Capnography is the monitoring of the partial pressure of alveolar carbon dioxide (CO2) in the respiratory gases. It is a useful noninvasive clinical tool for assessing efficiency and optimizing mechanical ventilation.1 The use of capnography for monitoring surgical patients during anesthesia and in the emergency department to confirm artificial airway placement is well-established and recommended.2 Capnography's importance as a standard of monitoring and patient safety in the ICU has been confirmed in recent years.1,3 Clinical uses of capnography in the ICU also include indirect assessment of cardiac output during weaning from cardiopulmonary bypass in patients without significant lung disease, monitoring of patients during changes in bed positioning, prognostic indicator of outcome in cardiac arrest, adjustment of the trigger sensitivity, and assessment of pulmonary circulation, recognizing the presence of pulmonary embolism as well as the effectiveness of chemical thrombolysis.4–8
Expiratory partial pressure of CO2 can be plotted against time (time-based capnography) or expired volume (volumetric capnography) on each breath. The capnogram is a graphical representation of the concentration or partial pressure of inhaled and exhaled CO2. Measurement of PaCO2 in arterial blood gas, although a true reflection of ventilatory efficiency, is far from ideal because of its invasive and intermittent nature. The main advantage of capnography over arterial blood gas is the ability to provide continuous CO2 monitoring, thus making trend assessments feasible. Although capnography does not replace arterial blood gas analysis (PaCO2), it may decrease the required frequency.
Physiologically based indices derived from the capnographic waveform give direct information about ventilatory misdistributions and functional disturbances. Changes in the morphology of the capnogram indicate ventilation disturbances, and several indices based upon the geometry of the curve were developed in order to quantify ventilation homogeneity distribution.9–11 Moreover, capnography provides an assessment of ventilation/perfusion ratio (V̇/Q̇) mismatch through measurements of end-tidal CO2 (PETCO2). The difference between PaCO2 and PETCO2 in healthy subjects is very small, so PETCO2 may reflect PaCO2.12 Many clinical situations could change PETCO2 values, influencing the arterial-alveolar CO2 difference, which in healthy individuals is <5 mm Hg.12 This difference can be significantly increased in the presence of lung diseases and some cyanotic heart diseases. Then, in an absence of V̇/Q̇ mismatch, the PETCO2 can be correlated to PaCO2.
PETCO2 is often used to optimize the ventilatory strategy and to assess potential changes in pulmonary blood flow.1 PETCO2 measures the expired gas at the end of tidal volume; therefore, it is more representative of alveolar CO2 than airway gas. The PETCO2 obtained from time-based capnography, when associated with arterial blood gas (PaCO2), can be used to calculate the end-tidal alveolar dead-space fraction (end-tidal alveolar dead-space fraction = [PaCO2 − PETCO2]/PaCO2) utilized as an indicator of alveolar VD. Hardman et al13 describe the end-tidal alveolar dead-space fraction as a reliable marker to quantify alveolar VD. According to these authors, the end-tidal alveolar dead-space fraction is easy to obtain, useful, and safe at the bedside of ICU, infirmary, or emergency rooms when used correctly.
The time-based capnogram is the capnography method most commonly used in clinical practice. It provides only a quantitative measurement of PETCO2, and it is not able to quantify directly alveolar VD or anatomic VD. The volumetric capnogram, besides quantitative monitoring of the PETCO2, provides extensive physiological information about production, transportation, and elimination of CO2 within the lungs through waveform patterns.9–11,14 Volumetric capnography, unlike time-based capnography, allows identification of CO2 from sequential emptying of alveoli, and anatomic VD measures, and evaluation of VD in each breath. Time-based capnography requires simpler equipment than does volumetric capnography, which involves a pneumotachometer attached to a capnograph, making it a more expensive apparatus. There are few volumetric capnographs commercially available, and most of them are attached to modern mechanical ventilators.
