<PAGE>   1
                                                                   EXHIBIT 10.46



                           SECOND AMENDMENT TO AMENDED
                          AND RESTATED CREDIT AGREEMENT

        This SECOND AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT (this
"AMENDMENT") dated as of September 26, 1997, is by and among Raytel Medical
Corporation (the "BORROWER"), Bank of Boston Connecticut and Banque Paribas
(collectively, the "BANKS") and Bank of Boston Connecticut, as agent for the
Banks (in such capacity, the "AGENT").

                                     W I T N E S S E T H:

        WHEREAS, the Borrower, the Banks and the Agent entered into a certain
Amended and Restated Credit Agreement dated as of August 14, 1996, as amended by
the First Amendment to Amended and Restated Credit Agreement dated as of June 4,
1997 (as amended from time to time, the "CREDIT AGREEMENT"); and

        WHEREAS, the Borrower has requested that the Credit Agreement be amended
in certain respects; and

        WHEREAS, the Agent and the Banks have agreed to amend the Credit
Agreement on the terms and conditions set forth herein.

        NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto agree as
follows:

        SECTION 1. DEFINITIONS. Capitalized terms used herein without definition
that are defined in the Credit Agreement shall have the same meanings herein as
therein.

        SECTION 2. REPRESENTATIONS AND WARRANTIES. The Borrower hereby repeats
on and as of the date hereof the representations and warranties made by it in
the Credit Agreement, provided that all references therein to the Credit
Agreement shall refer to the Credit Agreement as amended hereby.

        SECTION 3. RATIFICATION, ETC. Except as expressly amended hereby, the
Credit Agreement and all documents, instruments and agreements related thereto
are hereby ratified and confirmed in all respects and shall continue in full
force and effect. This Amendment and the Credit Agreement shall hereafter be
read and construed together as a single document, and all references in the
Credit Agreement or any related agreement or instrument to the Credit Agreement
shall hereafter refer to the Credit Agreement as amended by this Amendment.



<PAGE>   2
                                      -2-



        SECTION 4.1. AMENDMENT TO CREDIT AGREEMENT.


                     SECTION 4.1. AMENDMENT TO SECTION 8.3. Section 8.13 of the
        Credit Agreement is hereby amended by deleting it in its entirety and
        substituting therefor the following:

                     "SECTION 8.13 USE OF PROCEEDS. The Borrower will use the
        proceeds of the Loans (i) to finance the Acquisitions, (ii) to refinance
        on the Effective Date the Existing Indebtedness, (iii) to refinance
        certain indebtedness incurred in connection with the Borrower's
        acquisition of Cardiovascular Ventures, Inc. that has been previously
        disclosed to the Banks in writing and (iv) for working capital and
        general corporate purposes; provided, that the Borrower will not, and
        will not permit any of its Subsidiaries to, use the proceeds of any of
        the Loans to finance or pay for any Indebtedness or obligations arising
        in connection with the operations or business of any of the Joint
        Ventures, including, without limitation, any Indebtedness incurred by
        the Borrower or any of its Subsidiaries on behalf of any Joint Venture
        in its capacity as a general partner or joint ventures of such Joint
        Venture or otherwise or any Indebtedness incurred under any management
        or operating contract between the Borrower or any of its Subsidiaries
        and any of the Joint Ventures, and, provided, further, that nothing set
        forth in this ss.8.13 shall be deemed to restrict the ability of the
        Borrower and its Subsidiaries to pay Indebtedness permitted by the terms
        of ss.9. 1(h) hereof."

                     SECTION 4.2. AMENDMENT TO SCHEDULE 1. Schedule 1 of the
        Credit Agreement is hereby amended by deleting it in its entirety and
        substituting therefor a copy of Schedule 1 attached hereto.

                     SECTION 4.3. AMENDMENT TO SCHEDULE 2. The definitions of
        "Revolving Credit Termination Date" and "Maturity Date" appearing in
        Schedule 2 of the Credit Agreement are hereby amended by deleting each
        of them in their entirety and substituting therefor, respectively, the
        following:

                     "Revolving Credit Termination Date. August 1, 1999."

                     "Maturity Date. August 1, 2004."

                     SECTION 4.4. AMENDMENT TO SCHEDULE 7.2. Schedule 7.2 of the
        Credit Agreement is hereby amended by deleting items 1, 5, 6 and 7
        appearing therein.

