Mooney down in Atlantic

[peevee]

Any insight towards long term effects to a pilot's brain regarding the altitudes we fly and the lower O2 levels we are measuring?

Is there an SPO2 level we should be avoiding?  Or, that occurs at lower numbers than we are awake for?

thank you for sharing your insight,

-a-

Seems like the number thrown around most often is to stay above 90 for cognitive function. I don't know what you have to go to that would eventually cause damage

[carl]

 

Normal oxygen blood levels are 80-100 mmHg    which has saturation above 95%

Oxygen levels 60 -79mmHg are mild hypoxia   with a  saturation reading  between 95% -90%

I would be concerned if I were the pilot of an aircraft with a reading of less than 95%

I will reword that .I would not operate an aircraft with a saturation less than 95%

How do you correct a low saturation? Increase your oxygen.

 

You should never have tunnel vision, black spots, blurred visions, trouble seeing lights, colors. These are indicators you are already hypoxic.

 

 

Caveat:  I’m just a guy and not a good pilot.

 

The calculations above are based on the equation developed by Severinghaus*:

Oxgen saturation calculated

SO2 = (23,400 * (pO23 + 150 * pO2)-1 + 1)-1

http://www-users.med.cornell.edu/~spon/picu/calc/o2satcal.htm

[Hank]

I have read articles in aviation magazines about possible cumulative effects from flight 8-12K, where mild hypoxia may be present. The onset is, of course, highly variable between people, and also variable over time for a given individual as they age and their health and fitness levels fluctuate.

is this something that those of us who never fly in the Oxygen levels need to worry about? My C just won't go that high, and climb rates are generally T or below 500 fpm by 10K if not before. No, I do not own an oxygen system nor a pulse ox meter.

[DXB]

Any insight towards long term effects to a pilot's brain regarding the altitudes we fly and the lower O2 levels we are measuring?

Is there an SPO2 level we should be avoiding?  Or, that occurs at lower numbers than we are awake for?

thank you for sharing your insight,

-a-

I do think >=95% is a good guideline - in absence of CO-poisoning, this SaO2 level pretty much takes hypoxia concerns off the table for a healthy pilot. Any number below 90 is very alarming by the following reasoning: Our measurement of O2 sat% (SaO2) on our finger reflects  hemoglobin saturation, which is the single best measure of content of the blood.  This is because much more O2 is loaded on hemoglobin than dissolved free in the blood. The relationship between SaO2 and partial pressure of dissolved O2 in the blood (PaO2) is not linear but rather sigmoidal, such that small drops in PaO2 (governed by the amount of O2 you inspire)  below a certain level produce huge drops in in SaO2 (and thus the amount of O2 available to your highly O2-dependent brain tissue).  You are then on the vertical part of the curve.  So is an SaO2 of 90% with a PaO2 of 60mmHg necessarily impairing? No- after all, your arterial blood is carrying roughly 90% of the O2 that it does at sea level. But it is the edge of a  cliff, where a minor further drop in inspired O2 will produce a precipitous drop in SaO2.  Adding to the complexity here is that the edge of the cliff on this sigmoidal curve can move in different physiologic contexts - Google "hemoglobin-oxygen dissociation curve shift" if more info desired.  So it's hard to know precisely if the edge of the cliff at 89% or 91%, and a buffer of 95% is further warranted.   .  

Your question regarding long term health concerns is also partly answered by this sigmoidal curve. I would not expect any long term health impact at SaO2s in the low nineties in a healthy person.  This is because your blood's O2 content is largely maintained over a range of PaO2s on the top part of the scale.  Not sure on long term health concerns at SaO2s in the 80s, but there you've fallen off the flat part of the curve, and the short term health concerns related to a fatal accident are the prevailing ones.

 A caveat is that "healthy" is an operative term here.  For instance, a passenger with bad ischemic heart disease (or, in an extreme case, in the middle of a heart attack) operates with no reserve capacity - they need every last bit of O2 dissolved in their blood (PaO2) above that loaded on hemoglobin  (SaO2) to prevent further injury.  It might be similar with the cognitive ability of an elderly person with small vessel ischemic disease in their brain - for instance, the 80 year old who is typically sharp as a tack but gets confused with minor stressors. Or a person with severe anemia (i.e. adequately saturated hemoglobin, just not enough of it).  It is important to remember that hypoxia is really defined at the level of oxygen in the cells of individual tissues that make use of it, and  so we just use SaO2 and PaO2 as surrogates.  So if blood can't get to the organ either because the pump (heart) or the small vessels in the organ are no good, then a 100% SaO2 doesn't help you as much.

