<PAGE>
 
                      FIRST AMENDMENT TO CREDIT AGREEMENT
                      ------------------------------------



          FIRST AMENDMENT TO CREDIT AGREEMENT (this "Amendment"), dated as of
August 12, 1997, among Three Rivers Holding Corp. ("Holdings"), a Delaware
corporation, Three Rivers Acquisition Corp. ("Acquisition Corp."), a Delaware
corporation, SMT Health Services Inc. ("SMT"), a Delaware corporation,
(Acquisition Corp. and SMT each a "Borrower" and, collectively, "the
Borrowers"), the lender party to the Credit Agreement referred to below on the
date hereof and immediately before giving effect to this Amendment (the
"Existing Bank"), BANKERS TRUST COMPANY, as Agent (the "Agent") for the Banks
and each of the banks listed on Schedule A hereto (each, a "New Bank" and,
collectively, the "New Banks").  All capitalized terms used herein and not
otherwise defined herein shall have the respective meanings provided such terms
in the Credit Agreement referred to below.



                             W I T N E S S E T H :
                             - - - - - - - - - -  


          WHEREAS, Holdings, the Borrowers, the Existing Bank, and the Agent are
parties to a Credit Agreement, dated as of August 5, 1997 (the "Credit
Agreement");


          WHEREAS, the parties hereto wish to amend the Credit Agreement as
herein provided;


          NOW, THEREFORE, it is agreed:


          1.   The definition of "Scheduled Commitment Reduction Date" appearing
in Section 3.03(c) is hereby amended by (i) deleting the text "August 31, 2002"
appearing therein and inserting in lieu thereof the text "August 31, 2001" and
(ii) deleting the text "August 31, 2003" appearing therein and inserting in lieu
thereof the text "August 31, 2002".


          2.   The definitions of "Applicable RL Commitment Fee Percentage",
"Applicable Excess Cash Flow Percentage" and "Applicable Prepayment Percentage"
appearing in Section 11 are hereby amended by inserting the phrase "with respect
to Revolving Loans" immediately after the phrase "Interest Reduction Discount
then in effect" appearing therein.
<PAGE>
 
          3.  The definition of "Interest Reduction Discount" appearing in
Section 11 is hereby amended by:


          (i)  inserting the phrase "(1) for all purposes of determining
          interest with respect to Revolving Loans and Swingline Loans,"
          immediately prior to the phrase "(x) zero or (y) the respective
          percentage per annum set forth in clause (A), (B), (C), (D) or (E)
          below if," appearing therein;


          (ii)  deleting the period at the end of the first sentence thereof and
          inserting a semicolon in lieu thereof;


          (iii)   inserting the following text after clause (E) of, and as part
          of, the first sentence thereof:


               "or (2) for all purposes of determining interest with respect to
               Term Loans, (x) zero or (y) the respective percentage per annum
               set forth in clause (A) or (B) below, if, but only if, as of the
               Test Date most recently ended prior to such Start Date the
               conditions in clause (A) or (B) below are met:


                    (A) 1/4 of 1% if, but only if, as of the Test Date for such
                    Start Date, the Leverage Ratio for the Test Period ended on
                    such Test Date shall be less than 4.00:1.00 and the
                    conditions set forth in clause (B) below are not satisfied;
                    or


                    (B) 1/2 of 1% if, but only if, as of the Test Date for such
                    Start Date, the Leverage Ratio for the Test Period ended on
                    such Test Date shall be less than 3.00:1.00.";


          (iv)  deleting the word "clause" appearing in the final sentence of
          such definition and inserting in lieu thereof the following text: "(x)
          in the case of Revolving Loans and Swingline Loans, clause (1)"; and


          (v)  inserting the following text immediately at the end of, and as
          part of, the last sentence of such definition: "or (y) in the case of
          Term Loans, clause (2)(A) or (B), if any, contained in this definition
          shall be applicable."