Volumetric capnography is considered the best tool to measure dead-space volumes (physiological, alveolar, and anatomic), allowing a functional analysis and providing clinical clues about the efficiency of gas exchange.3 The VD increases when the alveolar-capillary interface is compromised (eg, COPD, atelectasis, and ARDS),15–17 during decreased pulmonary blood flow (eg, pulmonary embolism),18 or when alveoli are overdistended (eg, high PEEP). Physiological dead space (VD) involves the sum of anatomic VD, consisting of the portion of the airways that conducts gas without gas exchange, and alveolar VD, which consists of alveoli that are ventilated but not well perfused (V̇/Q̇ mismatch). The physiological VD/VT ranges from 0.30 to 0.35 in healthy children.19 The VD/VT > 0.4 can result in hypoxemia or hypercapnia due to V̇/Q̇ inequality.
Monitoring the VD and its ratio to tidal volume (VD/VT) has proven to be valuable for monitoring patients on mechanical ventilation and for optimizing ventilatory strategies. VD/VT has been studied as a measurement of extubation failure risk in children.20 Increased VD fractions also have been demonstrated to be strongly associated with an increased mortality in subjects with ARDS.21 Almeida-Junior et al22 monitored VD/VT in infants with acute bronchiolitis and reported an association between disease severity and VD/VT. Likewise, VD/VT could be predictive of survival in ventilated newborn infants with congenital diaphragmatic hernia.23 Arnold et al23 claim that VD/VT >0.6 is associated with a 15-fold increase in the rate of mortality.
VD/VT reflects disturbances in the V̇/Q̇ ratio and is affected by any kind of V̇/Q̇ mismatch. Volumetric capnography is also used for monitoring the progression of lung diseases and evaluating the response of VD/VT after changes in mechanical ventilation parameters, such as PEEP and VT.24,25 Furthermore, volumetric capnography can discriminate healthy individuals from those with lung disease, such as cystic fibrosis, bronchiectasis, COPD, lung fibrosis, and asthma, which presents a steeper slope of phase 3 of the capnogram.9–11
In this issue of Respiratory Care, Bhalla et al26 correlated volumetric capnography versus time-based capnography measurements of VD and its fractions. Researchers calculated the VD/VT and end-tidal alveolar dead-space fraction in 65 mechanically ventilated children and adolescents and found a higher correlation between the end-tidal alveolar dead-space fraction and alveolar VD/VT than with VD/VT. Monitoring changes in alveolar VD at the bedside in the ICU with time-based capnography could be simpler and easier than with volumetric capnography. According to Bhalla et al,26 the end-tidal alveolar dead-space fraction calculated by time-based capnography is as good as alveolar VD/VT measured by volumetric capnography.
The interpretation of capnography must take into account the stability of physiologic variables, such as minute ventilation, VT, cardiac output, and CO2 body storage. As a result, the evaluation and interpretation of capnographic data in children should be cautious, particularly in a critical care setting. Although the authors have described that data collection was made during a stable period, if capnography data had been recorded in a similar ventilation mode, bias could be reduced. Bhalla et al26 points out major factors that reduced the correlation between physiological and alveolar VD/VT measured by volumetric capnography with time-based capnography (end-tidal alveolar dead-space fraction): high VD/VT (>0.6), lower oxygenation (PaO2/FIO2 < 200), a large anatomic VD (≥3 mL/kg), and the administration of inotropes or vasopressors. Those results are probably due to a higher V̇/Q̇ mismatch caused by an increase in anatomic VD in subjects with worse lung disease severity.
To summarize, capnography is promising and feasible in the ICU, emergency room, and anesthesia care unit for monitoring the lungs' ventilation/perfusion in mechanically ventilated patients. Volumetric capnography is the best capnography method to guide ventilatory support according to VD fractions, but it requires specialized and expensive equipment. As evidenced by Bhalla et al,26 time-based capnography may be a feasible and less expensive option to assess alveolar VD during mechanical ventilation in the ICU and reduce it, allowing a better V̇/Q̇ match. Despite its potential, capnography remains rarely used because health professionals are unaware of its usefulness in the bedside routine. As stated by Suarez-Sipmann et al,3 the time has come to take capnography out of the experimental setting and introduce it to guide ventilatory strategies in clinical practice.
Footnotes
- Correspondence: Pricila Mara Novais de Oliveira PhD, Rua Engenheiro José Carlos Moraes Sarmento, 332/401. Juiz de Fora, MG 36036-100, Brazil. E-mail: pricila.mno{at}gmail.com.
The authors have disclosed no conflicts of interest.
See the Original Study on Page 1548
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