Bhavani Shankar Kodali MD
In the last two decades, measurement of carbon dioxide (CO2) in the expired air (capnography) has become increasingly popular in the operating room to monitor patients during anesthesia.1 Its use is strongly recommended in every patient requiring endotracheal intubation,2,3 because capnography can instantaneously identify potentially life threatening conditions such as failed intubation, failed ventilation, failed circulation, and failed circuits before irreversible damage is done to the patients.1,4,5 While the anesthesiologists have appreciated the value of capnography in the last decade, physicians in other specialties are beginning to appreciate its value as a reliable diagnostic, and monitoring aid.6,7
Pediatric critical care medicine has matured dramatically over the last two decades. The American Academy of Pediatrics (Committee on hospital care, 1991-92) has issued 'minimum guidelines and levels of care' required for pediatric intensive care units as a means of ensuring proper patient care and professional creditability.8 The guidelines require CO2 and oxygen (O2) monitoring be performed on all patients receiving care in level I and II pediatric critical care centers. In the past, the only method to quantify the adequacy of ventilation and oxygenation was by assessment of arterial blood gas (ABG). While ABG's remain the gold standard, limitations also exist. ABG's require either a painful, time consuming procedure or an invasive arterial line to obtain a specimen for evaluation. Further, ABG's have inherent fallacies such as the amount of heparin, the amount of time before analysis, and hyperventilation due to pain or breath holding if the child cries during percutaneous sampling. ABG's provide intermittent, not continuous data, which limits its use in documenting transient events. Therefore noninvasive monitors such as pulse oximetry to determine oxygenation, and transcutaneous CO2 (PctCO2) monitoring and end-tidal CO2 monitoring (capnography) to monitor the CO2 status of critically ill infants and children have become increasingly popular.9 They have lessened the need for invasive monitoring with indwelling catheters, with the subsequent reduction in complications due to transfusions, infections and vascular events. However, unlike pulse oximetry sensors, the heated electrodes of transcutaneous CO2 sensors are associated with complications such as burns in the neonate, damages to the skin by adhesive, excessive drift of electrodes, erratic behavior in the presence of acidosis, long calibration and stabilization intervals and the need to change the sensor every 2-4 h.10-14 These effects are more pronounced in infants with decreasing gestational age. In contrast, capnography is not associated with any deleterious effects. It provides continuous surveillance of arterial CO2 tensions and provides information which is not obtainable from ABG's or PtCO2's alone. For example, it serves as a reliable and instantaneous apnoea monitor. It also generates valuable information regarding the mechanical and gas exchanging functions of the child's lungs. Further, in conjunction with ABG analysis, capnography can provide information about ventilation/perfusion (V/Q) disturbances in the lung. Therefore, the value of capnography has recently been recognized in pediatric care, and its use is progressively increasing in pediatric and neonatal intensive care units,15-23 However, the potential diagnostic and therapeutic abilities of capnography are not often recognized, and thus capnography is not used as often as would seem indicated.18 This may be due to two reasons. First, capnography in pediatric patients is viewed skeptically due to limitations in obtaining accurate measurements of CO2 in expired air. In the past five years, though, newer techniques were developed that allowed the accurate measurement of CO2 in expired air even in neonates.16 One such example is the development of Microstream technology which could measure expiratory CO2 accurately in infants and children.24,25 Secondly, the information on capnography in pediatric practice is scattered in the pediatric literature, and a comprehensive paper describing applications and limitations of capnography in pediatric practice is currently lacking. The objective of this web based presentation is to provide a brief background of physiology of CO2 monitoring in expired air, enumerate its potential benefits in healthy children as well as a children with cardiopulmonary abnormalities at various levels of pediatric care, and consider the current limitations of capnography as a diagnostic and monitoring tool as applicable to infants and children.
The clinical usefulness of capnography in pediatric practice is best recognized if its usefulness is considered at various clinical circumstances encountered at different levels of care such as at community hospitals (secondary care centers), during transport from community hospitals to tertiary care centers and at the tertiary care centers.
Community hospitals: There are four important uses of capnography in secondary care pediatric centers:
1. Noninvasive predictor of PaCO2:
By far the most important use of capnography is its noninvasive ability to provide an instantaneous clue to the level of CO2 in the arterial blood. In infants and children breathing spontaneously, the PETCO2 values range from 36-40 mmHg.9 Normally PETCO2, as sampled from the nasal cavity in neonates, infants and children with healthy lungs breathing spontaneously is a good estimate of PaCO2.22,23 In infants and children the (a-ET)PCO2 gradient can vary from - 0.65 mmHg to 2.4 mmHg.22 In preterm infants the gradient may be 3.5 mmHg.22 Alveolar hypoventilation increases PaCO2 as well as PETCO2. Therefore PETCO2 monitoring serves as a important non-invasive monitor of PaCO2 and avoids repeated ABG's. Several factors though, affect the (a-ET)PCO2 difference and the predictability of PaCO2 from PETCO2 particularly in neonates and infants. This is highlighted in the next section. Hence, it is prudent that abnormal PETCO2 values should be confirmed by an immediate ABG.