          4.  The Existing Bank hereby sells and assigns to each of the New
Banks without recourse and without representation or warranty (other than as
expressly provided herein), and each New Bank hereby purchases and assumes from
the Existing Bank, that interest in and to each of the Existing Bank's rights
and obligations under the Credit Agreement as of the date hereof which in the
aggregate represents such New Bank's pro




                                      -2-
<PAGE>
 
rata share (for each such New Bank, its "Pro Rata Share") as set forth on, and
in respect of the credit facilities listed on, Schedule B hereto (calculated
after giving effect to this Amendment), and such Pro Rata Share represents all
of the outstanding rights and obligations under the Credit Agreement that are
being sold and assigned to each New Bank, including, without limitation, (x) in
the case of any assignment of all or any portion of the Total Term Loan
Commitment, all rights and obligations with respect to such New Bank's Pro Rata
Share of such Total Term Loan Commitment, including all such rights and
obligations with respect to such New Bank's Pro Rata Share of outstanding Term
Loans, and (y) in the case of any assignment of all or any portion of the Total
Revolving Loan Commitment, all rights and obligations with respect to such New
Bank's Pro Rata Share of such Total Revolving Loan Commitment, including all
such rights and obligations with respect to such New Bank's Pro Rata Share of
outstanding Revolving Loans, Swingline Loans and Letters of Credit.  After
giving effect to this Amendment, the Existing Bank's and New Banks' Term Loan
Commitment, outstanding Term Loans and Revolving Loan Commitment will be as set
forth on Schedule C hereto.


          5.  In accordance with the requirements of Section 13.04(b) of the
Credit Agreement, on the First Amendment Effective Date (as defined below), (i)
the Credit Agreement shall be amended by deleting Schedule I thereto in its
entirety and by inserting in lieu thereof a new Schedule I in the form of
Schedule C hereto and (ii) each Borrower agrees that it will issue an
appropriate Term Note and/or Revolving Note, as the case may be, to each Bank in
conformity with the requirements of Section 1.05 of the Credit Agreement.


          6.  On and after the First Amendment Effective Date, Schedule II to
the Credit Agreement shall be amended by deleting Schedule II in its entirety
and inserting in lieu thereof a new Schedule II in the form of Schedule D
hereto.


          7.  The Existing Bank (i) represents and warrants that it is the legal
and beneficial owner of the interest being assigned by it hereunder and that
such interest is free and clear of any adverse claim; (ii) makes no
representation or warranty and assumes no responsibility with respect to any
statements, warranties or representations made in or in connection with the
Credit Agreement or the other Credit Documents or the execution, legality,
validity, enforceability, genuineness, sufficiency or value of the Credit
Agreement or the other Credit Documents or any other instrument or document
furnished pursuant thereto; and (iii) makes no representation or warranty and
assumes no responsibility with respect to the financial condition of Holdings,
the Borrowers or any of their respective Subsidiaries or the performance or
observance by Holdings, the Borrowers or any of their respective Subsidiaries of
any of their respective obligations under the Credit Agreement or the other
Credit Documents to which they are a party or any other instrument or document
furnished pursuant thereto.





                                      -3-
<PAGE>
 
          8.  Each New Bank (i) confirms that it has received a copy of the
Credit Agreement and the other Credit Documents, together with copies of the
financial statements referred to therein and such other documents and
information as it has deemed appropriate to make its own credit analysis and
decision to enter into this Amendment; (ii) agrees that it will, independently
and without reliance upon the Agent or any other Bank and based on such
documents and information as it shall deem appropriate at the time, continue to
make its own credit decisions in taking or not taking action under the Credit
Agreement; (iii) confirms that it is an Eligible Transferee under Section
13.04(b) of the Credit Agreement; (iv) appoints and authorizes the Agent to take
such action as agent on its behalf and to exercise such powers under the Credit
Agreement and the other Credit Documents as are delegated to the Agent by the
terms thereof, together with such powers as are reasonably incidental thereto;
(v) agrees that it will perform in accordance with their terms all of the
obligations which by the terms of the Credit Agreement are required to be
performed by it as a Bank; and (vi) to the extent relevant, agrees to promptly
submit to the Borrowers and the Agent the appropriate Internal Revenue Service
Forms described in Section 13.04(b) of the Credit Agreement.


          9.  The Existing Bank, the New Banks and the Agent hereby agree that
all amounts accrued with respect to the Commitments, the Loans or the Letters of
Credit prior to the delivery by such New Bank of the amount referred to in
clause (ii) of Section 15 of this Amendment, shall be for the account of the
Existing Bank and that all such amounts accrued after the delivery of such
amounts referred to in clause (ii) of such Section 15 shall be for the account
of such New Bank based upon its Pro Rata Share.


