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Datei: plugin_base.dune Sprache: VDM

Original von: VDM^©

\section{AOO mode requirements}

The AOO code states

\begin{itemize}
\item[A] Pace the Atria
\item[O] Without sensing the chambers
\item[O] Without response to sensing
\end{itemize}

\noindent meaning that the pacemaker must pace the atria chamber discarding any sensed data from the chambers just regarding the programmable parameters.

\paragraph{Programmable parameters requirements}

\begin{itemize}
\item LRL\ref{sym:LRL} shall be 60 ppm
\item URL\ref{sym:URL} shall be 120 ppm
\end{itemize}

\paragraph{Purpose and abstraction level}

The purpose is to model and validate the requirements of the Pacemaker AOO mode.

\begin{itemize}
\item Atrial Amplitude, Atrial Pulse With and Sensitivity are discarded because they are not relevant for the propose and don't add any understanding of the mode of pacing.
\item In this mode the paced/sensed chamber is always the atria so the other chambers are discarded in the model.
\end{itemize}

\subsection*{VDM-SL module}

The model of the requirements start by defining the value of the LRL.

\begin{vdm_al}
module PacemakerAOO

definitions

values
LRL     : nat = 60;
\end{vdm_al}

The input is a sequence and each element of it corresponds to a time unit abstraction (in this case 1 millisecond).

\begin{vdm_al}
types
SenseTimeline = map Time to Sense;

Sense = <NONE> | <PULSE>;

Time = nat1;
\end{vdm_al}

The output will be a sequence of the reactions to the input that can be either do nothing or discharge a pulse on the Atria.

\begin{vdm_al}
ReactionTimeline = map Time to Reaction;

Reaction = <NONE> | <PULSE>;

functions
\end{vdm_al}

\subsubsection*{From LRL to ppm}

Considering that the ppm rate is given by the following formula

\begin{displaymath}
ppm = \frac{numberOfPulses}{timeInMinutes}
\end{displaymath}

\noindent and converting it to the modelled time unit: (milliseconds)

\begin{displaymath}
ppm = \frac{60000}{ms}
\end{displaymath}
We get
\begin{displaymath}
ms = \frac{60000}{ppm}
\end{displaymath}

The period of the LRL is

\begin{displaymath}
LRLperiod_{observed} = \frac{numberOfPulses}{numberOfElementsObserved}
\end{displaymath}

To compare the observed value to the LRL expressed in ppm, we need to convert it to milliseconds:

\begin{displaymath}
LRLperiod_{expected} = \frac{60000}{LRL}
\end{displaymath}

And because LRL is defined as a minimum, the pacemaker it is modelled as an implicit function stating that the ppm rate is larger or equal the LRL\cite{www:PacemakerCycles}.

\begin{displaymath}
LRLperiod_{observed} <= LRLperiod_{expected}
\end{displaymath}

\begin{vdm_al}
Pacemaker (inp : SenseTimeline) r : ReactionTimeline
post let m = {i | i in set dom r & r(i) = <PULSE>}
     in card dom r = card dom inp
        and
        card dom inp > 1 => r(1) = <PULSE>
        and
        forall x in set m & (
           (exists y in set m & y > x) =>
                 (exists y in set m & abs(x - y) <= 60000/LRL and x <> y));

end PacemakerAOO
\end{vdm_al}

\paragraph{Requirements review}

\begin{itemize}
\item LRL \textbf{Modelled}
\item URL is the maximum rate at which the paced ventricular rate will track sensed atrial events. The URL interval is the minimum time between a ventricular event and the next ventricular pace.\textbf{Not modelled}\footnote{We discovered this requirement is contradictory and its a minor error in a table from the specification.}
\end{itemize}

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