<PAGE>   1
                           THIRD AMENDMENT TO AMENDED
                          AND RESTATED CREDIT AGREEMENT

        This THIRD AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT (this
"AMENDMENT") dated as of July 24, 1998, is by and among Raytel Medical
Corporation (the "BORROWER"), BankBoston, N.A., formerly known as Bank of Boston
Connecticut ("BANKBOSTON") and Banque Paribas (collectively, the "BANKS") and
BankBoston, 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 and by the Second Amendment to Amended and Restated Credit Agreement dated
as of September 26, 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.

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                                      -2-



        SECTION 4.  AMENDMENTS TO CREDIT AGREEMENT.

                    SECTION 4.1. AMENDMENT TO SECTION 10.3. Section 10.3 of the
Credit Agreement is hereby amended in its entirety to read as follows:

                    "SECTION 10.3 CONSOLIDATED TOTAL FUNDED DEBT TO EBITDA. The
Borrower will not permit the ratio of Consolidated Total Funded Debt of the
Borrower and its Subsidiaries to Earnings Before Interest and Taxes,
Depreciation and Amortization of the Borrower and its Subsidiaries for any
period consisting of four (4) consecutive fiscal quarters of the Borrower ending
after the Effective Date to exceed 2.5:1.0."

                    SECTION 4.2. AMENDMENTS TO SCHEDULE 2.

                    (a) The definition of "Consolidated Total Funded Debt"
appearing in Schedule 2 of the Credit Agreement is hereby amended in its
entirety to read as follows:

                         "Consolidated Total Funded Debt. As to any Person and
        whether recourse is secured by or is otherwise available against all or
        only a portion of the assets of such Person and whether or not
        contingent, but without duplication:

                         (i) every obligation of such Person for money borrowed,

                         (ii) every obligation of such Person evidenced by
                    bonds, debentures, notes or other similar instruments,
                    including obligations incurred in connection with the
                    acquisition of property, assets or businesses,

                         (iii) every reimbursement obligation of such Person
                    with respect to letters of credit, bankers' acceptances or
                    similar facilities issued for the account of such Person,

                         (iv) every obligation of such Person issued or assumed
                    as the deferred purchase price of property or services
                    (including securities repurchase agreements but excluding
                    trade accounts payable or accrued liabilities arising in the
                    ordinary course of business which are not overdue or which
                    are being contested in good faith),

                         (v) every obligation of such Person under any
                    Capitalized Lease,

                         (vi) every obligation of such Person under any lease (a
                    "synthetic lease") treated as an operating lease under
                    generally 

<PAGE>   3
                                      -3-



                    accepted accounting principles and as a loan or financing
                    for U.S. income tax purposes,

                         (vii) all sales by such Person of (A) accounts or
                    general intangibles for money due or to become due, (B)
                    chattel paper, instruments or documents creating or
                    evidencing a right to payment of money or (C) other
                    receivables (collectively "receivables"), whether pursuant
                    to a purchase facility or otherwise, other than in
                    connection with the disposition of the business operations
                    of such Person relating thereto or a disposition of
                    defaulted receivables for collection and not as a financing
                    arrangement, and together with any obligation of such Person
                    to pay any discount, interest, fees, indemnities, penalties,
                    recourse, expenses or other amounts in connection therewith,

                         (viii) every obligation of such Person (an "equity
                    related purchase obligation") to purchase, redeem, retire or
                    otherwise acquire for value any shares of capital stock of
                    any class issued by such Person, any warrants, options or
                    other rights to acquire any such shares, or any rights
                    measured by the value of such shares, warrants, options or
                    other rights,

                         (ix) every obligation of such Person under any forward
                    contract, futures contract, swap, option or other financing
                    agreement or arrangement (including, without limitation,
                    caps, floors, collars and similar agreements), the value of
                    which is dependent upon interest rates, currency exchange
                    rates, commodities or other indices (a "derivative
                    contract"),

                         (x) every obligation in respect of Indebtedness of any
                    other entity (including any partnership in which such Person
                    is a general partner) to the extent that such Person is
                    liable therefor as a result of such Person's ownership
                    interest in or other relationship with such entity, except
                    (1) to the extent that the terms of such Indebtedness
                    provide that such Person is not liable therefor and such
                    terms are enforceable under applicable law and (2) any
                    obligation arising in connection with the Joint Ventures,

                         (xi) every obligation, contingent or otherwise, of such
                    Person guaranteeing, or having the economic effect of
                    guarantying or otherwise acting as surety for, any
                    obligation of a type described in any of clauses (i) through
                    (x) (the "primary obligation") of another Person (the
                    "primary obligor"), in any manner, whether directly or
                    indirectly, and including, without limitation, any
                    obligation of such Person (A) to purchase or pay (or advance
                    or supply funds for the 

<PAGE>   4
                                      -4-



               purchase of) any security for the payment of such primary
               obligation, (B) to purchase property, securities or services for
               the purpose of assuring the payment of such primary obligation,
               or (C) to maintain working capital, equity capital or other
               financial statement condition or liquidity of the primary obligor
               so as to enable the primary obligor to pay such primary
               obligation.

