Physiology of Capnography
Bhavani Shankar Kodali MD
![]() |
![]() |
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 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 |
![]() |
![]() |
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 |
![]() |
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.
There are actually lots of particulars like that to take into consideration. That is a nice level to bring up. I supply the thoughts above as general inspiration but clearly there are questions just like the one you carry up where the most important thing shall be working in honest good faith. I don?t know if finest practices have emerged round issues like that, but I am certain that your job is clearly identified as a fair game. Both girls and boys feel the influence of just a second’s pleasure, for the rest of their lives.
Hiya, I’m really glad I have found this information. Nowadays bloggers publish just about gossips and net and this is actually frustrating. A good web site with exciting content, that is what I need. Thanks for keeping this web site, I will be visiting it. Do you do newsletters? Cant find it.
I do like the way you have framed this particular situation and it does indeed offer me some fodder for thought. On the other hand, coming from what I have experienced, I simply just hope when the actual feedback pile on that folks continue to be on issue and in no way start upon a tirade regarding some other news du jour. Still, thank you for this exceptional point and while I do not necessarily concur with it in totality, I value the viewpoint.
you’re really a excellent webmaster. The web site loading speed is amazing. It kind of feels that you are doing any unique trick. Also, The contents are masterpiece. you’ve done a great job on this subject!
Thanks , I’ve recently been looking for information approximately this topic for a long time and yours is the greatest I’ve discovered so far. But, what in regards to the conclusion? Are you certain about the source?
Hey very nice site!! Man .. Beautiful .. Amazing .. I’ll bookmark your site and take the feeds also…I’m happy to find so many useful information here in the post, we need work out more techniques in this regard, thanks for sharing. . . . . .
Thanks for your marvelous posting! I truly enjoyed reading it, you’re a great author.I will be sure to bookmark your blog and definitely will come back in the future. I want to encourage you to definitely continue your great work, have a nice day!
I know this if off topic but I’m looking into starting my own weblog and was wondering what all is required to get setup? I’m assuming having a blog like yours would cost a pretty penny? I’m not very web smart so I’m not 100 sure. Any tips or advice would be greatly appreciated. Thank you
I’m impressed, I need to say. Actually rarely do I encounter a weblog that’s both educative and entertaining, and let me inform you, you may have hit the nail on the head. Your thought is excellent; the difficulty is one thing that not sufficient persons are talking intelligently about. I’m very blissful that I stumbled across this in my seek for something referring to this.
My developer is trying to persuade me to move to .net from PHP. I have always disliked the idea because of the costs. But he’s tryiong none the less. I’ve been using Movable-type on various websites for about a year and am nervous about switching to another platform. I have heard good things about blogengine.net. Is there a way I can transfer all my wordpress content into it? Any kind of help would be greatly appreciated!
I was just seeking this info for a while. After six hours of continuous Googleing, at last I got it in your website. I wonder what is the lack of Google strategy that don’t rank this type of informative web sites in top of the list. Generally the top web sites are full of garbage.
Hey there, You’ve done a fantastic job. I’ll definitely digg it and personally suggest to my friends. I am sure they’ll be benefited from this site.
Would you be occupied with exchanging links?
Today, I went to the beach with my children. I found a sea shell and gave it to my 4 year old daughter and said “You can hear the ocean if you put this to your ear.” She placed the shell to her ear and screamed. There was a hermit crab inside and it pinched her ear. She never wants to go back! LoL I know this is totally off topic but I had to tell someone!
Hi there just wanted to give you a quick heads up. The words in your post seem to be running off the screen in Ie. I’m not sure if this is a format issue or something to do with internet browser compatibility but I thought I’d post to let you know. The layout look great though! Hope you get the issue fixed soon. Thanks
Howdy very cool website!! Guy .. Beautiful .. Amazing .. I will bookmark your site and take the feeds additionally…I am glad to seek out numerous useful info here in the post, we need develop more techniques on this regard, thank you for sharing. . . . . .
Great work! This is the type of information that should be shared around the web. Shame on Google for not positioning this post higher! Come on over and visit my web site . Thanks =)
whoah this blog is excellent i love reading your articles. Keep up the great work! You know, lots of people are hunting around for this info, you can aid them greatly.
I loved as much as you will receive carried out right here. The sketch is attractive, your authored material stylish. nonetheless, you command get bought an shakiness over that you wish be delivering the following. unwell unquestionably come further formerly again since exactly the same nearly very often inside case you shield this hike.
When I initially commented I clicked the “Notify me when new comments are added” checkbox and now each time a comment is added I get several e-mails with the same comment. Is there any way you can remove me from that service? Many thanks!
Hello there, I found your web site by means of Google even as looking for a similar subject, your site came up, it looks great. I’ve bookmarked it in my google bookmarks.
I’d should examine with you here. Which is not one thing I normally do! I enjoy studying a submit that may make people think. Additionally, thanks for permitting me to comment!
Thanks for the sensible critique. Me & my neighbor were just preparing to do a little research on this. We got a grab a book from our local library but I think I learned more clear from this post. I’m very glad to see such great info being shared freely out there.
Hmm is anyone else having problems with the pictures on this blog loading? I’m trying to determine if its a problem on my end or if it’s the blog. Any suggestions would be greatly appreciated.
Hi there, I found your website via Google while searching for a related topic, your site came up, it looks great. I’ve bookmarked it in my google bookmarks.
Hello very nice website!! Man .. Excellent .. Superb .. I will bookmark your site and take the feeds also…I’m happy to search out a lot of useful information here in the submit, we want work out more techniques on this regard, thanks for sharing. . . . . .
I’m curious to find out what blog system you are working with? I’m having some small security problems with my latest blog and I’d like to find something more safe. Do you have any solutions?
I was wondering if you ever thought of changing the layout of your blog? Its very well written; I love what youve got to say. But maybe you could a little more in the way of content so people could connect with it better. Youve got an awful lot of text for only having 1 or 2 images. Maybe you could space it out better?
Sweet blog! I found it while surfing around on Yahoo News. Do you have any suggestions on how to get listed in Yahoo News? I’ve been trying for a while but I never seem to get there! Thanks
Generally I do not learn article on blogs, but I wish to say that this write-up very compelled me to take a look at and do so! Your writing style has been amazed me. Thanks, quite nice post.
Hi! Do you use Twitter? I’d like to follow you if that would be okay. I’m undoubtedly enjoying your blog and look forward to new posts.
Great write-up, I’m normal visitor of one’s blog, maintain up the excellent operate, and It is going to be a regular visitor for a long time.
Very nice post. I just stumbled upon your blog and wanted to say that I’ve really enjoyed browsing your blog posts. After all I’ll be subscribing to your rss feed and I hope you write again very soon!
F*ckin’ remarkable things here. I am very glad to see your post. Thanks a lot and i am looking forward to contact you. Will you kindly drop me a mail?