The invention relates to a halogen-free, pentane-blown, flame-retardant rigid polyurethane foam, wherein oxalkylated alkylphosphonic acids are present as flame retardants. If the local table does not contain a matching address, the server asks a routing server to identify a receiving server address. The ventricular sensing timer initiates timing of a lowest tachy rate zone interval and also a ventricular pace interval that is calculated using a lower rate limit.