A different type of caveat is lung disease (e.g. advanced COPD), where there is not adequate gas exchange between the lung and the blood- but this will be adequately reflected in the SaO2 number- and the 15,000 foot rule to offer this type of person O2 is inadequate and possibly dangerous.  They need continuous SaO2 monitoring too.  Lastly, a similar but smaller drop in this gas exchange efficiency is normal in the elderly.  Thus the healthy 70 year old pilot may need O2 at slightly lower altitudes, but again, their SaO2 monitor should be an adequate indicator of this.  Overall, an SaO2 monitor is an incredibly powerful tool.   

[carusoam]

Great responses! Thank you everybody.

I have a much better understanding of where I fit in world of GA and how and why I fit there.

The biology classes I took in school left off at a basic level.  They covered a few diseases, but inconveniently avoided the aging processes.

Best regards,

-a-

[Danb]

I have since this occurrence just happened to have a visit with my cardiologist who is also aa Ame  and pilot, I mentioned the vast difference in O2 saturation between me and my wife and that I generally go to O2 around 8,000.  Generally 95% for me, he highly recommended that I continue that practice and go on at the 95% reading. His statement was the cumulative effective below 95% could result in not being as alert as I should be when possibly shooting an approach a time when I should be my sharpest...

[Marauder]

I do think >=95% is a good guideline - in absence of CO-poisoning, this SaO2 level pretty much takes hypoxia concerns off the table for a healthy pilot. Any number below 90 is very alarming by the following reasoning: Our measurement of O2 sat% (SaO2) on our finger reflects  hemoglobin saturation, which is the single best measure of content of the blood.  This is because much more O2 is loaded on hemoglobin than dissolved free in the blood. The relationship between SaO2 and partial pressure of dissolved O2 in the blood (PaO2) is not linear but rather sigmoidal, such that small drops in PaO2 (governed by the amount of O2 you inspire)  below a certain level produce huge drops in in SaO2 (and thus the amount of O2 available to your highly O2-dependent brain tissue).  You are then on the vertical part of the curve.  So is an SaO2 of 90% with a PaO2 of 60mmHg necessarily impairing? No- after all, your arterial blood is carrying roughly 90% of the O2 that it does at sea level. But it is the edge of a  cliff, where a minor further drop in inspired O2 will produce a precipitous drop in SaO2.  Adding to the complexity here is that the edge of the cliff on this sigmoidal curve can move in different physiologic contexts - Google "hemoglobin-oxygen dissociation curve shift" if more info desired.  So it's hard to know precisely if the edge of the cliff at 89% or 91%, and a buffer of 95% is further warranted.   .  

Your question regarding long term health concerns is also partly answered by this sigmoidal curve. I would not expect any long term health impact at SaO2s in the low nineties in a healthy person.  This is because your blood's O2 content is largely maintained over a range of PaO2s on the top part of the scale.  Not sure on long term health concerns at SaO2s in the 80s, but there you've fallen off the flat part of the curve, and the short term health concerns related to a fatal accident are the prevailing ones.

 A caveat is that "healthy" is an operative term here.  For instance, a passenger with bad ischemic heart disease (or, in an extreme case, in the middle of a heart attack) operates with no reserve capacity - they need every last bit of O2 dissolved in their blood (PaO2) above that loaded on hemoglobin  (SaO2) to prevent further injury.  It might be similar with the cognitive ability of an elderly person with small vessel ischemic disease in their brain - for instance, the 80 year old who is typically sharp as a tack but gets confused with minor stressors. Or a person with severe anemia (i.e. adequately saturated hemoglobin, just not enough of it).  It is important to remember that hypoxia is really defined at the level of oxygen in the cells of individual tissues that make use of it, and  so we just use SaO2 and PaO2 as surrogates.  So if blood can't get to the organ either because the pump (heart) or the small vessels in the organ are no good, then a 100% SaO2 doesn't help you as much.

