<PAGE>   1
                                                                   EXHIBIT 10.12

                                 VITALCOM INC.

                         COMMON STOCK PURCHASE AGREEMENT


        THIS AGREEMENT is made as of July 14, 1998, between VitalCom Inc., a
Delaware corporation (the "Company"), and Irwin & Browning, Inc., a Georgia
corporation ("Purchaser").

        WHEREAS, Purchaser's continued affiliation with the Company is
considered to be important for the Company's growth; and

        WHEREAS in order to provide Purchaser an opportunity to acquire an
equity interest in the Company as an incentive for Purchaser to continue to
participate in the training of Company personnel and management organization
consulting to the Company, the Company is willing to sell to Purchaser and
Purchaser desires to purchase shares of Common Stock according to the terms and
conditions hereof;

        THEREFORE, the parties agree as follows:

1. PURCHASE AND SALE OF STOCK. Subject to the terms and conditions of this
Agreement, the Company hereby agrees to sell to Purchaser and Purchaser agrees
to purchase from the Company 10,000 shares of the Company's Common Stock (the
"Stock") at a price of $3.06 per share, for an aggregate purchase price of
$30,600.00. Payment of the aggregate purchase price shall be by check in the
amount of $10.00 and a non-recourse promissory note in the amount of $30,600.00
(the "Note") in the form attached hereto as Exhibit A. The Note shall bear
interest at a rate no less than the "prime rate" at time of purchase, and shall
be secured by a pledge of the Stock purchased by the Note pursuant to a Security
Agreement in the form attached hereto as Exhibit B (the "Security Agreement")
accompanied by executed stock powers in the form attached hereto as Exhibit C.
Upon such payment, the Company shall issue a duly executed certificate
evidencing the Stock in the name of Purchaser; said certificate to be held by
the Secretary of the Company subject to the terms and conditions of the Security
Agreement.

2. RESTRICTIVE LEGENDS AND STOP-TRANSFER ORDERS.

                (a) Legends. The share certificate evidencing the Stock issued
hereunder shall be endorsed with the following legends (in addition to any
legends required under applicable state securities laws):

                THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE BEEN
                ACQUIRED FOR INVESTMENT AND NOT WITH A VIEW TO, OR IN
                CONNECTION WITH, THE SALE OR DISTRIBUTION THEREOF. NO
                SUCH SALE OR DISPOSITION MAY BE EFFECTED WITHOUT AN
                EFFECTIVE REGISTRATION STATEMENT RELATED THERETO OR AN


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                OPINION OF COUNSEL SATISFACTORY TO THE COMPANY THAT
                SUCH REGISTRATION IS NOT REQUIRED UNDER THE SECURITIES
                ACT OF 1933.

                THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO THE
                TERMS AND CONDITIONS OF A COMMON STOCK PURCHASE AGREEMENT, DATED
                AS JULY 14,1998 BETWEEN THE COMPANY AND PURCHASER. COPIES OF
                SUCH AGREEMENT MAY BE OBTAINED AT NO COST BY WRITTEN REQUEST
                MADE BY THE HOLDER OF RECORD OF THIS CERTIFICATE TO THE COMPANY.

                (b) STOP-TRANSFER NOTICES. Purchaser agrees that, in order to
ensure compliance with the restrictions referred to herein, the Company may
issue appropriate "stop transfer" instructions to its transfer agent, if any,
and that, if the Company transfers its own securities, it may make appropriate
notations to the same effect in its own records.

3. PURCHASER'S REPRESENTATIONS AND COVENANTS. In connection with the purchase of
the Stock, Purchaser hereby represents and warrants to the Company as follows:

                (a) INVESTMENT INTENT; CAPACITY TO PROTECT INTERESTS. Purchaser
is purchasing the Stock solely for Purchaser's own account for investment and
not with a view to or for sale in connection with any distribution of the Stock
or any portion thereof and not with any present intention of selling, offering
to sell or otherwise disposing of or distributing the Stock or any portion
thereof. Purchaser also represents that the entire legal and beneficial interest
of the Stock is being purchased, and will be held, for Purchaser's account only,
and neither in whole or in part for any other person. Purchaser either (i) has a
pre-existing business or personal relationship with the Company or at least one
of its officers, directors or controlling persons, or (ii) by reason of
Purchaser's business or financial experience (or the business or financial
experience of Purchaser's professional advisors who are unaffiliated with and
who are not compensated by the Company or any affiliate or selling agent of the
Company, directly or indirectly), can be reasonably assumed to have the capacity
to evaluate the merits and risks of an investment in the Company and to protect
Purchaser's own interests in connection with this transaction.

                (b) RESIDENCE. Purchaser's principal business location is within
the State of Georgia and is located at the address indicated beneath Purchaser's
signature below.