2. Apnea monitor:
Apnea is a serious threat to the health of infants. it is defined as the cessation of respiration originating from the central nervous system or obstruction of the airway.19 Transthoracic impedance pneumography, which is generally used to monitor chest wall movements, can only detect central apnea and shows false-positive responses in the presence of shallow breathing. Further, impedance devices do not detect the presence of obstructive apnea as chest wall movements persist during obstructive apnea. Detection of obstructive apnea necessitates the evaluation of oral-nasal airflow. It is estimated that up to 60% of observed apnea in preterm infants may be obstructive.19 Accurate information about the rate and rhythm of respiration can be obtained by sampling CO2 from respired gases using nasal adaptors. During apnea of either type, the CO2 concentration at the sampling site falls rapidly and can be instantaneously detected by capnography. Therefore CO2 monitoring serves as a reliable apnea monitor in neonates, infants and children. Further, capnography can be used as reliable monitor to detect sleep apnea syndromes.26
In severe airway obstruction such as bronchial asthma and laryngotracheobronchitis, the shape of the capnogram can be altered, with a prolongation or slanting of phase II and increased slope of phase III, the expiratory plateau. With adequate treatment, the capnogram reverts to normal.
|Bronchial asthma||After treatment|
Therefore the effectiveness of bronchodilator treatments in a child with asthma, and racemic epinephrine treatments in the child with stridor can be assessed from the capnography.18
4. (a-ET)PCO2 gradient as an indicator of pulmonary disease:
As stated in the physiology section, the (a-ET)PCO2 is an indicator of V/Q mismatching resulting from pulmonary disease. In neonates with respiratory disease, the (a-ET)PCO2 difference becomes wider, as for example, in infants with bronchopulmonary dysplasia, where the gradient may be as much as 9mm Hg.22
5. Prehospital stabilization before transport to tertiary care centers.
Critically ill children often require endotracheal intubation before interhospital transportation. Unrecognized esophageal intubation may be catastrophic and can occur even in the hands of the most experienced personnel.20 When used with the standard techniques of chest asculstation, CO2 monitoring is probably the best way to detect esophageal intubation and displacement of endotracheal tube at a later stage. X-rays can only confirm the ETT placement at the time of the X-ray. The tube can be displaced at anytime following the X ray. Although CO2 may be present in the stomach, it is rapidly flushed out during ventilation of the stomach and PETCO2 would decrease, resulting in a flat capnogram. Recently, PETCO2 detectors, which change color on exposure to 4% CO2 have been used successfully to confirm ETT placement in children.27-29
Because of the nature of transport, inadvertent extubation may occur at any point enroute. The noisy environments of the ambulance or helicopter makes evaluation of ETT position difficult. Continuous use of portable CO2 monitors during transport would provide an effective visual check of ETT position and effectively reassure team members. Further, it indirectly confirms ventilation and circulation. Although oximetry would give an indication of hypoxia, presence of CO2 confirms ETT position and would assist in the differential diagnosis.20
In addition to the applications stated above, capnography plays a significant role in the ventilatory management of neonates and children in intensive care units at tertiary care centers.
1.Non-invasive monitor of adequacy of mechanical ventilation:
Capnography is not only a reliable non-invasive monitor to predict PaCO2 in awake infants and children who are breathing spontaneously, but it also serves a as useful device to monitor PaCO2 during mechanical ventilation of intubated children in intensive care units. In intubated neonates and infants with normal respiratory and cardiovascular physiology, PETCO2 values approximate PaCO2 values. In older children though, PETCO2 values are lower than PaCO2 by 2-5 mmHg.29-34 Changes in PETCO2 can often be regarded as indicative of changes in PaCO2. Several factors effect the relationship of PETCO2 and PaCO2 which are discussed in the physiology section. It is prudent to establish the relationship of PETCO2 to PaCO2 initially by blood gas analysis. Thereafter, changes in PaCO2 may be assumed to occur in parallel with those in PETCO2 thus avoiding repeated ABG's. During mechanical ventilation, children are frequently repositioned in their cribs or beds. These positional changes can greatly affect the delivered tidal volume. These alterations in tidal volume result in changes in PETCO2 which can be detected with capnography thus enabling the physician to institute corrective measures.
2. Integrity of Ventilation:
Capnography can identify disconnections in the ventilatory circuit instantaneously before O2 and CO2 levels change in the blood. During the course of IPPV in children with no spontaneous breathing, PETCO2 falls to zero instantaneously following the disconnections in the circuit and sounds an alarm. Corrective measures can be instituted immediately before irreversible damage is caused by prolonged hypoxia. However, in children breathing spontaneously, circuit disconnections distal to CO2 sampling site (towards the child) can be identified instantaneously as CO2 concentration falls to zero in the sampling adaptor, whereas, circuit disconnection proximal to the sampling site may not be detected instantaneously as PETCO2 values depends on the adequacy of spontaneous breathing. If spontaneous breathing is adequate, the PETCO2 values remains normal, whereas, PETCO2 values may rise gradually if spontaneous breathing is inadequate, and thereby alert the physician. Capnography is also useful in giving an early warning of CO2 retention caused by faulty ventilators and misconnections.