          10.  In accordance with Section 13.04(b) of the Credit Agreement, on
and as of the date upon which each New Bank delivers the amount referred to in
clause (ii) of Section 15 of this Amendment, such New Bank shall become a "Bank"
under, and for all purposes of, the Credit Agreement and the other Credit
Documents and, notwithstanding anything to the contrary in Section 13.17 of the
Credit Agreement, the Agent shall record the transfers contemplated hereby in
the Register.


          11.  This Amendment is limited as specified and shall not constitute a
modification, acceptance or waiver of any other provision of the Credit
Agreement or any other Credit Document.


          12.  This Amendment may be executed in any number of counterparts and
by the different parties hereto on separate counterparts, each of which
counterparts when executed and delivered shall be an original, but all of which
shall together constitute one and the same instrument.  A complete set of
counterparts shall be lodged with the Borrower and the Agent.





                                      -4-
<PAGE>
 
          13.  THIS AMENDMENT AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES
HEREUNDER SHALL BE CONSTRUED IN ACCORDANCE WITH AND GOVERNED BY THE LAW OF THE
STATE OF NEW YORK.


          14.  Subject to Section 15 of this Amendment, this Amendment shall
become effective on the date (the "First Amendment Effective Date") when (i)
Holdings, each Borrower, the Agent, the Existing Bank and each New Bank shall
have signed a counterpart hereof (whether the same or different counterparts)
and shall have delivered (including by way of facsimile transmission) the same
to the Agent at its Notice Office and (ii) each New Bank shall have delivered to
the Agent for the account of the Existing Bank, an amount equal to such New
Bank's Pro Rata Share of the principal amount of the outstanding Loans.


          15.  Notwithstanding Section 14 of this Amendment, if for any reason
any New Bank shall not have (i) signed a counterpart hereof and delivered the
same to the Agent at its Notice Office and (ii) delivered to the Agent an amount
equal to such New Bank's Pro Rata Share of the principal amount of the
outstanding Loans, in each case on or prior to August 12, 1997, then, if the
Existing Bank agrees, this Amendment shall become effective notwithstanding such
failure, provided that (x) Schedules A, B, C and D shall be modified to delete
         --------                                                             
any such New Bank and such New Bank's Pro Rata Share shall be retained by the
Existing Bank and (y) the signature pages of this Amendment shall be deemed
revised to delete such New Bank's name therefrom.


          16.  From and after the First Amendment Effective Date, all references
in the Credit Agreement and each of the other Credit Documents to the Credit
Agreement shall be deemed to be references to the Credit Agreement as amended
hereby.


                                *      *      *





                                      -5-
<PAGE>
 
          IN WITNESS WHEREOF, each of the parties hereto has caused a
counterpart of this Amendment to be duly executed and delivered as of the date
first above written.


                             THREE RIVERS HOLDING CORP.



                             By_________________________________________________
                               Name:
                               Title:



                             THREE RIVERS ACQUISITION CORP.



                             By_________________________________________________
                               Name:
                               Title:



                             SMT HEALTH SERVICES INC.



                             By_________________________________________________
                               Name:
                               Title:



                             BANKERS TRUST COMPANY,
                               Individually and as Agent



                             By_________________________________________________
                               Name:
                               Title:
<PAGE>
 
                             SALOMON BROTHERS HOLDING COMPANY INC



                             By_________________________________________________
                               Name:
                               Title:



                             LASALLE NATIONAL BANK



                             By_________________________________________________
                               Name:
                               Title:



                             PILGRIM AMERICA PRIME RATE TRUST



                             By_________________________________________________
                               Name:
                               Title:



                             PRIME INCOME TRUST



                             By_________________________________________________
                               Name:
                               Title:



                             VAN KAMPEN AMERICAN CAPITAL
                             PRIME RATE INCOME TRUST



                             By_________________________________________________
                               Name:
                               Title:
<PAGE>
 
                             THE ING HIGH INCOME PRINCIPAL
                             PRESERVATION OFFERING, L.P.



