Insulin pump overview

The embedded control software for a personal insulin
pump
Case study: Insulin pump overview 1

Medical systems
 More and more medical instruments now include embedded
control software.
 These software systems are often critical systems as a
patient’s life (or at least their health) may depend on the
correct and timely functioning of these systems
 The systems themselves are often relatively small and are
therefore understandable unlike, for example, industrial
control systems

Diabetes
 People with diabetes cannot make their own insulin, a
hormone that is normally secreted by the pancreas. Insulin is
essential to metabolise sugar and hence generate energy
 Currently most diabetics inject insulin 2 or more times per day,
with the dose injected based on readings of their blood sugar
level
 However, this results in artificial blood sugar fluctuations as it
does not reflect the on-demand insulin production of the
pancreas

A personal insulin pump
 A personal insulin pump is an external device that mimics the
function of the pancreas
 It uses an embedded sensor to measure the blood sugar level
at periodic intervals and then injects insulin to maintain the
blood sugar at a ‘normal’ level.
 I will draw on this example at various points in the course to
illustrate aspects of critical systems engineering

Insulin pump hardware schematic

Activity model of the personal insulin pump

Concept of operation
 Using readings from an embedded sensor, the system
automatically measures the level of glucose in the sufferer’s
body
 Consecutive readings are compared and, if they indicate that
the level of glucose is rising (see next slide) then insulin is
injected to counteract this rise
 The ideal situation is a consistent level of sugar that is within
some ‘safe’ band

Sugar levels
 Unsafe
 A very low level of sugar (arbitrarily, we will call this 3 units) is
dangerous and can result in hypoglaecemia which can result in a
diabetic coma and ultimately death.
 Safe
 Between 3 units and about 7 units, the levels of sugar are ‘safe’ and
are comparable to those in people without diabetes. This is the ideal
band.
 Undesirable
 Above 7 units of insulin is undesirable but high levels are not
dangerous in the short-term. Continuous high-levels however can
result in long-term side-effects.

Insulin injection
 The decision when to apply insulin does NOT depend on the
absolute level of glucose that is measured in the sufferer’s
blood.
 The reason for this is that insulin does not act instantaneously
and the change in sugar level does not simply depend on a
single injection but also on previous injections.
 A more complex decision based on previous levels and rate of
change of sugar level is used.

Injection scenarios
 Level of sugar is in the unsafe band
 Do not inject insulin;
 Initiate warning for the sufferer.
 Level of sugar is falling
 Do not inject insulin if in safe band. Inject insulin if rate of change of level is
decreasing.
 Level of sugar is stable
 Do not inject insulin if level is in the safe band;
 Inject insulin if level is in the undesirable band to bring down glucose level;
 Amount injected should be proportionate to the degree of undesirability ie
inject more if level is 20 rather than 10.

Injection scenarios
 Level of sugar is increasing
 Reading in unsafe band
• No injection.
 Reading in safe band
• Inject only if the rate of increase is constant or increasing. If constant,
inject standard amount; if increasing, compute amount based on increase.
 Reading in unsafe band
• Inject constant amount if rate of increase is constant or decreasing.
• Inject computed amount if rate of increase is increasing.

Glucose measurements
Undesirable area
Safe area
Time
Sugar level
Unsafe area
Inject
t1 t2 t3
Inject
Do not inject
Do not inject
Do not inject

System specification
 Functional specification
 How to carry out the computation to determine if insulin should be
administered
 Dependability specification
 Requirements to ensure safe operation of the pump

Functional requirements
 If the reading is below the safe minimum, no insulin shall be
delivered.
 If the reading is within the safe zone, then insulin is only delivered if
the level of sugar is rising and the rate of increase of sugar level is
increasing.
 If the reading is above the recommended level, insulin is delivered
unless the level of blood sugar is falling and the rate of decrease of
the blood sugar level is increasing.

Formal specification
 Because of the complexity of the functional specification,
there is considerable scope for misinterpretation
 This system is an example where formal specification can be
used to define the insulin to be delivered in each case

Dependability specification
 Availability
 The pump should have a high level of availability but the nature of
diabetes is such that continuous availability is unnecessary
 Reliability
 Intermittent demands for service are made on the system
 Safety
 The key safety requirements are that the operation of the system
should never result in a very low level of blood sugar. A fail-safe
position is for no insulin to be delivered
 Security
 Not really applicable in this case

System availability
 In specifying the availability, issues that must be considered
are:
 The machine does not have to be continuously available as failure to
deliver insulin on a single occasion (say) is not a problem
 However, no insulin delivery over a few hours would have an effect on
the patient’s health
 The machine software can be reset by switching it on and off hence
recovery from software errors is possible without compromising the
usefulness of the system
 Hardware failures can only be repaired by return to the manufacturer.
This means, in practice, a loss of availability of at least 3 days

