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Designed, Produced, and maintained (Edition 10, March 2019) by 

Bhavani Shankar Kodali MD

rebreathing

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The case report on  "Failure To Detect CO2-absorbent Exaustion:  Seeing and Believing" by Pond et al is an interesting reading.1  However, the interpretation of the abnormal capnogram as obtained by the authors (figure above) warrants further analysis.

First of all, the differential diagnosis of the abnormal capnogram could be a curare cleft capnogram (if the second peak occurred during expiration), or a rebreathing capnogram (if the second peak occurred during inspiration).2  Understandably, the authors were not certain if the second peak was a part of expiration or inspiration because the capnographs do not have a device yet for marking inspiration and expiration on the time capnogram, thereby delaying the diagnosis of the problem.3,4

Secondly, the mechanism underlying the causation of the abnormal capnogram needs further elucidation. If it was simple dilution of expired gas by the fresh gas, then it should have resulted in a capnogram similar to the computer generated capnogram (fig.3) with a uniform rise in the horizontal baseline.  However, the capnogram obtained by the authors resembles very much  a 'signature capnogram,'  which is classically obtained during the use of rebreathing circuits such as Bain anesthetic system or Mapelson D circuit.2 In the absence of CO2 absorption, the closed circuit functions as a rebreathing circuit. The only difference between the two circuits, under the circumstances described, is that the fresh gas flow is added to the circuit at the patient end in the Bain or Mepelson D system, whereas it is at the machine end in the circle system. Therefore, before the next inspiration, the inspiratory limb of the closed system consists of predominantly CO2 rich gases at the patient end, and predominantly fresh gases (gases accumulated  during the expiratory pause resulting in a further decrease of  the CO2 concentration) towards the machine end of the inspiratory limb.  During inspiration (shown in     ), the relative differences in the CO2 concentration in the inspired limb results in an inspiratory peak (which is less than the expiratory peak), followed by an inspiratory dip in the CO2 concentration, thereby producing the characteristic shape of the capnogram (fig 1).

Lastly, a faulty expiratory valve has been stated as one of the differential diagnosis of the abnormal capnogram. But a sharp decline in the CO2 concentration at the end of the expiratory plateau (the angle between the plateau and the inspiratory downstroke, also known as beta angle, of nearly 90 degrees) rules out this possibility.3,4,5 The beta angle is more obtuse in the capnograms obtained during valve dysfunction. Moreover, the expiratory plateau is usually prolonged in these circumstances to include a part of inspiration as well. 3,4

(Animated version of the capnogram produced with permission from authors reference 1)

 References:

1.   Pond D, Jaffe RA, Brock-Utne JG. Failure to detect CO2-absorbent exhaustion: Seeing and Believing. Anesthesiology 2000;92:1196-8.

2.   Bhavani-Shankar K, Moseley H, Kumar AY, Delph Y.  Capnometery and Anaesthesia: Review article.   Can J Anaesth 1992;39:6:617-32.

3.    Yasodananda Kumar A, Bhavani-Shankar K, Moseley  HSL, Delph Y.  Inspiratory valve malfunction in a circle system: pitfalls in capnography. Can J Anaesth 1992;39:997-9.

4.    Bhavani-Shankar K, Kumar AY, Moseley HSL, Ahyee-Hallsworth R.  Terminology and the current limitations of time capnography: A brief review. J Clin Monit 1995;11:175-82.
5.    Pyles ST, Berman LS, Modell JH. Expiratory valve dysfunction in a semiclosed circle anesthesia circuit - verification by analysis of carbon-dioxide waveform.  Anesth Analg 1984;63:536-7.

Phase 4

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The terminal upswing towards the end of expiratory plateau. This occurs in pregnant patients and also in obese patients during anesthesia and IPPV.

Interpret your capnogram

Interpret your capnogram

Bhavani Shankar Kodali MD
Compiled: by Ahalya Kodali
 

This section provides a variety of capnograms put together in one group. Several E-mails have prompted us to compile this section. If you happen to see a capnogram and you wanted to know what the underlying cause is, this section should provide an answer. Match your capnogram against the following; the explanation is either self explanatory or' click' on the 'detail' to open a new window. Close the window to return to this page. Majority of these capnograms have bee discussed elsewhere in the website. If you do not see a matching capnogram, please E-mail us (capnoman@gmail.com) with your explanation or explanation referenced elsewhere for inclusion.

 

 

Air-leak - Loose connection between sampling tube
and capnograph / broken connection or filter.

Details
Following one lung transplantation.
If the tip of the endotracheal tube is too close to the carina thereby resulting in differential lung ventilation due to partial obstruction. (see 15 and 23)

 

Rebreathing capnogram of Mapleson D circuit.
Bain circuit.

Details
Exhausted CO2 absorber. See below ()

 

 

Cardiogenic oscillations - Ripple effect - Seen during low frequency ventilation.

Details

 

 

Bronchospasm / COPD / Emphysema/ obstructed endotracheal tube -Slanting and prolonged phase 2 and increased slope of phase 3 (see 14)

 

 

Contamination of capnograph
Trend showing abrupt elevation of baseline and capnogram. See 43

 

 

Trend capnogram during cardiac arrest / resuscitation.

 

 

Upward slanting of phase 4. A normal variant in pregnant women during anesthesia.

