Trigger Works But Hangs Application

I have a trigger on a table which when I do a direct update to throught QA seems to work fine. When a user developed application updates the record it locks their screen and record. I can't see where it could be causing it, but I am pasting it below. Any thoughts/comments greatly appreciated

CREATE TRIGGER tr_Patient_Updt2
ON patient_
FOR UPDATE
AS
if update(df_last_cholestero)
begin
declare @tstyears int
declare @tdate datetime
declare @sdate datetime
declare @bdate datetime
declare @ptage int
declare @actid varchar(12)

select @bdate = convert(datetime,df_last_cholestero,101), @actid = account_id
from inserted
select @ptage = dbo.GetAge(convert(datetime,patient.Date_Of_Birth, 101), GETDATE())
from patient where patient.account_id = @actid
select @sdate = convert(datetime,patient.Date_Of_Birth,101)
from patient where patient.account_id = @actid
select @tstyears = datediff(month,@bdate,getdate())

if @ptage between 18 and 65
begin transaction
begin

if @tstyears < 12
begin
set @ptage = @ptage + 5
set @sdate = DATEADD(year, @ptage, @sdate)
update patient_
set df_next_cholestero = convert(char(08),@sdate,112)
where patient_.account_id = @actid
end

else if @tstyears >= 12 and @tstyears < 24
begin
set @ptage = @ptage + 4
set @sdate = DATEADD(year, @ptage, @sdate)
update patient_
set df_next_cholestero = convert(char(08),@sdate,112)
where patient_.account_id = @actid
end

else if @tstyears >= 24 and @tstyears < 36
begin
set @ptage = @ptage + 3
set @sdate = DATEADD(year, @ptage, @sdate)
update patient_
set df_next_cholestero = convert(char(08),@sdate,112)
where patient_.account_id = @actid
end

else if @tstyears >= 36 and @tstyears < 48
begin
set @ptage = @ptage + 2
set @sdate = DATEADD(year, @ptage, @sdate)
update patient_
set df_next_cholestero = convert(char(08),@sdate,112)
where patient_.account_id = @actid
end

if @tstyears > 48
begin
set @ptage = @ptage + 1
set @sdate = DATEADD(year, @ptage, @sdate)
update patient_
set df_next_cholestero = convert(char(08),@sdate,112)
where patient_.account_id = @actid
end



end
end
commit transaction