                (c) INFORMATION CONCERNING COMPANY. Purchaser has discussed the
Company and its plans, operations and financial condition with the Company's
officers and has received all such information as Purchaser has deemed necessary
and appropriate to enable Purchaser to evaluate the financial risk inherent in
making an investment in the Stock. Purchaser has received satisfactory and
complete information concerning the business and financial condition of the
Company in response to all inquiries in respect thereof.


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                (d) ECONOMIC RISK. Purchaser realizes that the purchase of the
Stock involves a high degree of risk. Purchaser is able, without impairing
Purchaser's financial condition, to hold the Stock for an indefinite period of
time and to suffer a complete loss on Purchaser's investment.

                (e) RESTRICTED SECURITIES. Purchaser understands and
acknowledges that:

                        (i) The Stock has not been registered under the 
Securities Act of 1933, as amended, in reliance upon a specific exemption
therefrom, which exemption depends upon, among other things, the bona fide
nature of Purchaser's investment intent as expressed herein. In this connection,
Purchaser understands that, in the view of the Securities and Exchange
Commission ("SEC"), the statutory basis for such exemption may be unavailable if
Purchaser's representation was predicated solely upon a present intention to
hold the Stock for the minimum capital gains period specified under tax
statutes, for a deferred sale, for or until an increase or decrease in the
market price of the Stock, or for a period of one year or any other fixed period
in the future.

                        (ii) The Stock must be held indefinitely unless it is
subsequently registered under the Securities Act or unless an exemption from
such registration is otherwise available. Purchaser further acknowledges and
understands that, except as contemplated by Section 4 of this Agreement, the
Company is under no obligation to register the Stock. In addition, Purchaser
understands that the certificate evidencing the Stock will be imprinted with a
legend which prohibits the transfer of the Stock unless it is registered or such
registration is not required in the opinion of counsel satisfactory to the
Company.

                (f) DISPOSITION UNDER RULE 144. Purchaser understands that:

                        (i) The shares of  Stock are restricted securities 
within the meaning of Rule 144 promulgated under the Securities Act; that the
exemption from registration under Rule 144 will not be available in any event
for at least one (1) year from the date of purchase and payment of the Stock,
and then will be available only if (i) a public trading market then exists for
the Common Stock of the Company, (ii) adequate information concerning the
Company is then available to the public, and (iii) other terms and conditions of
Rule 144 are complied with; and that any sale of the Stock may be made only in
limited amounts in accordance with such terms and conditions of Rule 144;

                        (ii) That at the time Purchaser wishes to sell the Stock
there may be no public market upon which to make such a sale; that, even if such
a public market then exists, the Company may not be satisfying the current
public information requirements of Rule 144; and that, in such event, Purchaser
would be precluded from selling the Stock under Rule 144 even if the one (1)
year minimum holding period had been satisfied; and

                        (iii) In the event all of the requirements of Rule 144
are not satisfied, registration under the Securities Act or compliance with
Regulation A or another registration exemption will be required; that,
notwithstanding the fact that Rule 144 is not exclusive, the


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Staff of the SEC has expressed its opinion that persons proposing to sell
private placement securities other than in a registered offering or pursuant to
Rule 144 will have a substantial burden of proof in establishing that an
exemption from registration is available for such offers or sales; and that such
persons and their respective brokers who participate in such transactions do so
at their own risk.

                (g) FURTHER LIMITATIONS ON DISPOSITION. Without in any way
limiting Purchaser's representations set forth above, Purchaser further agrees
that Purchaser shall in no event make any disposition of all or any portion of
the Stock unless and until:

                        (i)    Either:

                               (A) there is then in effect a Registration 
Statement under the Securities Act covering such proposed disposition, and such
disposition is made in accordance with said Registration Statement; or

                               (B) Purchaser may sell the stock pursuant to Rule
144; or

                               (C) (1) Purchaser shall have notified the Company
of the proposed disposition and shall have furnished the Company with a detailed
statement of the circumstances surrounding the proposed disposition; and (2)
Purchaser shall have furnished the Company with an opinion of Purchaser's
counsel, satisfactory to the Company, to the effect that such disposition will
not require registration of such shares under the Securities Act.

4. TERMINATION. This Agreement shall terminate upon payment in full of the Note,
except as to paragraphs 2, 3, 5, 6(d), 6(e), 6(g).

5. GOVERNING LAW. This Agreement shall be governed and construed by the laws of
the State of California.

6. MISCELLANEOUS.

                        (a) FURTHER ASSURANCES. The parties agree to execute 
such further instruments and to take such further action as may reasonably be
necessary to carry out the intent of this Agreement.

