Tuesday, July 20, 2010

Packing for spearfishing

I sometimes get the question - "How do you use packing in spearfishing"

My answer is - "Mostly I don't."

Reasons for this include that I generally dive well buffered for safety - i.e. not pushing it close to blackout. Sure I often dive with another person but most of the time we are away from one another and do the occasional check for the others presence - so essentially we still dive apart. Also packing is not efficient through a snorkel - the gulps (mouthfuls of air) that one can pump in are smaller that without a snorkel - they are probably only 30% of the pack without a snorkel.

I have occasionally rolled onto my back and watched the sky and as it is swept by albatrosses or gannets while packing without a snorkel. (A calm day is required or waves spill into your mouth). Once loaded with that extra air I roll over and duckdive in one motion.
With the extra air I am more buoyant and need to fin harder on the initial descent. Packing is normally accompanied by tachycardia(increased heart rate) at first so one may well feel uncomfortable on the initial descent. My observations are in agreement with research conducted on divers who packed. The researchers refer to packing as glossopharyngeal insufflation (GI). An article, Cardiovascular aspects of glossopharyngeal insufflation and exsufflation, is available in the journal of Undersea Hyperb Medicine. 2007 Nov-Dec;34(6):415-23.
web location:http://www.ncbi.nlm.nih.gov/pubmed/18251438

The researchers studied cardiovascular effects of packing and reverse packing(pumping out air from your lungs using your mouth as a pump). They found that during packing (GI):
- heart rate (HR) increased by 36% - that is pretty high as compared to a typical resting heart rate of 40-60 bpm. I readily felt this when having packed - normally about 5-10 seconds after the last pack.
- the divers experienced a 48% drop in arterial pressure to 50 mmHg, with that an 88% drop in pulse pressure.
- cardiac output (CO) dropped by 79% to 1.3 l/min.

So the research sheds some interesting light on my personal findings. In some persons, including myself, packing heavily(15-30 full packs)occasionally results in a semi blackout if done fast. Such "blackouts" seem to be caused by reduced blood pressure to the brain due to arrhythmia (irregular heartbeat). My experience is that these symptoms are transient and within 5-10 seconds the condition is alleviated. Samba symptoms are experienced - the shaking sensation. It is thought that the reduced flow of blood to the brain causes this apparent blackout.
At my high school some kids would take turns to hyperventilate for 30 seconds and then take a full breath - a buddy would squeeze his chest and out he would go and the other kids would laugh at the person who blacked out. Recovery was normally within about 10-15 seconds. Boy if I could go back there now I would play dead and wind them up a bit. I think packing simply does the same thing - if you pack fast enough then the blood flow is reduced to your brain.
High body temperature appears to enhance the "blackout" - some posts back I spoke of doing this in the bath when I was very hot - I was unable to complete 10 packs without the onset of the dizziness. The heat must lower blood pressure I guess.

Because packing and its side effects are not thoroughly studied and since individuals respond differently to packing it is an activity that should be treated with great caution and progress should be made in small increments rather than great leaps lest you "fall into the deep side before you can swim". Competent back-up is a must when trying out new activities.
Competent implies that the person should be able to retrieve you and administer CPR in even of trouble. Competent implies that the person should be able to retrieve you and administer CPR in even of trouble.

The aforementioned researchers also wave the flag of caution: "Because of their hemodynamic effects, these breathing maneuvers should be performed with caution, particularly in the case of GI[packing]."

Now back to a packed dive for spearfishing - the combination of the harder fining and the active heart and low blood pressure may make this exercise feel as if its use is suspect because you feel rather uncomfortable during the first part of the descent. It is temporary though and I have noticed that with practice the negative sensations are overcome and one adapts.

Another authoritative article covering the effects of packing on heart-related phenomena is:
Effects of glossopharyngeal insufflation on cardiac function: an echocardiographic study in elite breath-hold divers
website: http://jap.physiology.org/cgi/content/abstract/103/3/823?ijkey=40e45106771335a22e564d06a0ff070f563e07fe&keytype2=tf_ipsecsha

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