sample line Figure 15: Dispersion of gas in sample line. The plateau height may alter in either an upward or downward fashion. An increase in the plateau height is due to increased metabolic activity due to disease conditions like malignant hyperthermia or thyroid storm or due to hypoventilation. Figure 16: Plateau height increase. Close A decrease in plateau height may be due to hyperventilation, hypothermia, or an acute insults like thrombo- or air embolism. Figure 17: Decreased plateau height. An interesting case in point is CO2 embolism during laparoscopic surgery. The temporal pattern is one of an increase in plateau height, followed by a sudden decrease in the plateau height. Figure 18: Temporal progression of CO2 embolism during laparoscopic surgery. Leaks in the breathing circuit may show up as different waveforms, from no reading at all to abnormalities in phase II and III. Figure 19: Examples of graphs when substantial leakage occurs in sample line. Close INCREASED PETCO2 / PACO2 RATIO When ventilation and perfusion functions normally, PetCO2 should read 2-5mmHg lower than the arterial PCO2. The natural CO2 gradient exists between the level of CO2 in the artery and the end tidal point because every alveolus varies in its own rate of ventilation and perfusion. The gradient will widen (to > 5mmHg) in two clinical abnormal states: incomplete alveolar emptying, and alveolar hypo perfusion. In these states the PetCO2 will not be true reflexion of the PaCO2. Incomplete Alveolar Emptying When exhalation is interrupted, the CO2 detector measures both the exhaled portion of the CO2 free gases from the anatomic dead space and a portion of the CO2 containing gases from the alveoli. Thus the alveolar CO2 rich gas is incompletely measured. The plateau phase of the capnogram is interrupted and the reported CO2 will underestimate the patients true Pa CO2. The causes of incomplete alveolar emptying is summarized in Table 1. Causes for incomplete alveolar emptying Chronic obstructive airways disease -- Asthma -- Emphysema Malfunction of endotracheal tube -- Mucus plug -- kinked tube -- Leaking cuff or cuffless tube In these clinical scenarios true PetCO2 end point is never reached. Therefore, in the setting of wheezing, prolonged expiration, or early capnograph plateau incomplete alveolar emptying should be suspected. Alveolar Hypoperfusion Alveolar hypoperfusion that is significantly less than alveolar ventilation in a large number of alveolar/capillary units impairs CO2 transfer from blood to the lungs. This ventilation (V)/perfusion (Q) mismatch (high V/Q ratio) results in lower measured PetCO2. Both segmental (pulmonary emboli, prolonged lateral decubitus position) and global (cardiac arrest and shock) hypoperfusion of the lung causes the patient PetCO2 to read less than Pa CO2. Segmental hypoperfusion will widen the Pet CO2/PaCO2 gradient as follows. The alveoli in contact with a capillary that is not perfused cannot partake in gas exchange over the capillary membrane. During exhalation, gas from the perfused alveoli (containing CO2) mix with gas from non-perfused alveoli (containing no CO2). The eventual mixture being sampled by the detector is therefor diluted and the reading lower than the actual PaCO2. Likewise, global hypoperfusion will cause disparity between the measured PetCO2 and the actual PaCO2.ventilation is dramatically in excess of perfusion and the result is a diluted CO2 in the airways. PETCO2 TREND DATA While minute ventilation must kept constant before interpretation of the phenomenon, changes in the trend of capnograph reading can alert the clinician to certain adverse conditions developing. Close Increasing PetCO2 This may have two basic etiologies. First are conditions that increase the patient’s metabolic rate (thyroid storm, malignant hyperthermia, fever, seizures), and the second clinical conditions that result in decreased alveolar ventilation (respiratory depression COPD, neuro muscular blockade). Consistently elevated PetCO2 indicate persistent inadequate minute ventilation or CO2 retention to buffer metabolic alkalosis. A bicarbonate infusion or release of a tourniquet from an extremity may give rise to a sudden rise in the PetCO2 trend. Sudden return of spontaneous circulation during CPR also manifests as an increased PetCO2. When both baseline and amplitude increase, defective ventilator exhalation valve, excessive mechanical dead space or rebreathing due to a too low a fresh gas flow for a particular circuit are possible culprits. Clinical conditions increasing PetCO2 Increased production/delivery Bicarbonate administration Fever Seizures Malignant hyperthermia Thyroid storm Decreased alveolar ventilation COPD Hypoventilation Respiratory depression Muscle relaxants Equipment malfunction Rebreathing Ventilator leak or faulty exhalation valve Decrease in PetCO2 Decreased PetCO2 with a normal waveform is result of clinical states of decreased CO2 production as is the case with hyperventilation or hypothermia. An exponential decrease in PetCO2 is associated with extreme pulmonary hypoperfusion (cardiac arrest, massive pulmonary embolism or cardiopulmonary bypass). A sudden decrease to zero or near zero levels indicates disconnection, large leak in the breathing circuit, ventilator malfunction or accidental endotracheal tube displacement. A sustained low PetCO2 without a plateau phase should alert the clinician to reasons for incomplete alveolar emptying. Close Clinical conditions decreasing PetCO2 Decreased production/delivery Cardiac arrest Hypotension Hypothermia Pulmonary embolism Pulmonary hypoperfusion Equipment malfunction Complete airway obstruction Esophageal intubation