A different type of caveat is lung disease (e.g. advanced COPD), where there is not adequate gas exchange between the lung and the blood- but this will be adequately reflected in the SaO2 number- and the 15,000 foot rule to offer this type of person O2 is inadequate and possibly dangerous.  They need continuous SaO2 monitoring too.  Lastly, a similar but smaller drop in this gas exchange efficiency is normal in the elderly.  Thus the healthy 70 year old pilot may need O2 at slightly lower altitudes, but again, their SaO2 monitor should be an adequate indicator of this.  Overall, an SaO2 monitor is an incredibly powerful tool.   

Dev - you obviously didn't pick this knowledge up by staying at Holiday Inn Express, or did you?

Are you a physician? Or just the greatest Googler I ever met?

Sent from my iPad using Tapatalk

[Hank]

I have read articles in aviation magazines about possible cumulative effects from flight 8-12K, where mild hypoxia may be present. The onset is, of course, highly variable between people, and also variable over time for a given individual as they age and their health and fitness levels fluctuate.

is this something that those of us who never fly in the Oxygen levels need to worry about? My C just won't go that high, and climb rates are generally T or below 500 fpm by 10K if not before. No, I do not own an oxygen system nor a pulse ox meter.

dxb's last post makes me think I may not be too sharp on an approach after flying for a while, but that there are no long-term health effects to worry about.

please correct me if I am wrong.

<anyone else notice no capitalization for new paragraphs? It works on all other sentences.>

[Danb]

Come on Chris he stayed at the holiday inn last night

[mike_elliott]

Come on Chris he stayed at the holiday inn last night

He has over 500000 Holiday inn points by now, I am sure

[DXB]

Dev - you obviously didn't pick this knowledge up by staying at Holiday Inn Express, or did you?

Are you a physician? Or just the greatest Googler I ever met?

Chris- I am a physician, though not a kind that needs to think about cardiopulmonary physiology every day. I was really into critical care medicine during med school and residency, but ultimately went in another direction (so my interest is mostly amateur at this point, a bit akin to staying at a Holiday Inn Express ).

[DXB]

I have since this occurrence just happened to have a visit with my cardiologist who is also aa Ame  and pilot, I mentioned the vast difference in O2 saturation between me and my wife and that I generally go to O2 around 8,000.  Generally 95% for me, he highly recommended that I continue that practice and go on at the 95% reading. His statement was the cumulative effective below 95% could result in not being as alert as I should be when possibly shooting an approach a time when I should be my sharpest...

dxb's last post makes me think I may not be too sharp on an approach after flying for a while, but that there are no long-term health effects to worry about.

please correct me if I am wrong.

Feeling less sharp on approach probably has a few contributors that not related to hypoxia, including ordinary attention fatigue after trying to stay focused during a long flight, and dehydration from increased evaporative losses at altitude. As far as hypoxia is concerned, if the sat monitor has been reading 91%, a healthy person may not really be hypoxic at the tissue level, but their oxygenation is being maintained by slightly increased breathing. There's no way to increase breathing without blowing off more CO2, which raises blood pH above normal (respiratory alkalosis). This unintended effect is a big part of the feeling of altitude sickness, which is mild in most people starting at around 7-8000ft, and is noticed as decreased energy and just feeling a bit off.  In a minority of folks, the sickness builds up over several hours to a much more severe level, and oddly the risk of more severe effects is the same in elite athletes vs. healthy sedentary folks.  So in short, Danb's AME/cardiologist is right - running sats at 95% may make you a tad sharper and feel better by preventing respiratory alkalosis, even if you were getting enough O2 otherwise at 91%. 

As far as long term health effects of repeated altitude exposures for Mooney pilots?  I can't think of any- I'm sure someone here will correct me if I'm wrong.  Keep in mind there are people who live their whole lives in villages at 15,000 feet. The adaptations that allow this are fascinating but take more time to happen and are thus mostly not relevant here. One of the quickest ones is actually through the kidneys dumping bicarbonate from blood into urine so you can breathe more without getting alkalotic. This actually comes with the cost of a diuresis and thus dehydrates you more- not sure how much this happens during a 3-4 hr Mooney flight, but I bet it contributes to getting dehydrated while crossing the Pacific in a plane pressurized to 8000ft. 

[cujet]

Note: This is my personal experience and may not apply to anyone else. 

In 1990-91, a Gulfstream GIII I crewed had a bad outflow valve. It's an electric, 8 inch diameter, butterfly valve. It opens fully on the ground to allow enough flow for air conditioning, without pressurizing the aircraft. We had a few problems with this system, leading to multiple complete de-pressurization events at 45,000 feet. 