                     SECTION 4.5. AMENDMENT TO SCHEDULE 9.1. Schedule 9.1 of the
        Credit Agreement is hereby amended by adding thereto the information
        contained on Schedule 9.1 hereto.

<PAGE>   3
                                      -3-



                     SECTION 4.6. AMENDMENT TO EXHIBIT A. Exhibit A to the
        Credit Agreement is hereby deleted in its entirety and Exhibit A
        attached hereto is substituted therefor.

                     SECTION 4.7. AMENDMENT TO EXHIBIT F. Exhibit F of the
        Credit Agreement is hereby amended by deleting it in its entirety and
        substituting therefor a copy of Exhibit F attached hereto.

                     SECTION 4.8. AMENDMENT TO EXHIBIT G. Exhibit G of the
        Credit Agreement is hereby amended by deleting it in its entirety and
        substituting therefor a copy of Exhibit G attached hereto.

        SECTION 5. CONDITIONS TO EFFECTIVENESS. The effectiveness of this
Amendment is subject to the prior satisfaction, on or before October 15, 1997,
of the following conditions precedent (the date of such satisfaction herein
referred to as the "AMENDMENT EFFECTIVE Date"):

               (a) Execution and Delivery of New Notes. The Borrower shall have
        duly executed and delivered to each of the Banks amended and restated
        Notes in the aggregate principal amount of each such Bank's respective
        Commitment, which Notes shall be in form and substance satisfactory to
        the Banks.

               (b) Payment of Amendment Fee. The Borrower shall have paid to the
        Agent for the account of the Banks a non-refundable amendment fee of
        $100,000.

               (c) Compliance with Post-Funding Letter. The Borrower shall have
        satisfied all of the conditions of that certain post-funding letter
        agreement dated August 15, 1997 among the Borrower and the Agent, the
        form of which is attached hereto as Exhibit 5(c).

               (d) Representations and Warranties. The representations and
        warranties contained in paragraph 7 of the Credit Agreement shall have
        been correct at and as of the date made. Such representations and
        warranties shall also be correct at and as of the date thereof, except
        to the extent that such representations and warranties expressly related
        to a specific date or were based on facts which have changes in the
        ordinary course of business, which changes, either singly or in the
        aggregate, are not materially adverse.

               (e) No Event of Default. There shall exist no Event of Default or
        condition which, with either or both the giving of notice of the lapse
        of time, would result in an Event of Default upon the execution and
        delivery of this Amendment.

               (f) Corporate Action. The Banks and the Agent shall have received
        evidence reasonably satisfactory to the Bank and the Agent that all
        requisite corporate action necessary for the valid execution, delivery
        and performance by the Borrower and Guarantors of this Amendment and all
        other instruments and documents delivered by the Borrower and Guarantors
        in connection herewith.


<PAGE>   4
                                      -4-



               (g) Delivery of Amendment; Opinions. The parties hereto shall
        have executed and delivered this Amendment and Borrower shall have
        delivered to the Banks and the Agent such opinions of counsel as the
        Banks or the Agent may request, such opinions to be in form and
        substance satisfactory to the Banks and the Agent.

               (h) UCC Search Results. The Borrower shall have delivered to the
        Agent and the Banks results of Uniform Commercial Code searches with
        respect to the Collateral, indicating no liens other than Permitted
        Liens and otherwise in form and substance satisfactory to the Agent and
        the Banks.

               (i) No Liens. The Borrower shall have delivered to the Agent and
        the Banks evidence reasonably satisfactory to the Agent and the Banks
        that all liens and encumbrances with respect to the property of the
        Borrower and its Subsidiaries, other than Permitted Liens, shall have
        been discharged in full.

               (j) Charter Documents, etc. The Borrower and each Subsidiary
        shall have delivered to the Agent and the Banks a certificate of a duly
        authorized officer of such Person attesting that its charter or other
        incorporation or partnership documents and/or bylaws have not been
        amended, modified or supplemented since the Effective Date.

               (k) Reaffirmation of Security Documents. Each Guarantor shall
        have executed and delivered to the Agent and the Banks a reaffirmation
        letter reaffirming their respective obligations under their Security
        Documents.