Details

 

 

Trend showing gradual elevation of baseline.
Rebreathing

 

 

Curare cleft (see 10)

 

 

Resembling curare cleft due to an artifact created by surgeon leaning on the chest, or pushing against the diaphragm during expiration.
Partial disconnect of main stream capnometer

Details

 

 

Dilution of expiratory gases by the forward flow of fresh gases during the later part of expiration when expiratory flow rate decreases below the forward gas flow rate.

 

 

Elevation of base line- A classic representation of rebreathing.

Exhausted CO2 absorber

 

 

A gradual decrease in end tidal carbon dioxide can occur during reduced metabolism, hypothermia, hyperventilation, small tidal volume ventilation due to inadequate alveolar sampling, and leaks in the sampling system,decrease in cardiac output see 13

 

 

Occasionally, there can be a reverse phase 3 slope seen in patients with emphysema. Most like this may be due to destruction of alveolar capillary system in emphysematous lungs resulting in the delivery of carbon dioxide to expired gases.

 

 

Endobronchial intubation may not result in a characteristic waveform. However, occasionally, it may be like the one seen in COPD or the above. Read the 3 sections.

Details Details Details Details see (15)

 

 

The CO2 waveform has two humps. Kyphoscoliosis resulted in a compression of the right lung. Differential lung emptying. Details

 

 

Esophageal intubation.

 

 

Esophageal intubation: Small CO2 spikes. Esophageal or gastric CO2 due to mask ventilation

 

 

Warming up CO2 analyzer is necessary before it begins recording CO2 wave forms.

 

 

The monitor will zero periodically. Will show CO2 numerical value with no waveform.

 

 

Ventilator IMV breath during spontaneous ventilation.

 

 

Sticking inspiratory valve - Inspiratory flip - Red indicates possible rebreathing

 

 

Air leak due to a broken connection between sampling tube and capnograph Details see (1)

Lung transplant - Dual capnogram Details see 1, 15 and 23

 

 

Increased CO2 due to hypoventilation, hypermetabolic states and rebreathing. See 8, 39

 

 

Capnogram during spontaneous ventilation in adults (see 27)

 

 

These capnograms can occur in children and neonates. Variations are normal and due to faster respiratory rates, smaller tidal volumes, relatively longer response time of the capnographs. Recent technological advances such as micro-stream analyzers are reducing the artifacts due to faster response time and thereby producing normal looking capnograms even in children and neonates.

 

 

Sampling problems such air or oxygen dilution during nasal or mask sampling of carbon dioxide in spontaneously breathing patients.

 

 

Slit sampling tube can result in a pig tail capnogram. A variation of 23 Details

 

 

A terminal upswing at the end of phase 3, known as phase 4, can occur in pregnant subjects, obese subjects and low compliance states. Details see also physiology section-phase 4.

Expiratory valve malfunction can result in prolonged abnormal phase 2 and phase 0.Details

Inspiratory valve malfunction predominantly results in abnormal phase 0. Details

Esophageal intubation resulting few abnormal capnograms with relatively normal initial CO2 numerical values. Details

Hyperventilation gradually results in lowering of ETCO2 values. see 13

Hypoventilation gradually increases CO2 values with normal base line. see 8 and 25

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Rebreathing producing gradual elevation of base line and ETCO2 values. See 8

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Carbonated beverages in the stomach can result in abnormal capnograms with progressively decreasing CO2 values following esophageal intubation. See other variations 17, 18, 37

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Unrecognized exhaustion of CO2 absorber resulted in substantial rebreathing and rising ETCO2 values. The closed circuit without functioning absorber mimicked Mapleson D circuit.   Details

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Contamination of capnometer results in the sudden elevation of base line as well as ETCO2 values. See 5

Curare cleft

Curare cleft 

Bhavani Shankar Kodali MD

Curare Cleft as a result of partial disconnection of main stream CO2 sensor

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Tripathi et al reported that a partial disconnect of a main stream capnometer also can present a "cleft" on the expiratory plateau during controlled ventilation.

After induction of anesthesia, they noticed a "cleft" ( a sudden dip in CO2) during expiratory plateau phase on capnogram. Additional vecuronium was administered. There was no obvious inspiratory efforts by the patient nor was the negative deflection of the airway pressure gauge typical of inspiratory effort. The ETCO2was observed to be increased from control value of 33 mm Hg. There was also a decrease in peak airway pressure as well as a decrease in tidal volume. A leak in the breathing circuit was suspected and found between the capnograph main stream sensor assembly and endotracheal tube. Reconnection of the transducer eliminated the leak and restored the capnogram to normal in the next exhalation.

The authors provide the following hypothesis.

In the presence of the disconnection, the inspiratory capnogram is unchanged. During expiration, the initial high expiratory flow rate will be sufficient to close the inspiratory valve and allow exhaled gas to flow through the CO2 analyzer giving rise to the normal looking expiratory plateau. However, a portion of expiratory gases exit through the leak. This reduces the measure expired tidal volume. As the expiratory flow rate decreases, leak flow will cause pressure to decrease in the circuit, allowing expiratory valve to close prematurely and facilitate flow of inspiratory gases.  The flow of fresh gases flow from the inspiratory limb reduces the measured CO2  at the analyzer. As the fresh gas flow causes pressure to rebuild in the circuit, the expiratory valve will reopen, allowing the further movement of alveolar gases pass through the CO2 analyzer.

Reference:
Tripathi M, Tripathi, M. Partial disconnection at the main stream CO2 tansducer mimics

'curare cleft" capnogram. Anesthesiology 1998;88:1117-9.