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Volume Capnogram verses Time capnogram

Physiology of capnography

Bhavani Shankar Kodali MD
 
 
Volume Capnogram / Expirogram/ SBTCO2CO2 Curve Time Capnogram
volumecap.gif timecap.gif
Time capnography is simple, popular and adequate for clinical use

h6: There is no inspiratory segment in a volume capnogram


The expiratory segment is divided into three phases

Phase III of a volume capnogram is a better representation of V/Q status of lung than the phase III of time capnogram
 

Carbon dioxide concentration can be plotted against time (time capnogram) or expired volume (SBT-CO2 curve / Volume capnogram / CO2 expirogram / CO2 spirogram) during a respiratory cycle.1-5 A volume capnogram has only an expiratory segment. There is no inspiratory segment in a volume capnogram. Whereas, a time capnogram has both inspiratory (0) as well as expiratory segment. The expiratory segment of a volume capnogram is divided into three phases, phase I, II, and III.3,4

Advantages of time capnography:

1. Simple and convenient:

2. Monitor non-intubated patients

3. Monitors dynamics of inspiration as well as expiration:

1. Simple and convenient:

The time capnogram is convenient and adequate for clinical use; it is the method most commonly used by capnographs. More elaborate equipment is necessary for plotting SBT-CO2 trace. The CO2 analyzer should be designed to operate with the ventilator, which provides a flow signal and a timing pulse. A computer relates the the instantaneous CO2 signals to expired volume and an SBT-CO2 curve is plotted.3
 
2. Monitor non-intubated patients:

The CO2 analyzer used in SBT-CO2 tracings are mainstream capnometers, where the cuvette containing the CO2 sensor is inserted between the endotracheal tube and breathing circuit. Hence, endotracheal intubation is required for plotting an SBT-CO2 curve, whereas a time capnogram does not require a ventilator and can be used to monitor spontaneous ventilation without breathing through the ventilator. This is because time capnographs make use of main-stream sensors or side stream sensors. Time capnographs with side-stream sensors have the sensor located in the main unit itself; the sample of gas is aspirated from the patient’s airway, via a tiny pump, through a 6-ft capillary tube, into the unit. This enables time capnography to monitor non-intubated patients, as the sampling of respired gases is obtained from the nasal cavity using nasal adapters.

3. Monitors dynamics of inspiration as well as expiration: Time capnography can be used to monitor the dynamics of expiration as wellasinspiration, whereas the SBT-CO2 curve monitors expiration exclusively.

Disadvantages of time capnography:

1. Poor estimation of V/Q status of the lung

2. Can not be used to estimate components of physiological deadspace.

1. Poor estimation of V/Q status of the lung.
The V/Q status of the lung is more accurately reflected in the slope of phase III in an SBT-CO2 trace than in that of a time capnogram, in which the gradient of the phase III slope is usually less obvious (see figure above). This may be because a smaller volume of expired gases (approximately the final 15%) occupies half the time available for expiration, so that a similar change in FCO2 is distributed over a greater length of time in the time capnogram than in the SBT-CO2 trace.3,4


2. Components of physiological deadspace cannot be determined: /h14: Although the time capnogram grossly can be related to a tidal volume and its components, thephysiological deadspace, CO2 output, the components of a tidal volume cannot be determined from a time capnogram as currently recorded, which is further discussed in the following physiology section.

* References


1. Bhavani Shankar K, Moseley H, Kumar AY, Delph Y. Capnometry and anaesthesia. Review articel. Can J Anaesth 1992;39:517-32.

2. Bhavani Shankar K, Kumar AY, Moseley H, Hallsworth RA. Terminology and the current limitations of time capnography. J Clin Monit 1995;11:175:82.

3. Fletcher R. The single breath test for carbon dioxide. Thesis, Lund, 1980.

4. Fletcher R, Jonson B, Cumming G, Brew J. The concept of deadspace with special reference to the single breath test for carbon dioxide. Br J Anaesth 1981;53:77-88.

5. Breen PH, Bradley PJ. Carbon dioxide spirogram (but not capnogram) detects leaking inspiratory valve in a circle circuit. Anesth Analg 1997;85:1372-6.
 
 
 
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Components of a time capnogram

Physiology of Capnography

Bhavani Shankar Kodali MD

 

Components of a time capnogram
logicalterm.gif descap.gif

 

Inspiratory segment Expiratory segment

Phase 0: Inspiration

Beta Angle - Angle between phase III and descending limb of      

inspiratory segment.

