Ensuring Patient Safety in Wireless Medical Device

Networks

Presented by:

Eric Flickner

Chris Hoffman

Speed vs. Safety

WMDNs provide many alarms and related clinical data that are life-critical. To avoid exposing patients to serious injuries or death, these systems must be protected from data delays, distortions, loss, or other erratic delivery problems.

WDN (Wireless Device Network)

Based upon existing popular IEEE 802.1x technologies Wi-Fi (IEEE 802.11a/b/g) Wi-Max (IEEE 802.11n) Bluetooth (IEEE 802.15.1) Zigbee (IEEE 802.15.4)

Each of these has their own pros/cons in Speed, interoperability, security, coexistence,

battery life, and building/object penetration

Business Networks

Simple CSM (Collision Sense Method)Random delay intervals to resequence data

ProblemsUnpredictable CSM delay lengthRandomization of message transfers

Both are tolerable in this kind of network

Medical Networks

Unpredictable CSM delay length Ex: delay can exceed max delay allowed in

arrhythmia monitoring applications Causes corruption of real-time patient waveforms

leads to misdiagnosis, interfering with therapeutic interventions

Randomization of message transfers Invalidates intelligent alarm monitoring (IEC/ISO

60601-1-8)

Problems during WMDN Life Cycle

Delayed or lost WMDN data is the major problem

Any change or interference can seriously affect other WMDN during its life cycle

Nonproprietary WMDN verification and validation (V2) techniques do not exist

Problems during WMDN Life Cycle Absence of industry standards or regulations Unconstrained mobility of patients and devices Rapid changes in the underlying wireless network

modalities No single proprietary V2 strategy can assure safe

and reliable WMDN systems Solution: Propose developing a V2 toolkit for use by

clinical and biomedical engineering departments to ensure safe and reliable WMDN operation.

Formal Methods

Definition A notation or technique, based on some

mathematical theory, for modeling and analyzing systems.

Advantages Making sure that it behaves according to

specifications Helps developers identify potential problems or

misunderstandings

Petri Nets

A petri net (a.k.a. place/transition net) is one of several mathematical representations of discrete distributed systems.

Graphically depicts the structure of a distributed system as a directed bipartite graph

Petri Nets

States Ready to accept $$ (Ready) $$ accepted (Accepted)

Events Insert coin (Coin) Soda dispense button (Soda) Gum dispense button (Gum)

Requirements Gum costs 1 coin Soda costs 2 coins

Current state indicates Ready

Healthcare Scenario

For example, suppose a heart alarm goes off while a large image file is being transmitted over the same wireless network.

How will this affect the network’s behavior?

Will the alarm signal reach the station in time?

A formal modeling and analysis technique can answer these questions.

Sample Patient Monitoring System

Sample Patient Monitoring System

10 patients with heart monitors and pulse oximeters

Heart monitors can generate a low battery alarm

2 nurses at nurse’s station

Connected via wireless network

Colored Petri Net (CPN)

CPNs trace and control the path and timing of each token (alarm) in the net

CPN ML is a the programming language used to edit, model, simulate, and analyze CPNs

Colored Petri Net (CPN) Model

Red – infrequent heart alarms

Orange – frequent pulse oximetry alarms

Yellow – very infrequent heart monitor battery alarm

Colored Petri Net (CPN) Model

Colored Petri Net (CPN) Model

Pulse oximetry alarms began to queue up, exposing a bottleneck in network

CPN allows priority to be given to individual tokens in a IEEE802.11e-style QoS technique

Critical heart alarm and battery alarms given priority over pulse-oximetry alarms

Conclusions

QoS compliant network equipment necessary for life-critical applications

CPN Tools predict and avoid life-threatening data delays, insufficient bandwidth, and inadequate priority management

Model does not address RF interference