                        (b) NOTICES. Any notice required or permitted hereunder
shall be given in writing and shall be deemed effectively given upon personal
delivery (including by express courier) or upon deposit in the United States
Post Office, by First Class mail with postage and fees prepaid, addressed to
Purchaser at his address shown on the Company's employment records and to the
Company at the address of its principal corporate offices (attention: Secretary)
or at such other address as such party may designate by ten (10) days advance
written notice to the other party.

                        (c) ASSIGNMENT. The Company may assign its rights and
delegate its duties under this Agreement. This Agreement shall inure to the
benefit of the successors


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and assigns of the Company and, subject to the restrictions on transfer herein
set forth, be binding upon Purchaser, his heirs, executors, administrators,
successors and assigns. The rights of Purchaser under this Agreement may be
assigned only with the prior written consent of the Company.

                        (d) WAIVER. Either party's failure to enforce any
provision or provisions of this Agreement shall not in any way be construed as a
waiver of any such provision or provisions, nor prevent that party thereafter
from enforcing each and every other provision of this Agreement. The rights
granted both parties herein are cumulative and shall not constitute a waiver of
either party's right to assert all other legal remedies available to it under
the circumstances. (e) ADVICE OF COUNSEL. Purchaser has reviewed this Agreement
in its entirety, has had an opportunity to obtain the advice of counsel prior to
executing this Agreement and fully understands all provisions hereof.

                        (f) COUNTERPARTS. This Agreement may be executed in any
number of counterparts, each of which shall be an original and all of which
together shall constitute one instrument.

                        (g) ENTIRE AGREEMENT. This Agreement together with the 
Security Agreement and the Note represent the entire agreement between the
parties with respect to the purchase of Common Stock by Purchaser, may be
modified or amended only in writing signed by both parties, and satisfies all of
the Company's obligations to Purchaser with regard to the issuance or sale of
securities.


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        IN WITNESS WHEREOF, the parties hereto have duly executed this Agreement
as of the day and year first above written.

VITALCOM INC.                                IRWIN & BROWNING, INC.
a Delaware corporation                       a Georgia corporation


By:  s/ Frank T. Sample                      By: s/ Tim Irwin
     -----------------------------               ----------------------------


Title:  CEO                                  Title: President
      ----------------------------                  -------------------------


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                                    Exhibit A

                          NON-RECOURSE PROMISSORY NOTE


$30,600.00                                                   Tustin, California

                                                             July 14, 1998


        FOR VALUE RECEIVED, the undersigned, Irwin & Browning, Inc., a Georgia
corporation, promises to pay to VitalCom Inc., a Delaware corporation (the
"Company"), or order, the principal sum of Thirty thousand six hundred dollars
and no cents ($30,600.00), together with interest on the outstanding principal
amount from the date hereof at the rate of 8.5% per annum. The principal amount
hereof and all accrued and unpaid interest on the outstanding principal amount
shall be due and payable to the holder hereof at 15222 Del Amo Avenue, Tustin,
California 92780, or such other place as the holder hereof may designate.

        The principal amount, to the extent not sooner paid, shall be due and
payable on July 14, 2001. All accrued and unpaid interest shall be due and
payable on the first day of each fiscal quarter of the Company commencing on
July 14, 1998. Should the undersigned fail to make full payment of principal or
interest for a period of 30 days or more after the due date thereof, the whole
unpaid balance on this Note of principal and interest shall become immediately
due at the option of the holder of this Note. Payments of principal and interest
shall be made in lawful money of the United States of America.

        The undersigned may at any time prepay all or any portion of the
principal or interest owing hereunder.

        This Note is subject to the terms of the Common Stock Purchase
Agreement, dated as of July 14, 1998. This Note is without recourse to the
undersigned, provided that this Note is secured by a pledge of the Company's
Common Stock under the terms of a Security Agreement of even date herewith (a
copy of which is on file with the Secretary of the Company) and is subject to
all the provisions thereof.

        The undersigned hereby waives presentment, protest, demand for payment,
notice of dishonor and all other notices or demands in connection with the
delivery, acceptance, performance, default or endorsement of this Note.

        In the event the Purchase Agreement dated July 14, 1998 between the
undersigned and the Company shall be terminated pursuant to the terms of the
Purchase Agreement, this Note shall, at the option of the Company, be
accelerated, and the whole unpaid balance on this Note of principal and accrued
interest shall be immediately due and payable.


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        The reasonable costs and attorneys' fees of the holder incurred in the
collection of this Note shall be paid by the undersigned. This Note has been
made and delivered in the State of California and shall be governed and
construed in accordance with the laws of the State of
California.

