$$ \newcommand \Fee {\mathrm{fee}} \newcommand \MinTxnFee {T_{\Fee,\min}} \newcommand \RekeyTo {\mathrm{RekeyTo}} \newcommand \Heartbeat {\mathrm{hb}} \newcommand \PayoutsChallengeBits {\Heartbeat_\mathrm{bits}} $$

Heartbeat Transaction Semantics

If a heartbeat transaction’s group is empty, and the fee is less than \( \MinTxnFee \), the transaction FAILS to execute unless:

• The note \( N \) is empty;

• The lease \( x \) is empty;

• The rekey to address \( \RekeyTo \) is empty;

• The heartbeat_address, \( a \), is Online;

• The heartbeat_address, \( a \), eligibility flag (ie) is True;

• The heartbeat_address, \( a \), is at risk of suspension.

An account is at risk of suspension if the current round (\( r \)) is

$$ 100\mod1000 \leq r \leq 200\mod1000, $$

and the block seed of the most recent round that is \( 0 \mod 1000 \) matches \( a \) in the first \( \PayoutsChallengeBits \) bits.

If successful, the LastHeartbeat of the specified heartbeat address \( a \) is updated to the current round.