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| REVIEW ARTICLE |
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| Year : 2017 | Volume
: 18
| Issue : 2 | Page : 33-38 |
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Volume capnography: A narrative review
Shubhangi Singh, Bhavani Shankar Kodali
Department of Anesthesiology and Critical Care, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, USA
| Date of Submission | 18-Aug-2017 |
| Date of Acceptance | 16-Oct-2017 |
| Date of Web Publication | 12-Dec-2017 |
Correspondence Address:
Dr. Bhavani Shankar Kodali
Department of Anesthesia, Brigham and Women's Hospital, Boston, MA 02115
USA
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/TheIAForum.TheIAForum_27_17
Volume capnography is the graph of expired carbon dioxide concentration against the expired volume. It often requires special and bulky equipment to be recorded. It can be used to estimate the dead space with fair amount of precision. Various formulae and equations have been described to estimate the dead space. While the Bohr formula is likely the most accurate for measurement of dead space, the Enghoff's equation is likely the most popular and convenient to use. Volume capnography has found uses in both the operating room (OR) and the Intensive Care Unit setting. It can be used to identify the optimal level of positive end-expiratory pressure in patients suffering from the acute respiratory distress syndrome as well as to identify its effect on the ventilation. In the OR, it can be invaluable to monitor ventilation and alveolar recruitment in the obese population. It is also a useful diagnostic adjunct in medical emergencies like pulmonary embolism. In the pediatric population, it finds uses in the monitoring of infants suffering from bronchiolitis. In spite of its multiple and diverse uses, it remains an underutilized technology; the main reasons for this being lack of experience of the providers with volume capnography and the expensive and bulky equipment that is often required. However, volume capnography has a great deal of potential and with further advances in technology, is likely to gain popularity.
Keywords: Capnography, dead space, volume capnography
How to cite this article:
Singh S, Kodali BS. Volume capnography: A narrative review. Indian Anaesth Forum 2017;18:33-8 |
| Introduction | |  |
Volume capnography is becoming more and more popular these days, although it continues to be an underutilized technique. It is gaining popularity both in as well as outside the operating room (OR) such as in the Intensive Care Units (ICU). It is the plot of the concentration/partial pressure of expired CO2 against the expired volume. The ability of this technique to calculate the dead space to tidal volume ratio (VD/VTidal) is what makes it unique and invaluable. Dead space basically refers to the volume in a breath that does not participate in gas exchange. In practical terms, the lower the amount of dead space the more “efficient” the ventilation. Quantification of dead space is especially important in already compromised lungs (e.g., in patients of acute respiratory distress syndrome [ARDS]), where, by minimizing the dead space (e.g., by the use of optimal positive end-expiratory pressure [PEEP] as explained later), we can not only improve ventilation but also minimize the injury to the lungs.
| Principles of Capnography | | |
Physics of end-tidal CO2 measurement
With the advances in technology, there are now many ways to measure or detect CO2 in the expired breath. The capnometry technique used most commonly in the OR and the ICU is based on the Beer-Lambert law [Figure 1] and involves the use of infrared rays.[1] | Figure 1: Showing the beer lambert law. I0:Incident light intensity; It: transmitted light intensity; C: concentration; l: length of the path of light
Click here to view |
We know from Beer's studies that, there is a logarithmic relationship between the intensity of light entering (Incident light or Io) and leaving the solution (Transmitted light or It), and the concentration of the solute (c) in the solution.
I.e., log Io/It∞ c
Absorbance (A) refers to the log of the ratio of the light entering and leaving a sample:
I.e., A = log Io/It
Adding to this, the relation of the length of the path of light (l):[2]
A = log Io/It= εlc
Where A is the absorbance, ε is the molar absorptivity or the molar absorption coefficient (which is specific to the substance being investigated), l is the length of the path travelled by light and c is the concentration of the substance [Figure 1].
Polyatomic gases such as CO2, water vapor and nitrous oxide absorb the infra-red rays in the 4-5 μm wavelength range. Specifically, the wavelength absorbed by CO2 is 4.3 μm [3] and hence, this wavelength is used in the modern capnometers.
| Nomenclature | | |
Time and volume capnography
As the name suggests, time capnography refers to the plot of expired carbon dioxide concentration or partial pressure against time. Volume capnography is, similarly, the plot of the expired carbon dioxide concentration or partial pressure against the expired volume.
Mainstream and side-stream capnography
Mainstream capnography refers to the measurement of expired CO2 with the sensor being interposed between the circuit and the endotracheal tube. A pneumotachograph placed in the same adapter can measure the flow at the same time, thus allowing graphical representation in the form of volume capnogram.[4],[5] There is also no time lag in the display of the waveform. This technique, can, however, be used only in intubated patients. Recently, technical changes to mainstream technology have enabled mainstream sensors to monitor CO2 in nonintubated patients. Side-stream capnography, on the other hand, involves the use of 6-feet long tubing attached to a T-piece adapter that is inserted into the circuit and the endotracheal tube. This tubing does lead to a delay in displaying the waveform. Hence, this technology as currently available in the side stream capnographs cannot be used to obtain volume capnogram. Recent developments in side-stream technology have enabled the development of side-stream capnographs to display real-time volume capnogram. An inbuilt algorithm provides automatic correction of delay resulting from side-stream sampling. Furthermore, the new developments have also enabled plotting respiratory waveforms on time capnogram to delineate phases of the time capnogram more accurately.