                             By_________________________________________________
                               Name:
                               Title:
<PAGE>
 
                                                                      SCHEDULE A
                                                                              to
                                                                 First Amendment
                                                                 ---------------


                                   NEW BANKS
                                   ---------



SALOMON BROTHERS HOLDING COMPANY INC


LASALLE NATIONAL BANK


PILGRIM AMERICA PRIME RATE TRUST


PRIME INCOME TRUST


VAN KAMPEN AMERICAN CAPITAL
 PRIME RATE INCOME TRUST


THE ING HIGH INCOME PRINCIPAL
 PRESERVATION OFFERING, L.P.
<PAGE>
 
                                                                      SCHEDULE B
                                                                              to
                                                                 First Amendment
                                                                 ---------------



                                PRO RATA SHARE
                                --------------



<TABLE>
<CAPTION>
================================================================================
Bank                        Total Term Loan     Total Revolving Loan
                            Commitment and      Commitment and
                            Outstanding Term    Outstanding Revolving
                            Loans               Loans
<S>                        <C>                  <C>
--------------------------------------------------------------------------------
Bankers Trust Company              24%                 33.3333%
--------------------------------------------------------------------------------
Salomon Brothers                    0%                 33.3333%
Holding Company Inc
--------------------------------------------------------------------------------
LaSalle National Bank               0%                 33.3333%
--------------------------------------------------------------------------------
Pilgrim America Prime              20%                       0%
Rate Trust
--------------------------------------------------------------------------------
Prime Income Trust                 20%                       0%
--------------------------------------------------------------------------------
Van Kampen American                16%                       0%
Capital Prime Rate
Income Trust
--------------------------------------------------------------------------------
The ING High Income                20%                       0%
Principal Preservation
Offering, L.P.
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
TOTAL                             100%                     100%
================================================================================
</TABLE>
<PAGE>
 
                                                                      SCHEDULE C
                                                                              to
                                                                 First Amendment
                                                                 ---------------



                                  COMMITMENTS
                                  -----------



<TABLE>
<CAPTION>
================================================================================
Bank                 Term Loan       Outstanding       Revolving Loan
                     Commitment      Term Loans        Commitment
<S>                  <C>             <C>               <C>
--------------------------------------------------------------------------------
Bankers Trust        $ 2,633,852.40  $9,366,147.60     $10,000,000
 Company
--------------------------------------------------------------------------------
Salomon Brothers                  0              0     $10,000,000
 Holding Company
 Inc
--------------------------------------------------------------------------------
LaSalle National                  0              0     $10,000,000
 Bank
--------------------------------------------------------------------------------
Pilgrim America      $ 2,194,877.00  $   7,805,123               0
 Prime Rate Trust
--------------------------------------------------------------------------------
Prime Income         $ 2,194,877.00  $   7,805,123               0
 Trust
--------------------------------------------------------------------------------
Van Kampen           $ 1,755,901.60  $6,244,098.40               0
 American Capital
 Prime Rate
 Income Trust
--------------------------------------------------------------------------------
The ING High         $ 2,194,877.00  $   7,805,123               0
 Income Principal
 Preservation
 Offering, L.P.
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
Total                $10,974,385.00  $  39,025,615     $30,000,000
================================================================================
</TABLE>
<PAGE>
 
                                                                      SCHEDULE D
                                                                              to
                                                                 First Amendment
                                                                 ---------------



Bankers Trust Company
130 Liberty Street
New York, NY  10006


     Contact:  Gregory Shefrin
     Tel:  212-250-1724
     Fax:  212-250-7218



Salomon Brothers Holding Company Inc
Seven World Trade Center
32nd Floor
New York, NY  10048


     Contact:  Townsend U. Weekes, Jr.
     Tel:  212-783-7628
     Fax:  212-783-2823



LaSalle National Bank
135 South LaSalle Street
Suite 242
Chicago, IL 60674

 

     Contact:  Ward Nixon
     Tel:  312-904-8339
     Fax:  312-904-4779
<PAGE>
 
Pilgrim America Prime Rate Trust
Two Renaissance Square
40 North Central Avenue
Suite 1200
Phoenix, AZ 85004


     Contact:  Howard Tiffen
     Tel:  602-417-8259
     Fax:  602-417-8327



Prime Income Trust
c/o Dean Witter Intercapital, Inc.
72nd Floor
Two World Trade Center
New York, NY  10048


     Contact:  Louis Pistecchia
     Tel:  212-392-5845
     Fax:  212-392-5345



Van Kampen American Capital Prime
Rate Income Trust
One Parkview Plaza
Oakbrook Terrace, IL  60181


     Contact:  Jeffrey Maillet
     Tel:  630-684-6438
     Fax:  630-684-6740



The ING High Income Principal
Preservation Offering, L.P.
333 S. Grand Avenue, Suite 4250
Los Angeles, CA  90071


     Contact:  Kathleen Lenarcic
     Tel:  213-346-3971
     Fax:  213-346-3995




                                      -2-