1. Positional Changes in Arterial Oxygen Saturation and End-Tidal Carbon Dioxide at High Altitude - Medex 2015
By Arlena Kuenzel (MBChB, DiMM)
Ben Marshall (BSc, MBBS)
James D Anholm (MD)

2. Overview
Background
Hypothesis
Methods
Results
Recommendations

3. Background
Incidental observation on previous expedition of changes of arterial oxygen saturation with posture
No published data on changes in oxygen saturation in differing body positions at altitude
Haven’t gone into detail about the studies that James sent us here but could maybe be briefly mentioned when talking? Yes, I agree.
Also, I indicate there are “No” data on changes in SpO2 at altitude – this is true as far as we know, but thought we should keep it simple here.
JA: As you’re showing this slide, I would simply say something like: “We could find no published data systematically evaluating oxygen saturation in differing body positions at high altitude.”

4. Hypothesis
On ascent to altitude; SUPINE positioning would result in LOWER OXYGEN saturation than the sitting position due, in part, to DECREASED VENTILATION and INCREASED PETCO2

5. Preparation Subjects self selecting Consent obtained
Ethics committee approved
You could also verbally state that we had more subjects who volunteered but due to time constraints could not be included.

6. Demographics 28 healthy subjects: 10 females, 18 males
Ave. age 39.3 years, SD 15.5 (range 21-69)
Ave. BMI 22.5, SD 3.8 (range )
Mention initial 35 but reduced due to factors such as time constraints and illness
JA: I’m inclined not to confuse everyone, just say we studied 28 subjects at both SL and HA – in the paper we can expand on this but keep it simple for the presentation.
Similarly I’ve rounded off the BMI

7. Methods 10 minute rest prior to testing
Prone, sitting & supine positions tested each for 5 minutes at sea-level
compared to altitude at 5050 meters
JA: Let’s keep altitude as 5050m throughout. This is the best I can come up with comparing it with maps of the area, etc.

8. Measurements SpO2 VE PETCO2
continuously for each subject in a quiet environment
JA: Here or perhaps even better in the discussion, you could also say that we ensured all subjects remained awake throughout all testing. This is particularly important as we know respiratory control changes when we fall asleep and PaCO2 goes up a few Torr in normal healthy subjects when asleep.

9. Back up data recorded
manually
Data recorded using Cosmed K4b2 instrument, saturation probe and heart rate monitor
-mention of equipment difficulties at altitude
-some equipment kept in sleeping bags overnight to reduce environmental impact (impact of cold)

10. Results

11. Sitting Saturation compared to Supine is significantly higher
81.0%
79.4%
Sitting Saturation compared to Supine is significantly higher
p < 0.01

12. 22 of 28 subjects had a fall in SpO2
-1.6%
This is the same data, but eliminates the 1 outlier, Subj #8. n=27 here. Notice how the confidence intervals get narrower and the upper Conf. limit (-0.66) is actually further away from the “0” line than when Subj 8 is included (see previous slide, where upper confidence limit is -0.52)
JA: I would only show this figure, not the similarly looking one. This excludes the outlier (George). IF you have lots of time, you can show this figure and just say that one outlier is not shown and this outlier subject had a decrease in SpO2 of 24% -- we don’t know why, but suspect it is an error. He was not sick with altitude illness and subsequent SpO2 data (from Sam Verges study) didn’t show a low SpO2 while supine.
22 of 28 subjects had a fall in SpO2

13. VE not different in supine vs. upright positions
At altitude,
VE not different in supine vs. upright positions
(13.1 ± 2.4 l/min sitting and 12.0 ± 3.0 l/min, p = NS)
(Can I just check on this, 13.1 is sitting and 12.0 supine?)
JA: Yes, those numbers are correct! They look different, but they aren’t, because the mean change is about 1.05 l/min but the SD of the CHANGE in VE is ~2.8 l/min. In other words, the variability in their responses in VE (to different body positions) was quite variable

14. PETCO2 increased from-
22.8 ± 3.1 mmHg sitting to
23.5 ± 3.3 mmHg supine (p < 0.001)
PETCO2 not correlated with positional change in SpO2 (r = 0.14, p = NS)
PETCO2 not different between supine & prone

15. Further results
O2 consumption and CO2 production not affected by body position at sea level or high altitude SL data generally mimics findings at altitude for changes in end-tidal O2 and CO2 but there was no positional change in SpO2 at SL (p = NS)
Have split this slide over two to make it easier to follow during the presentation

16. Discussion
Changes in alveolar ventilation (as reflected by end-tidal CO2) are not the primary cause of positional changes in oxygen saturation at high altitude
Other factors such as positional changes in dead space and ventilation-perfusion matching in the lungs are likely major contributors
JA: I think “Theory” should be “Discussion”. This is where you can discuss what your findings mean – which is exactly what your statement above does.

17. Summary
When going from sitting to supine position at altitude there is a:
significant decrease in SpO2
variable changes in VE
significant increase in PETCO2
JA: have changed the “significant” decrease in VE to “variable” decrease in VE since it wasn’t significant.

18. Recommendations
We recommend that altitude studies report body position along with SpO2 measurements

19. Questions?