Page
19
of
24
M1 Data Sheet
Dimensions
Length
Width
Height
3.5”
2.5”
1”
88mm
64mm
25mm
Weight
Ounces
Grams
4oz
113.4g
Display
Daylight readable TFT-
color, 2.4”
The Compensatory Reserve Index (CRI)
Healthy human organ systems are dynamically coupled and physiologically stable. Compensatory
mechanisms (e.g. preload, baroreflexes, respiration, etc.), which serve to maintain physiological stability
in the setting of acute blood loss, vary in effectiveness among individuals: Some patients reach
Cardiovascular (CV) collapse after relatively little blood loss (<1 liter); Some can tolerate 30%+ blood loss
before CV collapse. However, as blood loss continues compensatory mechanisms eventually breakdown.
Due to these compensatory mechanisms, traditional vital signs (HR, BP, RR, SpO
2
) do not deteriorate until
the patient is already near collapse and at risk for poor tissue perfusion, progressive acidosis and sudden,
unexpected hemodynamic decompensation.
The Compensatory Reserve Index (CRI) provides a single number between normovolemia (1) and CV
collapse (0) for monitoring central volume changes in patients. CRI is formalized for Effective Volume Loss
(EVL) as:
CRI
= 1 −
𝐸𝑉𝐿
𝑐𝑢𝑟𝑟𝑒𝑛𝑡
𝐸𝑉𝐿
𝑐𝑜𝑙𝑙𝑎𝑝𝑠𝑒
𝐸𝑞(1)
Specifically,
CRI represents the proportion of remaining volume loss the individual can tolerate before
collapse
(see Figure 9).
Figure 9 - The Compensatory Reserve Index
As we can see from the equation above, in order to directly compute CRI, we would need to know both
the current volume of blood lost by the patient, and the total volume loss that patient could ultimately
tolerate before experiencing CV collapse. We can do this retrospectively for controlled experiments,
where
𝐸𝑉𝐿
𝑐𝑢𝑟𝑟𝑒𝑛𝑡
and
𝐸𝑉𝐿
𝑐𝑜𝑙𝑙𝑎𝑝𝑠𝑒
can be reliably estimated, for example in controlled blood draws or
simulated hemorrhage experiments using Lower Body Negative Pressure (LBNP).