Availability
 A general specification of availability suggests that the
machine should not have to be returned to the manufacturer
more than once every year years (this repair time dominates
everything else) so
 System availability = 727/730 *100 = 0.99
 It is much harder to specify the software availability as the
demands are intermittent. In this case, you would subsume
availability under reliability

Reliability metric
 Demands on the system are intermittent (several times per
hour) and the system must be able to respond to these
demands
 In this case, the most appropriate metric is therefore
Probability of Failure on Demand
 Other metrics
 Short transactions so MTTF not appropriate
 Insufficient number of demands for ROCOF to be appropriate

System failures
 Transient failures
 can be repaired by user actions such as resetting or recalibrating the
machine. For these types of failure, a relatively low value of POFOD
(say 0.002) may be acceptable. This means that one failure may occur
in every 500 demands made on the machine. This is approximately
once every 3.5 days.
 Permanent failures
 require the machine to be repaired by the manufacturer. The
probability of this type of failure should be much lower. Roughly once a
year is the minimum figure so POFOD should be no more than
0.00002.

System hazard analysis
 Physical hazards
 Hazards that result from some physical failure of the system
 Electrical hazards
 Hazards that result from some electrical failure of the system
 Biological hazards
 Hazards that result from some system failure that interferes with
biological processes

Insulin system hazards
 insulin overdose or underdose (biological)
 power failure (electrical)
 machine interferes electrically with other medical equipment
such as a heart pacemaker (electrical)
 parts of machine break off in patient’s body(physical)
 infection caused by introduction of machine (biol.)
 allergic reaction to the materials or insulin used in the
machine (biol).

Risk analysis example

Software-related hazards
 Only insulin overdose and insulin underdose are software
related hazards
 The other hazards are related to the hardware and physical
design of the machine
 Insulin underdose and insulin overdose can be the result of
errors made by the software in computing the dose required

Software problems
 Arithmetic error
 Some arithmetic computation causes a representation failure (overflow
or underflow)
 Specification may state that arithmetic error must be detected and an
exception handler included for each arithmetic error. The action to be
taken for these errors should be defined
 Algorithmic error
 Difficult to detect anomalous situation
 May use ‘realism’ checks on the computed dose of insulin

General dependability requirements
 SR1: The system shall not deliver a single dose of insulin that is
greater than a specified maximum dose for a system user.
 SR2: The system shall not deliver a daily cumulative dose of insulin
that is greater than a specified maximum for a system user.
 SR3: The system shall include a hardware diagnostic facility that
should be executed at least 4 times per hour.
 SR4: The system shall include an exception handler for all of the
exceptions that are identified in Table 3.
 SR5: The audible alarm shall be sounded when any hardware
anomaly is discovered and a diagnostic message as defined in
Table 4 should be displayed.

Safety proofs
 Safety proofs are intended to show that the system cannot
reach in unsafe state
Weaker than correctness proofs which must show that the
system code conforms to its specification
 Generally based on proof by contradiction
 Assume that an unsafe state can be reached
 Show that this is contradicted by the program code

Insulin delivery system
 Safe state is a shutdown state where no insulin is delivered
 If hazard arises,shutting down the system will prevent an accident
 Software may be included to detect and prevent hazards such
as power failure
 Consider only hazards arising from software failure
 Arithmetic error The insulin dose is computed incorrectly because of
some failure of the computer arithmetic
 Algorithmic error The dose computation algorithm is incorrect

Arithmetic errors
 Use language exception handling mechanisms to trap errors
as they arise
 Use explicit error checks for all errors which are identified
 Avoid error-prone arithmetic operations (multiply and divide).
Replace with add and subtract
 Never use floating-point numbers
 Shut down system if exception detected (safe state)

Algorithmic errors
 Harder to detect than arithmetic errors. System should always
err on the side of safety
 Use reasonableness checks for the dose delivered based on
previous dose and rate of dose change
 Set maximum delivery level in any specified time period
 If computed dose is very high, medical intervention may be
necessary anyway because the patient may be ill

Insulin delivery code

Informal safety argument

System testing
 System testing of the software has to rely on simulators for
the sensor and the insulin delivery components.
 Test for normal operation using an operational profile. Can be
constructed using data gathered from existing diabetics
 Testing has to include situations where rate of change of
glucose is very fast and very slow
 Test for exceptions using the simulator

Safety assertions
 Predicates included in the program indicating conditions
which should hold at that point.
 May be based on pre-computed limits e.g. number of insulin
pump increments in maximum dose.
 Used in formal program inspections or may be pre-processed
into safety checks that are executed when the system is in
operation.

Safety assertions

Insulin pump overview

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