Earlier this year I built a custom heart rate monitor to wear when diving. It's based around a sealed Polar chest strap and a waterproofed Arduino unit with relevant sensors and data logging. Yes, I know such devices do already exist (ironically a few weeks after I finished my device I picked up a Garmin Descent watch), but I wanted something that I could tinker with and which would provide complete detail with no rounding or smoothing of the data.
Out of many thousands of drops in the past few months, here is a selection of ten dives in which I wore the monitor, presented with commentary and a graph of HR and depth against time. As always, I have to thank my various safety divers and training buddies who make all this possible.
For reference my normal resting HR is about 50 - 55 BPM, and on a dry static from a rested state I typically experience very little bradycardia or hypoxia until 3 or more minutes have passed. On all the dives shown I was weighted to be neutral at about 12M on a normal peak inhale.
DIVE #1, APRIL 21 2018:
The above free immersion drop was done in late spring, when we enjoy our coldest water temps here in Southern California. Surface temperature this day was 57F, and at 10M it was only about 50F. Perfect temperatures to test a little cold water induced bradycardia. A dive mask will block a lot of the facial receptors which trigger mammalian dive reflex, so I did a drop without a mask and for good measure lost my noseclip on the way down, flooding my nostrils and sinuses with icy water, and as usual for me in such a situation experienced instant gut wrenching contractions. Bradycardia is particularly strong on this dive since it was one of my warm-up dives.
In the graph you can see very sudden onset of bradycardia, and about 30 seconds into the dive HR is in the low 20's. About midway through the dive there is a giant spike, but based on conversation with other divers, we suspect the HR just got low enough to confuse the device; detecting HR in the teens are outside the engineering scope of most HR monitors, and they start to count the same beat multiple times. One interesting note about this particular dive was that I had told my safety diver that I would be doing a 1 minute hang, but once I got down there I realized I had no idea when a minute would be up since I was blind and had even forgotten my dive watch that day. I hung for what I assumed was about 20 extraordinarily uncomfortable seconds, trying to just relax and focus on my own heart beats, then went up to discover from my safety that I'd been down over a minute.....
DIVE #2, APRIL 21, 2018:
This was a dive did as a safety/shadow for another diver. What is noteworthy about this particular graph? Notice the spike right around 48 seconds? Since I was shadowing another diver I did not have the benefit of using the line to slow myself, and when they turned at the bottom of their dive, I did one of the most metabolically expensive things a diver can do at depth--put the brakes on my own freefall using my fins, then open the throttle to stay even with the diver I was shadowing. Since this was a working safety dive, those were within my own dive parameters but when doing my own target dive I'd never want to waste that much energy at depth: The secret to bottom time is to use the bottom. When spearing, don't hover a few feet above it, and when line diving, use the line to turn yourself at the bottom instead of your fins.
DIVE #3, APRIL 21, 2018:
This is a stress exercise we sometimes do which can be the basis for a grueling workout. It's like a reverse variable weight dive. For this one, a 10lbs weight is left on the plate, and the diver goes down wearing their normal diving weight to fetch the 10lbs and brings it to the surface. This sort of exercise is great for divers training in areas where their depth is limited.
I dive regularly enough that 25M is nothing strenuous, but the addition of kicking 10lbs up makes it much harder, particularly when doing multiples of this exercise. You can see the spike in HR as I am coming off the bottom and kicking harder than usual to get going.
DIVE #4, JULY 17, 2018:
This was a target constant weight dive that shows some fairly classic curves. Notice the spike in HR as I am starting descent--clearing the first 5M is always one of the most metabolically expensive portions of a full lung dive. Also notice the small spike around 35M? This was me doing a final very large mouthfill starting right around 35M. Finally, there is a bit of a flattening out near the bottom of the dive which shows the slow down in descent speed as I started to struggle a bit with EQ and conditions (in southern California it is awfully dim and cold down there!). At the bottom I am also well below residual lung volume and we can see this reflected in the very low HR at that point in the dive.