A typical volume capnogram [Figure 2] Shows expired CO2 plotted against expired tidal volume. The individual volume capnogram represents one tidal volume. The CO2 waveform is delineated into phase 1 (dead space gases), phase II (mixing of dead space gases with alveolar gases, and phase III (alveolar gases). The CO2 concentration at the end of the alveolar plateau is end-tidal CO2 (ETCO2). The volume of gases below the CO2 curve represents effective alveolar ventilation. Whereas, the volume of gases above the CO2 curve and below the arterial CO2(PaCO2) represent dead space gases (physiological dead space). The physiological dead space can be subdivided into anatomical and alveolar dead space by a vertical line drawn across phase II so that the two triangles [orange and green in [Figure 2] are equal. Anatomical dead space is left to the vertical line. Alveolar dead space is above the alveolar plateau and below the PaCO2 line.[6] The volume capnograph also enables measuring mean alveolar carbon dioxide concentration, or partial pressure (FACO2, PACO2) on the slope of phase III. Therefore, the physiological dead space can be obtained noninvasively, breath by breath, using volume capnography.
Of note, the slope of Phase III of a volume capnogram is much steeper than that of the corresponding segment of the time capnogram. The main reason behind this is the exponential decrease in flow during expiration.[7]
VAE/VTidal or alveolar ejection to tidal volume ratio was studied by Romero et al. It is a new computerized physiologically based index. They found that this index does not change with VTidal even in patients suffering from ARDS and can be used to reliably monitor the ventilatory status (in terms of the fraction of tidal volume actually participating in gas exchange) of ARDS patients.[8]
Essentially, even the in the healthiest lungs, ventilation and perfusion are not equally distributed. There are areas in the upper part of the lungs that are better ventilated as compared to than perfused, and the lower part of the lung is better perfused than ventilated. The dependent parts of the lungs may also have areas of collapsed alveoli that are only perfused with no gas exchange, thus contributing to the shunt fraction.
Total lung ventilation can be divided into alveolar ventilation and dead space ventilation. Dead space refers to the volume of the lungs that is ventilated but not perfused, and hence that it does not participate in gas exchange. This is known as physiological dead space. Dead space can also be categorized into anatomical dead space and alveolar dead space.[9] Anatomical dead Constituted by space (also known as airway dead space) is constituted by the conducting airways that end at the alveoli. Alveolar dead space refers to the alveoli that are ventilated but not perfused. This corresponds to the West Zone 1 of the lungs.
The Bohr equation gives the most precise estimate of dead space.
Bohr equation:
VD/VTidal = (FACO2-FECO2)/FACO2
Where VD is the physiological dead space; VTidal is the tidal volume; FACO2 is the alveolar concentration of CO2; and FECO2 is the concentration of CO2 in the mixed expired gas or the mean concentration of CO2 in the expired gas. [Figure 3] displays the use of volume capnography to graphically measure the VD.
Enghoff equation simplifies this further by assuming that FACO2 is essentially similar to FaCO2(i.e., the arterial partial pressure of CO2. [Figure 4] shows the contrast between the Bohr and Enghoff's dead spaces. | Figure 4: Comparing the Bohr's and Enghoff's approaches in measuring dead space
Click here to view |
VD/VTidal = (FaCO2-FECO2)/FaCO2
However, PACO2 is never equal to PaCO2, since there is usually a small but finite shunt fraction. Thus, being dependent on the FaCO2 Enghoff's equation fails to take into account the difference created by shunt effects. The difference between FACO2 and FaCO2 is further exaggerated in the diseased lung, for example, lungs affected by ARDS, where the shunt fraction is likely higher.
FECO2 is calculated using the following equation:
FECO2= VCO2 total/VTidal
Where VCO2 is the total amount of CO2 expired in a single breath.[8]
Severinghaus and Stupfel's [10] findings further simplified this equation to:
VD/VTidal = (FaCO2-FETCO2)/FaCO2
Where FETCO2 is the ETCO2 concentration. This is the most popular version for the estimating dead space and is the basis of the most commonly used version of volume capnogram.
| Applications of Volume Capnogram | | |
Estimation of dead space
As is clear from the discussion above, a volume capnogram can be used to fairly accurately estimate both the physiological and anatomical dead pace. On the other hand, a time capnogram can only give a subjective idea of the dead pace depending on the difference between the ETCO2 and PaCO2.
Estimation of PaCO2 from end-tidal CO2
It follows from the relation between the ETCO2, the PaCO2 and the dead space, that if two of these three variables are known, the third can be estimated. McSwain et al.[11] found that PaCO2 can be estimated if both ETCO2 and dead space fraction are known. As the dead space fraction increase the difference between the ETCO2 and PaCO2 also increases, however, in a fairly predictable manner. Both these parameters are available from a volume capnogram. As the dead space fraction increases from <0.4 to >0.7 the correlation becomes weaker, however, continues to be at least moderately strong [Table 1].
