In avalanche victims the four most important factors that decide survival are degree of burial, duration of burial, the severity of trauma and finally presence of a free airway and the presence of an air pocket.
Degree of burial
This is a no-brainer. If you are deeply buried you are harder to find, it takes longer to have you dug out and you are likely to have been mauled by larger masses of snow.
The overall mortality rate in avalanche victims is 23%, but rises dramatically to 52% for completely buried victims while being only 4% in the partially buried. The mean depth of burial in completely buried victims is around 1 meter.
Duration of burial
The importance of the duration of burial should also be obvious. The longer you stay under the more likely you are to succumb to hypoxia, hypercapnia or trauma.
In 2001 Haegeli published a survival curve plotting duration of burial against survival.
Several phases can be identified.
There is a precipitous drop in survivability during the first 30 minutes. This drop can be subdivided into two phases. Deaths during the first 15 minutes are most likely to be attributable to trauma. Then there is the steep drop between 15 to 35 minutes. These patients likely die from asphyxia.
After 35 minutes the victims start dying off more slowly from hypoxia, hypercapnia and hypothermia. Hypothermia is a rare cause of death due to snow’s excellent insulating properties.
Haegelis curve tells us to dig like hell to reach the victims. You need to get at them within 0-30 minutes to get at the ones that are quickly dying off from potentially reversible conditions.
The severity of trauma in avalanche victims varies depending on the topography of the terrain. An area with wide open snow fields will have less deaths than an avalanche prone area strewn with rocks or trees.
As always there are not a lot of data, but a retrospective study from 2007 demonstrated how the majority, 75%, had minor injuries (ISS<8), 17% had moderate injuries (ISS 8-13), 2% had serious injury (ISS 14-20) and 7% had critical injuries (ISS>20). Injuries were primarily located in the extremities, chest and spine.
So trauma in avalanches is a big deal. Still the majority of patients have survivable injuries.
This is perhaps a bit surprising, given the enormity of the forces unleashed. However, the few studies that exist confirm how mortality from trauma is in the region of 5-25%. Significant, but still clearly second to asphyxia which is the big killer.
A free airway and an air pocket.
Mountain rescue groups and ski-patrol know what they are doing and know there is more to digging than you would think. It is interesting to watch them do practice scenarios.
First of all they will dig like crazy. You will also notice how they use a conveyor belt method rather than digging straight down. This in contrast to untrained helpers who will tend to dig straight down. This has proven to be an inefficient method.
This will result in the rescuers working in a deep small hole allowing only one or two diggers working at the same time. Eventually they will be lifting snow above their heads in order to empty the hole, which is incredibly exhausting. Finally the victim will have to be evacuated up this deep hole with steep walls. Gentle evacuation is impossible.
Organising a conveyor belt dig is proven to be a lot more effective. One or two persons dig from below towards the patient. The rest of the group are positioned in a v-formation ´downstream´to shovel away the snow produced by the primary diggers.
Then, as they localize the patient you will notice how they will slow down and dig very, very carefully around the head. Not only to avoid injury from the shovels but also, importantly, to assess the airway and look for an air pocket.
An air pocket in this setting is defined any air-filled cavity around the victims mouth and it’s presence or absence is vital for avalanche victims dug out in cardiac arrest.
An air pocket in front of the patient’s mouth and an unobstructed airway means he had a patent airway, and might have been breathing after being buried. It implies he didn’t die from immediate asphyxia or trauma. His time of death simply can’t be determined unlike victims without an air-pocket who died within minutes after being buried.
So then, why is this important? First of all, consider how most victims are found by ski-patrol only after 20-30 minutes at best. Scrambling and responding takes time. Second, consider how the resuce teams resources and manpower is limited. There can thousands of tons of snow to probe through. There might be more victims.
It doesn’t make sense for the rescue team use up two rescuers for doing futile CPR on a victim who has been dead for thirty minutes all the while there might be other victims buried that haven’t been found yet. Victims with an air-pocket, on the other hand, are candidates for CPR.
I included two youtube videos. The first one is unique helmet cam footage of what an avalanche looks from within. The skier survived and has added his own analysis of what went down. As I write this it’s going viral amongst ski-freaks.
The second one is video of a canadian ski-patrol demonstrating the conveyor belt technique. Note how they dig like they are on fire. Digging like mad in a race against time.