DIVE #5, JULY 29, 2018:
Here's a no fins dive I did during a relatively heavy portion of my training schedule. Doing no-fins dives into cold twilight water is nobody's idea of paradise but I find it to be excellent mental and physical conditioning. When compared to the dives with fins we see big spikes in HR near the begining of the dive and during the negatively bouyant portions of my ascent. Also noteworthy is the shape of the depth curve--no fins--particularly in a heavy westuit--can be a bit of a stop and go stroke so we can see stair-steps in the curve, both at the beginning of the dive as I am fighting to get down, and then for nearly the entire ascent as I am coming up. Also, when compared to my dives with fins you can see the ascent time is longer than the descent time.
DIVE #6, AUGUST 5, 2018:
When training on the line I like to warm up with a few exhale dives to failure, emptying my lungs at the surface to a level somewhere between my FRV and FEV lung volume, which in PFI parlance would be somewhere between a level 1 and 2 exhale. This was one such dive. Please note this is an advanced technique and can lead to grave injury if done incorrectly. You can see in the graph a pretty steady curve on the descent since I'm negatively buoyant at the surface on such a dive simply slip immediately into a gentle freefall. On ascent there are some stair steps since I am pulling myself up on the line, very stop and go, and really taking my time as I always do any exhale dive. Starting with such an empty lung I reach residual volume very shallow, and this is reflected in the HR line as well as it falls very quickly.
DIVE #7, AUGUST 5, 2018:
Here is the second exhale warm up I did on same day as as the previous dive, although it was done CWT. In this drop I experience less bradycardia than in the previous drop, which is again typical; I will experience strongest dive response (although the least amount of comfort) on my first warm-up dive. What is interesting here is that due to dive response related bradycardia there is very little rise in HR as I am kicking up heavy from 22M, and then we see the typical spikes in HR at the surface as I start to breathe again.
DIVE #8, AUGUST 5, 2018:
This was the third dive of my warm-up, done on the same day as the preceding two drops. For this one I kicked down to 20M and hung on the line for about 2 minutes. I did 10 packs on this one which is about as much as I ever do, and the HR curve--higher than noraml--at the top of the dive reflects the physical stress and added effort of the over pressurized lungs at the beginning of the dive. With the extended hang and of course water pressure it starts to fall and after 2 minutes is about 30 BPM. These type hangs are not hypoxic for me, but CO2 levels get elevated and I typically get some contractions which will get me ready for subsequent target dives.
DIVE #9, August 5:
This was a target I did after the previous three warm-up dives. I'd been in dietary ketosis for about three days preceeding all these August 5 dives, and also been working my legs very hard in the days prior to this dive with lots of aggressive spearing--loads of underwater swimming and drops to 25M-30M with short surface intervals. Very early on ascent during this dive I felt like my legs were failing and so I switched from my typical stereo kick to doing dolphin kick with some arm strokes. This is reflected in the HR on ascent, particularly when compared to the 57M constant weight dive from a few weeks ago which, even though shallower was done with my legs in a more rested state, and that earlier dive reflects more steady heart rate profile. The muscle failure I felt in my legs I attribute to intentional training fatigue as well as the lack of glycogen due to very low carbohydrate diet.
Also notable on this graph is depth (past 50M) where my HR drops in the 20's. Temperature on the bottom of the dive was a balmy 57F and surface temps were nearly 70F..
DIVE #10, AUGUST 5, 2018:
Here is another CNF dive, done on the same day as the previous, however I felt very strong on this dive. Heading into taper phase of my training schedule, I'd been giving my upper body more time to rest so muscles were more recovered and I had a bit more mental focus and practice on the CNF discipline. Dietary ketosis on this dive was not a noticeable factor, I think because I'd been resting my upper body and also just in general doing a lot of upper body muscle conditioning over the last year. When compared to the CNF dive just a week ago on July 29 you can see marked improvement in effort and much steadier and lower HR throughout the dive.