The "cheap" company mechanic only purchased a new "controller" (called a pressurization transducer) and the system functioned properly, for a while. The outflow valve's motor was drawing too much current. It would eventually overload and "short out" the drive transistors in the "controller" and motor-itself fully open at 45,000 feet. At which point, I would scramble to switch to the manual control DC motor and manually close the outflow valve to re-gain pressurization. 

My points:

1) One has well more than 5 seconds of useful "life" during a rapid decompression at 45,000 feet. It took me some time just to get the O2 mask on and set to 100%. Other crew members either did not consider it immediately necessary or "forgot". I never knew. But no one passed out. One guy did get completely dopey and did things incorrectly. But that was kind of normal for him anyway. 

2) You cannot see anything as "fog" briefly forms in the air as the pressure departs the airframe. 

3) You cannot hear anything, as eardrums are affected. 

4) You will not see red warning indications, or notice other "emergency" issues for some time. It's absolute chaos. 

5) Once it happens a couple of times, it becomes second nature to "don" the O2 mask, hit the 100% button and take a couple of deep breaths. 

6) If I were to guess, I'd say that it took 5-10 seconds for all the air to leave the cabin. Couple that with the quoted 15 seconds of useful consciousness and what we experienced was at least a 20 second window. Which really is plenty of time to get your mask on. 

7) If you are already on 100% oxygen, the time of useful consciousness is longer.

 

 

[Shadrach]

All of the above makes absolute sense ar relates to depressurization. The specter of hypoxia for the high altitude non-pressurized   operations is not how much useful consciousness you have when your O2 system fails. It how much useful consciousness you have after realizing your O2 system has failed.  At 25K, I'm betting that's not a lot of time.

I agree some of the claims are outrageous. Having spent days between 10,500' and 13,000' doing strenuous hiking and climbing, I don't think I'd be hypoxic at 14500 sitting on my a$$ in an airplane, but I don't know that to be the case. When I lived in the Bay Area (pretty close to SL) and traveled in a day to the Cascades or Sierra, I could easily go from sea level to 11k + in less than 14hrs using an intense amount of aerobic activity with short bursts of anaerobic climbs. Was I operating in mild hypoxia for 3 days?

[Danb]

Ross ..my wife and I went on vacation about 6-7 yrs ago and were in a hotel in Utah I believe 8000 ft or so not sure. I was complaining of headaches , tiredness and a few more items can't quite remember but we shortened..actually left after a day and went to Denver, even though driving I had symptoms once at lower msl I began to feel better...I believe there are sometimes vast differences between us as to when each of us needs  O2.. As stated earlier at 8000ft on the oximeter I start below 90. But at 12000 ft. My wife is at 95+.., makes me wonder the criterior that is set up for us to use. O2

Edited September 21, 2015 by Danb

[DXB]

Ross ..my wife and I went on vacation about 6-7 yrs ago and were in a hotel in Utah I believe 8000 ft or so not sure. I was complaining of headaches , tiredness and a few more items can't quite remember but we shortened..actually left after a day and went to Denver, even though driving I had symptoms once at lower msl I began to feel better...I believe there are sometimes vast differences between us as to when each of us needs  O2.. As stated earlier at 8000ft on the oximeter I start below 90. But at 12000 ft. My wife is at 95+.., makes me wonder the criterior that is set up for us to use. O2

The type of impairment you describe at 8000 ft is very common in a subset of perfectly healthy people.  I once went  to a conference in Colorado where the lodging was around 8500 ft, and an unfortunate attendee from Japan died in his sleep the first night from cerebral edema - a very rare extreme case. Three things reliably help- coming down as you did, supplemental O2, a mild diuretic called acetazolamide, and time acclimating first at a slightly lower altitudes (not really practical for aviation).  

I had assumed that lower SaO2 would be a clear predictor of impairment at such moderate altitudes and looked in the literature to confirm - there is a good amount of work on the question, but to my surprise, the data is all over the place. Instead people have tried to come up with more complex models with modest success to predict altitude sickness because SaO2 is only weakly correlated. Here's one paper that reviews a large amount of the data and concludes SaO2 may still have some value:  http://www.ncbi.nlm.nih.gov/pubmed/18057977  There are some newer papers that show a stronger relationship, but they used very controlled hypoxic challenges in the lab that may not reflect how it's experienced in the real world.

