Page 40 - Journal of Special Operations Medicine - Spring 2014

the two curves is the underlying assumption that jus- which classification receives priority should depend on
tifies the different treatment of immediate and delayed the relative availability of the resources in comparison
patients. with the size of the mass-casualty event. In particular,
in severely constrained environments, giving priority to
Main Findings From the the delayed as opposed to the immediate patients would
Mathematical and Computational Analysis maximize the total number of patients who survive. In-
Figure 1 communicates the higher degree of urgency for creased specificity can be gained by examining whether
immediate patients. However, our mathematical and the resource restriction is caused mostly by a large num-
computational analysis, details of which can be found ber of immediate patients, a large number of delayed
8
in a previously published article, revealed that, depend- patients, or both. Adding this distinction to our model
ing on resource limitations, giving priority to immedi- leads to Figure 2, which describes the solution for our
ate patients may in certain cases lead to poor outcomes. mathematical representation, and illustrates the rela-
For a relatively small-scale incident in which resources tionship among the number of patients, their composi-
are not too restrictive, giving priority to immediate pa- tion, resource availability, and the priority policy that
tients would be reasonable because delayed patients has a better chance of maximizing the expected number
will not experience a very significant decline in their of survivors. The reader can refer to the technical ver-
survival probability while they wait for the immediate sion of this report for more details on our mathematical
patients to be treated or transported. However, if the analysis and its results. 8
resources are constrained (because of having either too
many patients or too few ambulances), by the time the Figure 2 A model for taking resource limitations into
disposition of immediate patients is over, it might be too account when determining priorities.
late for at least some of the delayed patients to have a
realistic chance of surviving. If the goal of emergency e Prioritize immediate.
a t i Prioritize delayed.
response is to maximize the total number of survivors, d e m Priority switches from immediate to
this result would indicate a poor use of resources be- m i delayed after some time elapses.
y
cause, compared with the immediate patients who re- n a m
ceived priority, these delayed patients (who ended up A

having low survival probabilities due to the delay in re- s t n C
sponse) in fact had a higher chance of survival to begin e i t a
p
with and therefore were likely to benefit more from the e t a
d i
use of limited resources. This observation suggests that e m
m B
i

w
e
f
Figure 1 Structure of survival probability curves for
immediate and delayed patients. The curves indicate the few delayed patients many delayed patients
probability that a patient classified as immediate/delayed will
survive if he/she is transported/treated at the corresponding
time. The figure is used for the purpose of illustrating the Figure 2 establishes how the prioritization strategy will
shape of the curves; probabilities chosen are arbitrary. T depend on the number of patients in each triage class as
represents the time point when the rate of deterioration of prescribed by our mathematical findings. In particular,
probability of survival in the delayed patient group begins to there are three different types of scenarios, each corre-
accelerate and the slope of the survival curve in the delayed sponding to one of the three regions in the figure:
patients is now worse than the slope of the survival curve in
the immediate patient.
Scenario 1. When there are many delayed patients and/
or few ambulances (region A), delayed patients should
0.9
get priority. In this case, the number of immediate pa-
0.8 Survival probability of delayed patients
tients is irrelevant because there are so many delayed
0.7
patients relative to the available ambulances that there
0.6 is no reason to move resources away from the delayed
Probability of Survival 0.5 patients even for a short period of time. Transportation
resources are severely restricted even when only delayed
0.4
0.3 patients are considered. Without prioritizing delayed
patients from the start, there will be no feasible way of
Survival probability of immediate patients
0.2 avoiding the survival deflection point where delayed pa-
tients begin dying at a faster rate. Given that, in this
0.1
scenario, there are a large number of delayed patients
0
0 30 60 90 120 150 180
T Time to begin with, this results in the decreased probability
32 Journal of Special Operations Medicine Volume 14, Edition 1/Spring 2014