Phase I - Anatomical dead space

Phase II - Mixture of anatomical and alveolar dead space

Phase III
- Alveolar plateau

Alfa angle - Angle between phase II and phase III (V/Q status of lung).

 

 
Alveolar Plateau (phase III) has positive slope due to

Continuous excretion of CO2 into the alveoli becoming progressively smaller.

Late emptying of alveoli with low V/Q ratio containing relatively higher CO2 concentration.

 
 
 
Lower part of the lung is better ventilated Lower part of the lung is also better perfused
LungV.gif LungQ.gif
 
 
Lower part of lung is relatively more perfused than ventilated, resulting in spectrum of alveoli with higher V/Q ratio (lower CO2) on the top of the lung, and lower V//Q (higher CO2) at the bottom of the lung
LungVQ.gif
 
 
 
 
 Any factors (such as cardiac output, CO2 production, airway resistance, functional residual capacity) that affects the V/Q ratio of the lung can influence the height and slope of phase III
 
A typical time capnogram (shown above) can be considered as two segments and two angles;an inspiratory segment and an expiratory segment, and alpha and beta angles.1,2

Expiratory segment.

The expiratory segment of a time capnogram is divided into three phases: I, II, III. Occasionally, at the end of phase III, a terminal upswing, phase IV, may occur.


Phase I

Represents the CO2-free gas from the airways (anatomical and apparatus dead space).

Phase II

Consists of a rapid S-shaped upswing on the tracing (due to mixing of dead space gas with alveolar gas).

Phase III

Consists of an alveolar plateau representing CO2-rich gas from the alveoli. It almost always has a positive slope, indicating a rising PCO2 and is due to the following reasons:

(I) Steady excretion of CO2 into the alveoli.

Carbon dioxide is being continuously excreted into the alveoli which are becoming progressively smaller as expiration continues. This results in a steady increase in alveolar PCO2 towards the end of expiration, and hence contributes to a rising positive slope of phase III as expiration proceeds.

(ii) The late emptying of alveoli with lower ventilation/perfusion (V/Q) ratios and, therefore, relatively higher PC02.

If all the alveoli had the same PC02, then irrespective of the emptying patterns, phase III would be nearly horizontal. However, this ideal situation does not occur, even in normal lungs which have a wide range of V/Q ratios. Some alveoli have a higher V/Q ratio (over ventilated) than ideal alveoli and hence they have a relatively lower PC02. Others have a lower V/Q ratio than ideal alveoli (under ventilated) resulting in a relatively higher PC02. The delayed emptying of these alveoli with low V/Q (high PC02) contributes to the rising slope of phase III. The mechanisms producing this effect are:-3,4


(A) Within the terminal respiratory unit.

Ventilation:pelfusion (V/Q) mismatch within the unit may be due either to incomplete gas mixing (alveolar mixing defect) or to the fact that the maximum ventilation and maximum perfusion to that unit are out of sequence in respect to time (temporal mismatching - perfusion is highest during the latter part of expiration when ventilation is lowest). The scatter of V/Q ratios produced as a result is axially distributedwiththosealveoli having low V/Q ratio (higher PC02) being distributed distally and emptying later.

(B) Between respiratory units.


There may be a regional variation in ventilation per unit perfusion producing a spectrum of V/Q ratios (spatial mismatching). Under these circumstances, the slope of phase III is determined by the nature of emptying of the alveolar units: synchronous or asynchronous. If the units empty synchronously, the gas from well-perfused and underperfused alveoli is expired simultaneously, resulting in a horizontal phase 111 or else a phase 111 with minimal slope. However, if the units empty asynchronously, units with longer time constants, hence higher PC02, would empty later (sequential emptying) resulting in a rising slope of Phase III. The slope of the phase III is dependent, therefore, on the emptying patterns of various alveoli with different V/Q ratios as well as continuous C02 excretion into the alveoli. The relative contributions of all of the above mechanisms cannot be separated and all occur simultaneously, influencing the height or the slope of phase III. Factors, such as changes in cardiac output, CO2 production, airway resistance and functional residual capacity (FRC) may further affect the V/Q status of the various units in the lung, and thus influence the height or the slope of phase III. This attribute makes capnography a useful diagnostic tool to detect abnormalities in ventilation perfusion mismatch of the lung.