Still at first pass, I don't think it would be crazy to develop guidelines on using an SaO2 monitor at 6000-12,500 to guide supplemental O2 use. Of course I'm sure there are issues and practicalities that I haven't thought of here.  

[carl]

Having spent days between 10,500' and 13,000' doing strenuous hiking and climbing, I don't think I'd be hypoxic at 14500 sitting on my a$$ in an airplane, but I don't know that to be the case. When I lived in the Bay Area (pretty close to SL) and traveled in a day to the Cascades or Sierra, I could easily go from sea level to 11k + in less than 14hrs using an intense amount of aerobic activity with short bursts of anaerobic climbs. Was I operating in mild hypoxia for 3 days?

 

Ross, yes you were hypoxic. Your hike is a classic med school blood gas scenario. 

 

USMLE World Test-Taking Strategy: Acid Base

Posted by Dr. Christopher Carrubba, USMLE Tutor & Senior Contributing Editor ~ Med School Tutors

Jan 7, 2015 11:31:00 AM

A healthy 45 year–old male travels to Switzerland for a mountain climbing trip. During the trip, he stays in a high-altitude camp in the mountains at over 14,000 feet above sea level. After 6 days, he has blood drawn as part of a research study. Which of the following arterial blood gas values would you expect to see with this sample:

	pH	PaO2 (mm Hg)	PaCO2 (mmHg)	Plasma HCO3
A:	7.25	98	21	11
B:	7.32	60	55	31
C:	7.38	70	40	22
D:	7.49	60	20	15
E:	7.50	96	30	17
F:	7.6	72	50	32

When I was studying for Step 1, I would initially panic when I saw questions like this, thinking, "Not only are you asking me to pick between six answers, you’re asking me to work through several variables to arrive at the correct choice!" Now, as a tutor, I see a similar form of anxiety in my students when they encounter questions like these or the dreaded up/down arrows often seen in Endocrinology. However, what I am here to show you is that with some very simple test-taking strategies, you can make questions like these a strength when taking Step 1. Let’s break it down into a series of steps: 

Step I:

I always recommend that students start by reading the question at the end of the vignette first, and then very quickly looking at the answer choices. This can help put the question in context from the start and allow you to more easily decipher clues along the way. Here, we quickly see that this is an acid base question. Now, going through the vignette, it's clear that this is an acid base question about the effects of high altitude. 

Step II:

Make your own answer. In questions like this, I encourage my students to ask, “What would I expect to see?” In this question, that requires understanding the physiological effects of high altitude. Briefly, remember that at high altitude, due to the declining PaO2, our body develops a compensatory tachypnea. As a result, we are blowing off more CO2, which will subsequently lead to an increase in pH. This is respiratory alkalosis. As with all acid base questions, the next issue becomes assessing for compensation. At this point, 6 days in to his trip, we would expect the patient in this vignette to have metabolic compensation by decreasing his bicarbonate. Now, returning to the four variables in this vignette, we can say that the correct answer will have a pH > 7.4, a low PaO2, a low PaCO2, and a low HCO3. 

Step III:

Now it’s time for process of elimination. As we have diagnosed a respiratory alkalosis, we can instantly eliminate answer choices A, B and C as these all have acidotic pH’s. Moving on to PaO2, we can now reasonably eliminate answer choice E as we would not expect a PaO2 of 96 at a 14,000 foot elevation. In the span of a few seconds, we have taken the amount of possible answers from six to two! Now, let’s make the final choice.

Step IV:

We are now left with answer choices D and F. So, which is it? Again, as we have been dealing with a respiratory alkalosis, we know that the correct answer will have a low PaCO2. Thus, we can eliminate F and are left with D. However, before selecting this answer, we must confirm that the final variable, HCO3, fits with our choice. Here, given that the patient has been at altitude for 6 days, we would expect that metabolic compensation has occurred, thus, we would expect to see a low HCO3. A HCO3 of 15 as seen in answer choice D allows us to definitively know that we have made the correct choice. 

As you can see, creating our own answer before picking one of the many choices kept us from panicking at so many options and allowed us to rely on process of elimination to confirm a correct response. 