        SECTION 6. EFFECTIVE DATE This Amendment shall become effective among
the parties hereto as of the Amendment Effective Date. Until the Amendment
Effective Date, the terms of the Credit Agreement prior to its amendment hereby
shall remain in full force and effect.

        SECTION 7. COUNTERPARTS. This Amendment may be executed in any number of
counterparts and by different parties hereto on separate counterparts, each of
which when so executed and delivered shall be an original, but all of which
counterparts taken together shall be deemed to constitute one and the same
instrument.

        IN WITNESS WHEREOF, the parties have caused this Amendment to be
executed by their duly authorized officers, as of the day and year first above
written.


                                            RAYTEL MEDICAL CORPORATION


                                            By: /s/ E. PAYSON SMITH, JR.
                                               ---------------------------------
                                                    E. Payson Smith, Jr.


                                            Its: Chief Financial Officer
                                               ---------------------------------


                                            BANQUE PARIBAS




<PAGE>   5
                                      -5-



By:  /s/ RAYMOND T. BAXTER                  By:  /s/ SEAN T. CONLON
   -------------------------------             ---------------------------------

Its:  Group Vice President                  Its:  Director
    ------------------------------              --------------------------------



                                            BANK OF BOSTON CONNECTICUT,
                                            individually and as Agent


                                            By:  /s/  CHRIS PHELAN
                                               ---------------------------------

                                            Its:  Vice President
                                                --------------------------------



Each of the undersigned Guarantors acknowledges and accepts the foregoing and
ratifies and confirms in all respects such Guarantor's obligations under the
Guarantees:

RAYTEL CARDIAC SERVICES, INC.


By:  /s/ E. PAYSON SMITH, JR.
   ----------------------------------
        E. Payson Smith
        Its Chief Financial Officer



<PAGE>   6
                                      -6-



RAYTEL MEDICAL IMAGING, INC.

By: /s/ E. PAYSON SMITH, JR.
   -----------------------------------
        E. Payson Smith
        Its Chief Financial Officer

MEDICAL IMAGING PARTNERS L.P.
        By: RAYTEL MEDICAL IMAGING, INC.,
            Its General Partner


        By: /s/ E. PAYSON SMITH, JR.
           -----------------------------------
              E. Payson Smith
              Its Chief Financial Officer

RAYTEL IMAGING HOLDINGS, INC.


By: /s/ E. PAYSON SMITH, JR.
   -----------------------------------
        E. Payson Smith
        Its Chief Financial Officer


RAYTEL CARDIOVASCULAR LABS, INC.


By: /s/ E. PAYSON SMITH, JR.
   -----------------------------------
        E. Payson Smith
        Its Chief Financial Officer


RAYTEL IMAGING NETWORK, INC.


By: /s/ E. PAYSON SMITH, JR.
   -----------------------------------
        E. Payson Smith
        Its Chief Financial Officer


RAYTEL IMAGING WEST INC.


By: /s/ E. PAYSON SMITH, JR.
   -----------------------------------
        E. Payson Smith
        Its Chief Financial Officer




<PAGE>   7
                                      -7-



RAYTEL IMAGING EAST INC.


By:  /s/ E. Payson Smith, Jr.
   -------------------------------------------
        E. Payson Smith
        Its Chief Financial Officer


RAYTEL IMAGING MID-ATLANTIC INC.


By:  /s/ E. Payson Smith, Jr.
   -------------------------------------------
        E. Payson Smith
        Its Chief Financial Officer


RAYTEL GRANADA HILLS INC.


By:  /s/ E. Payson Smith, Jr.
   -------------------------------------------
        E. Payson Smith
        Its Chief Financial Officer


<PAGE>   8



                                   SCHEDULE 1



<TABLE>
<CAPTION>
                                  COMMITMENT              COMMITMENT
BANK                                AMOUNT                PERCENTAGES
----                              ----------              -----------
<S>                               <C>                         <C>
Bank of Boston Connecticut        $27,000,000                 60%

Banque Paribas                    $18,000,000                 40%
</TABLE>




<PAGE>   9

                                    EXHIBIT A
                                 [Form of Note]





<PAGE>   10

                                    EXHIBIT F

                               RII JOINT VENTURES

                                      None.



<PAGE>   11

                                    EXHIBIT G

                               RMI JOINT VENTURES

                                      None.