ETT leak Incomplete exhalation Poor sampling Disconnection in breathing circuit Increased alveolar ventilation Hyperventilation CLINICAL APPLICATIONS OF CAPNOGRAPHY Monitoring Ventilation In normal healthy patients, capnography is an invaluable monitor in assessing a patient’s ventilation- and cardiac output status, and for verification of endotracheal tube placement. Under normal circumstances the PetCO2 correlates well with the PaCO2. As noted previously, patients in states of segmental or global hypoperfusion, patients with underlying lung disease or pathology and in patients with multisystem disease or injury show poor correlation. Even without lung disease PetCO2 to PaCO2 gradient varies in the critically ill. The use of capnography as a guide to mechanical ventilation in the severely injured or critically ill is unreliable and cannot replace serial PaCO2 monitoring. PetCO2 is a good indicator of PaCO2 in head injured and neurosurgical patients except when they require mechanical ventilatory assistance with PEEP >5mmHg, if they are not paralysed, or if they have low PaO2/FiO2 ratio. Other application in ICU includes aiding in weaning from the ventilator where minimal disease is present. In adult patients suffering from acute asthma, capnograms correlate well with spirometry. This can be an extremely useful tool in monitoring severity of attacks, as PetCO2 is non-invasive and effort independent. It can be used successfully to monitor patients suffering from sleep apnea. In children suffering seizures, nasal capnometry can help the clinician to decide whether ventilatory support needs to be provided. The use of capnography is gaining widespread appeal in transporting critically ill patients. Not only does it allow continuous monitoring of ET-tube position, it also assists in optimal ventilation during the transport period. The ability to measure PetCO2 during transport allows pre hospital care providers to be more cognitive of the ventilation status of the patient Volumetric Capnography Volumetric capnography (VCap) represents the measurement of expired CO2 content in function of the expired volume. It is recognized as a useful tool in the ICU as it allows for the separation Close of the tidal volume (Vt) into its three main components, effective Vt contributing to gas exchange (Vtalv), the airway dead space (VDaw) and the alveolar dead space (VDalv). Vt = Vtalv + VDaw + VDalv (1) Thus it provides a breath-by-breath analysis of ventilation perfusion imbalances and dead space volumes. VCap represents a particular interest in respiratory disorders where changes in dead space volumes, V/Q ratios or pulmonary blood flow are expected. The VCap curve is shown in Figure. 20. It is similar to the curve obtained in the single breath nitrogen washout test. The horizontal axis represents expiration volume and the vertical axis the respective gas CO2 content. The slope of the plateau phase of the curve allows the creation of a vertical line defined by setting the areas p and q equal, according to work done on expired nitrogen. This vertical line divides the volume axis into airway dead space VDalv to the left of the line, and alveolar tidal volume Vtalv to the right of the line. Airway dead space is the derived from the formula (1). Clinical application of VCap shows most promise for use as reference method in measuring dead space and ventilation-perfusion mismatch. VCap measurement in healthy and ill patients, spontaneously breathing or ventilated, is comparable with current gold standard reference methods in measuring tidal volume and airway dead space. VCap may well become the next non-invasive standard bedside method of dead space measurement Figure 20: The Volumetric capnography curve. See text for detail. The first use of this measurement is in the diagnosis of pulmonary embolism, where VDalv is increased from unperfused but well ventilated lung regions. When used in conjunction with D-dimer analysis, it has an outstanding sensitivity (98.4%) for exclusion of pulmonary embolism if D-dimer and VCap VDalv are within normal limits. It can also be used for monitoring the improvement in V/Q mismatch during thrombolytic therapy in the treatment of massive pulmonary embolism. Secondly, is has been shown that an increase in physiological dead space (VDaw + VDalv) measured early in ARDS, correlates very well with mortality in ventilated ICU patients. Close Monitoring Cardiac Perfusion Capnography is a helpful adjunct in guiding the adequacy of cardiopulmonary resuscitation (CPR). PetCO2 is a better predictor of cardiac output than PaCO2. It has also been show to correlate well with coronary perfusion, efficacy of chest compression during CPR, and even survival. A drop in cardiac output is accompanied by a parallel decrease in the plateau phase and PetCO2 level. Return of spontaneous circulation or improved CPR results in sustained increase in PetCO2. More recently, is has been demonstrated that PetCO2 has the ability to predict survivability from cardiac arrest. In patients with asystole or pulseless electrical activity (PEA) in which the PetCO2 remained below 10mmHg after 20 minutes of advanced life support, none survived. The patient that survived had a sustained PetCO2 of at least 25mmHg during resuscitation Monitoring PaCO2 During Cardiopulmonary Bypass (CPB) Investigation into the correlation between PCO2 measured by oxygenator exhaust capnograph and blood gas analyser rendered consistently good results when used in an in vitro CPB model. When capnograph results were corrected for 95% oxygen, the accuracy improved further. There was no correlation between oxygenator exhaust PCO2 and blood or gas flow.