 

[Danb]

Carl

you stole my multiple choice theory I utilized 30+ yrs. ago when taking the coa exam...I practiced hundreds of hours on just multiple choice questions and based on your rational I generally could eliminate 50-75% of the wrong answers to questions I had no clue about..the rational I believe most people haven't figured out works wonderfully, I taught college courses aimed at future Cpa's and at the end of each course, since my goal was for them to pass the exam I attempted to show the method ...mostly to no avail..seems you have mastered the technique..

[carl]

Ha ha , 

No , this is not mine, it is Dr. Carruba.   I just use it. I also teach it,but my students don't get it.

carl

[DXB]

I enjoyed Carl's blood gas problem because it lets you integrate concepts relating short term and long term adaptations to altitude. 

All other physiologic factors being equal, Ross should have an even lower PaO2 while cruising along at 14,000 than the guy in the problem above, who's had 6 days to acclimate by dumping some HCO3 into his urine so he can blow off more CO2 without raising his pH too much. 

But Ross might still feel pretty good at 14,000 and only be subtly impaired. Meanwhile Danb might be getting into real trouble at the same altitude without O2, even if he manages to maintain the same PaO2  as Ross. How individual people fare at these intermediate altitudes seems kinda unpredictable - clearly SaO2 and PaO2 as correlates of hypoxia are part of it, but they don't tell the whole story. Susceptibility to altitude sickness seems rather poorly understood. 

[Tony Armour]

Yesterday I got to play at 14,000'

Seems my O2 must have developed a leak because we went from 700psi to 100 in a hurry. First thing I did was turn the wife's O2 off. Seeing we were going to be out soon and I didn't want to descend into the clouds just below while still 300+ miles from home I decided to tough it out. Normal breathing and I was in the high 70's, I could concentrate and continue deep deep breathing and bring it back to 90 O2 sat. For a while it was uncomfortable but got better....euphoric maybe ? I know I sure felt better at 90+ just after a few hits of O2 than I did while deep breathing. All while thinking I should have the portable with me instead of in the hanger ! A leak will be tough to track down.

[carusoam]

That brings up another question...

With the finger based portable O2 meter (oxymiter), what is it reading when people pass out in the hypobaric chamber?

Tony, thanks for supplying the details of your recent O2 system experience.  I am really interested in hearing what happened to the hardware when you find out.

Best regards,

-a-

[chrisk]

Yesterday I got to play at 14,000'

Seems my O2 must have developed a leak because we went from 700psi to 100 in a hurry. First thing I did was turn the wife's O2 off. Seeing we were going to be out soon and I didn't want to descend into the clouds just below while still 300+ miles from home I decided to tough it out. Normal breathing and I was in the high 70's, I could concentrate and continue deep deep breathing and bring it back to 90 O2 sat. For a while it was uncomfortable but got better....euphoric maybe ? I know I sure felt better at 90+ just after a few hits of O2 than I did while deep breathing. All while thinking I should have the portable with me instead of in the hanger ! A leak will be tough to track down.

Not the choice I would have made unless terain restricted or the weather below was likely to be dangerous (ice)

[DXB]

That brings up another question...

With the finger based portable O2 meter (oxymiter), what is it reading when people pass out in the hypobaric chamber?

Tony, thanks for supplying the details of your recent O2 system experience.  I am really interested in hearing what happened to the hardware when you find out.

Best regards,

-a-

Not experienced with the hypobaric chambers - I would imagine they monitor very closely and never let it go to full unconsciousness?   I'd expect some variability in SaO2 needed.  Still, hearing of someone in the 70s is really scary. Tony's description of feeling better with adding O2 over forced hyperventilation to come up to the the same SaO2 illustrates the physiology - beyond the hypoxia, the alkalosis from hyperventilation is a big problem unto itself.  Adding O2 by another mechanism (particularly by coming down) is much preferable.  Irrespective of being in the cockpit, I'd consider someone with a sat in the 70's from any cause a true medical emergency, particularly if it couldn't be improved immediately with O2. SaO2 in the 60's is certainly immediately life threatening.  50's means heart could stop working at any second.  When it gets to a certain point, the rest of your physiology enters a downward spiral and the precise SaO2 on the monitor means less.   

[chrisk]

I've learned a few things in this thread.  One of them was pressure breathing.  If the video is to be believed, there is some value to it.  https://www.youtube.com/watch?v=U-ZPjlWK0jc  One more tool in the box should things go wrong.