               The "amount" or "principal amount" of any Indebtedness at any
        time of determination represented by (u) any Indebtedness, issued at a
        price that is less than the principal amount at maturity thereof, shall
        be the amount of the liability in respect thereof determined in
        accordance with generally accepted accounting principles, (v) any
        Capitalized Lease shall be the principal component of the aggregate of
        the rentals obligation under such Capitalized Lease payable over the
        term thereof that is not subject to termination by the lessee, (w) any
        sale of receivables shall be the amount of unrecovered capital or
        principal investment of the purchaser (other than the Borrower or any of
        its wholly-owned Subsidiaries) thereof, excluding amounts representative
        of yield or interest earned on such investment, (x) any synthetic lease
        shall be the stipulated loss value, termination value or other
        equivalent amount, (y) any derivative contract shall be the maximum
        amount of any termination or loss payment required to be paid by such
        Person if such derivative contract were, at the time of determination,
        to be terminated by reason of any event of default or early termination
        event thereunder, whether or not such event of default or early
        termination event has in fact occurred and (z) any equity related
        purchase obligation shall be the maximum fixed redemption or purchase
        price thereof inclusive of any accrued and unpaid dividends to be
        comprised in such redemption or purchase price."

        (b) The following new definition of "Earnings Before Interest and Taxes,
Depreciation and Amortization" is hereby added to Schedule 2 of the Credit
Agreement in alphabetical order to read as follows:

               "Earnings Before Interest and Taxes, Depreciation and
        Amortization. For any period, an amount equal to the sum of (a) the
        consolidated earnings (or loss) from the operations of any Person for
        such period, after all expenses and other proper charges but before
        payment or provision for any income taxes or interest expense for such
        period, plus (b) depreciation, amortization and other non-cash charges
        for such period, all determined in accordance with generally accepted
        accounting principles."

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

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                                      -5-



               (a) 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.

               (b) 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.

               (c) 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 Subsidiaries of this Amendment and
        all other instruments and documents delivered by the Borrower and
        Subsidiaries in connection herewith.

               (d) Delivery of Amendment. The parties hereto shall have executed
        and delivered this Amendment in form and substance satisfactory to the
        Banks and the Agent.

        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.

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                                      -6-



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


                                          BANQUE PARIBAS, now known as Paribas


                                          By: /s/ Clare Bailhe
                                              --------------------------
                                          Its: Director



                                          BANKBOSTON, N.A.,
                                          individually and as Agent


                                          By: /s/ Christopher Phelan
                                              --------------------------
                                          Its: Vice President



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

RAYTEL CARDIAC SERVICES, INC.


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

<PAGE>   7
                                      -7-



RAYTEL MEDICAL IMAGING, INC.


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

RAYTEL IMAGING HOLDINGS, INC.


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


RAYTEL CARDIOVASCULAR LABS, INC.


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


RAYTEL IMAGING NETWORK, INC.


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


RAYTEL IMAGING WEST INC.


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


RAYTEL IMAGING EAST INC.


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

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                                      -8-



RAYTEL IMAGING MID-ATLANTIC INC.


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


RAYTEL GRANADA HILLS INC.


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


CARDIOVASCULAR VENTURES, INC.


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


RAYTEL MANAGEMENT HOLDINGS, INC.


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


RAYTEL CALIFORNIA PHYSICIAN
SERVICES, INC.


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

<PAGE>   9
                                      -9-



RAYTEL TEXAS PHYSICIAN SERVICES, INC.


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


CARDIOVASCULAR VENTURES OF
TEXAS, INC.


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


CARDIOVASCULAR VENTURES OF
WEST HOUSTON, INC.


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


CARDIOVASCULAR VENTURES OF
CENTRAL SAN ANTONIO, INC.


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


CARDIOVASCULAR VENTURES OF
TOWSON, INC.


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

<PAGE>   10
                                      -10-



CARDIOVASCULAR CENTERS OF
PORT ST. LUCIE, INC.


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


CARDIOVASCULAR VENTURES OF
ALEXANDRIA, INC.


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


CARDIOVASCULAR VENTURES OF
EAST NEW ORLEANS, INC.


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


FORT WORTH CARDIAC LABORATORY,
INC.


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


PHYSICIAN PARTNERS OF PORT ST.
LUCIE, INC.


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

<PAGE>   11
                                      -11-



HEART INSTITUTE OF PORT ST.
LUCIE, INC.


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