Inspiratory segment - Phase 0

After phase III is complete, the descending limb makes an almost right angle turn and rapidly descends to the base line. This represents the inspiratory phase during which the fresh gases (CO2 -free gases) are inhaled and CO2 concentration falls rapidly to zero.1,2 The segment of the CO2 trace from the beginning of inspiration to the beginning of expiration, which includes the descending limb and the initial part of the horizontal base line, can be designated as phase 0. The later part of the horizontal base line is the phase I of expiratory segment. Phase 0 represents dynamics of inspiration.

Alfa Angle:

The angle between phases 11 and III, which has been referred to as the alpha angle, increases as the slope of phase 111 increases. The alpha angle (primarily linked to variations in time constants within the lung) is thus an indirect indication of V/Q status of the lung.6

Beta Angle:

The nearly 90 degrees angle between phase III and the descending limb in a time capnogram has been termed as the beta angle.5 This can be used to assess the extent of rebreathing.5 During rebreathing, there is an increase in beta angle from the normal 90 degrees. As rebreathing increases, the horizontal baseline of phase 0 and phase I can be elevated above normal.5,7-9 Occasionally, other factors, such as prolonged response time of the capnometer compared to respiratory cycle time of the patient, particularly in children, can produce increase in the beta angle with the elevation of the baseline of phase 0 and phase I, as observed in rebreathing.10 The details of capnograms under these circumstances are discussed in section 'Pitfalls of capnography'.

References:

1. Bhavani Shankar K, Moseley H, Kumar AY, Delph Y. Capnometry and anaesthesia. Review articel. Can J Anaesth 1992;39:517-32.

2. Bhavani Shankar K, Kumar AY, Moseley H, Hallsworth RA. Terminology and the current limitations of time capnography. J Clin Monit 1995;11:175:82.

3. Fletcher R. The single breath test for carbon dioxide. Thesis, Lund, 1980.
 
 
 
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Basic Physiology of a Capnogram

Physiology of capnography

Bhavani Shankar Kodali MD

Basic Physiology

 

 

 


At the end of inspiration, assuming that there is no rebreathing, the airway and the lungs are filled with CO2-free gases. Carbon dioxide diffuses into the alveoli and equilibrates with the end-alveolar capillary blood (PACO2 = PcCO2 = 40 mm Hg). The actual concentration of CO2 in the alveoli is determined by the extent of ventilation and perfusion into the alveoli (V/Q ratio). The alveoli with higher ventilation in relation to perfusion (high V/Q alveoli) have lower CO2 compared to alveoli with low V/Q ratio that would have higher CO2. As one moves proximally in the respiratory tract, the concentration of CO2 decreases gradually to zero at some point. The volume of CO2-free gas is termed respiratory dead space and here there is no exchange of oxygen (O2) and CO2 between the inspired gases and the blood. As the patient exhales, a CO2 sensor at the mouth will detect no CO2 as the initial gas sampled will be the CO2-free gas from the dead space. As exhalation continues, CO2 concentration rises gradually and reaches a peak as the CO2 rich gases from the alveoli make their way to the CO2 sensing point at the mouth. At the end of exhalation, the CO2concentration decreases to zero (base line) as the patient commences inhalation of CO2 free gases. The evolution of CO2 from the alveoli to the mouth during exhalation, and inhalation of CO2 free gases during inspiration gives the characteristic shape to the CO2 curve which is identical in all humans with healthy lungs.1 Any deviation from this identical shape should be investigated to determine a physiological or a pathological cause producing the abnormality.

 

Reference:

1. Kalenda Z. Mastering infrared Capnography. The Netherlands: Kerkebosch-Zeist 1989.
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Trend Capnogram

Physiology of Capnography

Bhavani Shankar Kodali MD

 

Fast or Regular Capnogram

A Trend Capnogram

Fastcapno.gif trend.gif
 
 
A time capnogram may be recorded at two speeds. A high speed capnogram (about 7mm.sec-1) gives detailed information about each breath whereas the overall CO2 changes (trend) can be followed at a slow (about 0.7 mm.sec-1) speed.1
 
Reference:
 
1.Bhavani Shankar K, Moseley H, Kumar AY, Delph Y. Capnometry and anaesthesia. Can J Anaesth 1992;39:6:617-32.
 
 
 
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