3. Occlusion and displacement of endotracheal tube:
In addition to the value of end-tidal CO2 monitoring to confirm the endotracheal tube placement in the trachea, capnography can detect a total occlusion or accidental extubation. Total occlusion or displacement of ETT produces loss of CO2 waveform in capnography. Ventilation through partially kinked or obstructed tube produces distortions in CO2 waveform (prolonged phase II and steeper phase III, and irregular height of the CO2 tracings.35 This would enable capnography to identify a partially kinked or obstructed tube. Therefore, capnography is a useful monitor to detect accidental kinking or displacement of ETT during positioning the child while bathing, cleaning or changing bed covers. In a busy ICU, the need for suctioning of children who are on ventilators is not always noticed immediately. The high pressure alarm often alerts the nurse to a problem. With continuous CO2 monitoring, the need for suctioning can usually be detected before high pressure alarm is activated as partial obstruction of ETT results in inadequate ventilation and CO2 waveform distortions.35
Capnography along with pulse oximetry can be used to monitor alveolar ventilation during weaning from mechanical ventilation. Using ABG's alone may not be an adequate guide to decide on weaning because occasionally pain from blood sampling will cause patients to hyperventilate and thus decrease the PCO2 level in the blood, which may not be a true indicator of patients ventilatory status. Capnography can be used to evaluate the trend of PaCO2, breathing pattern, and importantly the consistency of breathing before extubation.36 Ventilator rates can be gradually decreased to the lowest point at which the patient can comfortably breathe and maintain adequate alveolar ventilation. If the child becomes distressed and increases the work of breathing, or PETCO2 rises, ventilatory rates can be returned to the previously acceptable settings. Evaluation of CO2 waveforms produced by spontaneous ventilation during weaning gives information about the depth and consistency of spontaneous ventilation. The stability of PETCO2 and increasing similarity of capnograms between the patient and the ventilator breaths indicate the patient's readiness to wean from the mechanical ventilation.18
5.PETCO2 as a non-invasive monitor of pulmonary blood flow.
A reduction in the cardiac output (pulmonary blood flow) produces high V/Q ratio (increased alveolar dead space) resulting in lower PETCO2 and an increased (a-ET)PCO2 gradients. As pulmonary blood flow increases, thereby improving V/Q ratio, the PETCO2 increases and (a-ET)PCO2 gradient becomes small. Thus PETCO2 is a function of cardiac output for a given ventilation.37 Hence PETCO2 is a noninvasive monitor of pulmonary blood flow. Utilizing this principle, PETCO2 monitoring can be used to monitor the effectiveness of cardiopulmonary resuscitation. During cardiac arrest, circulation ceases and PETCO2 gradually disappears, reappearing only when circulation is restored either by effective cardiopulmonary resuscitation or cardiac function. PETCO2 monitoring enables the physician to change the technique of pediatric advanced cardiac life support to produce effective pulmonary circulation.38-40 Further, the PETCO2 monitoring may have a prognostic significance. It has been observed that non-survivors had lower PETCO2 than survivors and no patient with PETCO2 < 10 mmHg could be successfully resuscitated.41
6. CO2 production:
Under normal respiratory conditions, changes in CO2 production are usually accompanied by changes in minute ventilation. Thus PETCO2 levels should remain constant If a child is unable to alter minute ventilation sufficiently, increased CO2 production will be manifested by an increase in PETCO2 whereas decrease CO2 production will be manifested by decreased PETCO2. Malignant hyperpyrexia, sepsis, thyrotoxicosis, seizures, shivering, bicarbonate injection and parenteral nutrition increases CO2 production and increase PETCO2, whereas, hypothermia leads to a decrease in PETCO2. Hence, increasing PETCO2 may, therefore, be an early warning sign of an impending hypermetabolic crisis.42
7. Monitoring the course of Pulmonary Disease:
Progress of pulmonary disease can be monitored by improved oxygenation with reduced oxygen requirements, X-ray improvement of lung fields and by serial (a-ET)PCO2. As the pulmonary disease improves (eg., resolving RDS) the initial wider (a-ET)PCO2 gradient is progressively reduced due to improvements in V/Q status of the lung. The (a-ET)PCO2 gradient has been used to assess the effectiveness of diuretic therapy in the improvement in V/Q status of the lung in infants with chronic lung disease.43 The gradient may also be used to assess the improvement in lung function following surfactant therapy in newborns with RDS. Therefore serial (a-ET)PCO2 gradient can be used as a trend monitor to assess the progress of pulmonary disease.44 Further the shape of capnogram also gives information about V/Q status of the lung. Increased V/Q mismatch is suggested by an increase in the slope of phase III. In the presence of abnormal V/Q ratios in lungs, for example during bronchospasm, the emptying pattern of alveoli with various time constants produces a characteristic capnogram with prolonged phase II and a steeper phase III. As bronchospasm improves with therapy, the capnogram reverts to normal as V/Q ratios normalize.18,36
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