Estimation of optimal positive end-expiratory pressure
During the gas exchange, the alveoli open and close - The alveoli at the dependent areas of the lung, by virtue of being already collapsed and below their lower inflection point, may be more difficult to open. Alveoli in lungs affected by ARDS have lost their elasticity and are more difficult to open up. Application of PEEP can help keep these alveoli open at a volume where expansion and collapse allow for easy gas exchange (this is also called the lower inflection point of the alveoli). PEEP higher than this point will result in increases in dead space and below this point will not overcome atelectasis and thus increase the shunt fraction. This is known as an optimal PEEP. Thus, it follows that both shunt and dead space are minimum at this PEEP level. Suter et al. demonstrated that the lowest dead space as measured by the Enghoff's equation actually corresponded to the optimal PEEP required to obtain not only the best pulmonary compliance but also the highest oxygen supply to the tissues.[12] Therefore, the volume capnogram, in the ICU can be used to determine the optimal PEEP level for ARDS patients, thus helping to optimize the FiO2 and ventilator settings for them. A volume capnogram can then be used to monitor the ventilatory status of patients suffering from ARDS [8] and thus, the recovery of their lungs by keeping track of the dead space and alveolar ventilation.
Response to peep
With disease progression as well as improvement in ARDS the compliance of the lungs changes, thus changing the optimal PEEP level. Volumetric capnography can be used to monitor this change and the response to application of different levels of PEEP. As the alveoli open up with the application of PEEP the slope of Phase III of the capnogram also decreases.[13] Thus monitoring the lope of Phase III can also give an idea of how the lungs are responding to the application of PEEP.
Prognostication of acute respiratory distress syndrome/acute lung injury patients
Lucangelo et al.[14] conducted a prospective observational study to see if the indices measured from volume capnography can be used to prognosticate patients with acute lung injury (ALI) and ARDS. They found that VAE/VTidal at admission and at 48 h were best predictors of survival in these patients. The interaction between VAE/Vtidal at admission and the PaO2/FiO2 ratio had a sensitivity of 100% and a specificity of 57% to predict survival.
Recruitment of the lungs in obese population
Obese patients are prone to atelectasis under anesethsia, especially in the presence of muscle paralysis. This can be overcome to an extent by intra-operative application of PEEP. Tusman et al.[15] showed that volume capnogram can be especially useful in obese patients undergoing laparoscopic bariatric surgery in the intra-operative setting. Volume capnography can be used to determine the opening and closing pressures of the alveoli in these lungs and thus help identify the PEEP at the lungs are maximally recruited or the optimal PEEP. When combined with pulse oximetry the reliability of identifying optimal PEEP increases further.
Diagnosis of pulmonary embolism
Pulmonary embolism results in sudden cessation of blood to the pulmonary arteries, thus cutting off the blood flow to a part of the lungs. This leads to increase in the dead space as there are areas of the lungs that are ventilated but not perfused. On a volume capnogram if Phase III is extrapolated to 15% of total lung volume then the CO2 at that volume should be close to the PaCO2. A difference of >12 mmHg can indicate pulmonary embolism.[16] When D-dimer is combined with dead space estimation by the volumetric capnography, the negative predictive ruling out pulmonary embolism becomes very high.[17],[18]
Diagnosis of chronic obstructive pulmonary disease
As with the time capnogram, the shape of the volume capnogram differs significantly between normal subjects and patients with chronic obstructive pulmonary disease (COPD). Qi et al.[19] showed that volumes between 25% ETCO2 and 50% ETCO2 and the slopes of Phase II (dC2/dV) [Figure 5] and Phase III (dC3/dV) [Figure 5] can be used to diagnose COPD. Although these tests alone are fairly reliable on their own, when combined, their ability to distinguish patients with obstructive lung disease increases significantly. | Figure 5: The use of volume capnogram for the diagnosis of chronic obstructive pulmonary disease
Click here to view |
Monitoring infants with bronchiolitis
The utilization of the volume capnogram to estimate the degree of lung injury in RDS and ALI can be extended to the pediatric population as well. Almeida-Junior et al.[20] used volume capnography to measure VD/VTidal in infants with bronchiolitis. They found that there was a statistically significant correlation between this ratio and the degree of ALI (defined as PaO2/FiO2). Thus, serial volume capnography can be used as a marker of severity of pulmonary disease in infants.
Noninvasive estimation of cardiac output
Since the volume capnogram plots the concentration of CO2 eliminated against the volume, it can be used to quantify the amount of CO2 eliminated per minute. If the mixed venous and the arterial concentration of CO2 are known, then it can be used to estimate the amount of blood flowing through the lungs each minute, i.e., the cardiac output.
Limitations of volume capnogram
One very important limitation of the volume capnography is that it requires expensive equipment. Furthermore, experience and knowledge of volume capnography are required to properly use this technology and obtain volume capnograms as accurately as possible. Nevertheless, it is a unique and useful tool that should be used more often in not only the ORs but also in the ICUs and the emergency departments to assess the progress of physiological dead space in critically ill patients.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1]
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