Tax-exempt healthcare bond volume climbed 19.3% in the first six months of 1991 compared with the same period last year. Hospitals sold 227 separate issues worth a total of $6.5 billion during the period, compared with 195 issues worth $5.4 billion in 1990. The increase was accelerated in part by anticipation of an Internal Revenue Service rule change expected to take effect in September. The change in "reimbursement bond" guidelines could restrict some hospital financings.