6. The apparatus of claim 1 , comprising a first comparator configured to compare an impedance determined by the processor circuit against a specified range; and in response, wherein the processor circuit is configured to declare an alert if the impedance falls outside the specified range.
6. The apparatus of claim 1 , comprising a first comparator configured to compare an impedance determined by the processor circuit against a specified range; and in response, wherein the processor circuit is configured to declare an alert if the impedance falls outside the specified range. 8. The apparatus of claim 6 , wherein the specified range includes an upper threshold and a lower threshold; and wherein one or more of the upper or lower thresholds are adjusted using an adjusted threshold specified relative to a central tendency of previously-determined impedances.