                                          IRWIN & BROWNING, INC.,
                                          a Georgia corporation

                                          By: s/ Tim Irwin
                                              ---------------------------

                                           Title: President
                                                  -----------------------


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                                    Exhibit B

                               SECURITY AGREEMENT


        This Security Agreement is made as of July 14, 1998 between VitalCom
Inc., a Delaware corporation ("Pledgee"), and Irwin & Browning, Inc., a Georgia
corporation ("Pledgor").


                                    Recitals

        Pursuant to Pledgor's election to purchase Stock under the Stock
Purchase Agreement dated July) 14, 1998 (the "Agreement") and Pledgor's election
to pay for such shares with a promissory note (the "Note"), Pledgor has
purchased 10,000 shares of Pledgee's Common Stock (the "Stock") at a price of
3.06 per share, for a total purchase price of $30,600.00. The Note and the
obligations thereunder are as set forth in Exhibit A to the Agreement.

        NOW, THEREFORE, it is agreed as follows:

        1. Creation and Description of Security Interest. In consideration of
the transfer of the Stock to Pledgor under the Agreement, Pledgor, pursuant to
the California Commercial Code, hereby pledges all of such Stock (herein
sometimes referred to as the "Collateral") represented by certificate number
_______, duly endorsed in blank or with executed stock powers, and herewith
delivers said certificate to the Secretary of Pledgee ("Pledgeholder"), who
shall hold said certificate subject to the terms and conditions of this Security
Agreement.

        The pledged stock (together with an executed blank stock assignment for
use in transferring all or a portion of the Stock to Pledgee if, as and when
required pursuant to this Security Agreement) shall be held by the Pledgeholder
as security for the repayment of the Note, and any extensions or renewals
thereof, to be executed by Pledgor pursuant to the terms of the Agreement, and
the Pledgeholder shall not encumber or dispose of such Stock except in
accordance with the provisions of this Security Agreement.

        2. Pledgor's Representations and Covenants. To induce Pledgee to enter
into this Security Agreement, Pledgor represents and covenants to Pledgee, its
successors and assigns, as follows:

                a. Payment of Indebtedness. Pledgor will pay the principal sum
of the Note secured hereby, together with interest thereon, at the time and in
the manner provided in the Note.


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                b. Encumbrances. The Stock is free of all other encumbrances, 
defenses and liens, and Pledgor will not further encumber the Stock without the
prior written consent of Pledgee.

                c. Margin Regulations. In the event that Pledgee's Common Stock
is now or later becomes margin-listed by the Federal Reserve Board and Pledgee
is classified as a "lender" within the meaning of the regulations under Part 207
of Title 12 of the Code of Federal Regulations ("Regulation G"), Pledgor agrees
to cooperate with Pledgee in making any amendments to the Note or providing any
additional collateral as may be necessary to comply with such regulations.

        3. Voting Rights. During the term of this pledge and so long as all
payments of principal and interest are made as they become due under the terms
of the Note, Pledgor shall have the right to vote all of the Stock pledged
hereunder.

        4. Stock Adjustments. In the event that during the term of the pledge
any stock dividend, reclassification, readjustment or other changes are declared
or made in the capital structure of Pledgee, all new, substituted and additional
shares or other securities issued by reason of any such change shall be
delivered to and held by the Pledgee under the terms of this Security Agreement
in the same manner as the Stock originally pledged hereunder. In the event of
substitution of such securities, Pledgor, Pledgee and Pledgeholder shall
cooperate and execute such documents as are reasonable so as to provide for the
substitution of such Collateral and, upon such substitution, references to
"Stock" in this Security Agreement shall include the substituted shares of
capital stock of Pledgor as a result thereof.

        5. Options and Rights. In the event that, during the term of this
pledge, subscription Options or other rights or options shall be issued in
connection with the pledged Stock, such rights, Options and options shall be the
property of Pledgor and, if exercised by Pledgor, all new stock or other
securities so acquired by Pledgor as it relates to the pledged Stock then held
by Pledgeholder shall be immediately delivered to Pledgeholder, to be held under
the terms of this Security Agreement in the same manner as the Stock pledged.

        6. Default. Pledgor shall be deemed to be in default of the Note and of
this Security Agreement in the event:

                a. Payment of principal or interest on the Note shall be 
delinquent for a period of 30 days or more;

                b. Pledgor fails to perform any of the covenants set forth in
the Agreement or contained in this Security Agreement for a period of 10 days
after written notice thereof from Pledgee; or

                c. The purchase agreement dated July 14, 1998 between Pledgee
and Pledgor shall be terminated for any reason.


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        In the case of an event of Default, as set forth above, Pledgee shall
have the right to accelerate payment of the Note upon notice to Pledgor, and
Pledgee shall thereafter be entitled to pursue its remedies under the California
Commercial Code.

        7. Release of Collateral. Subject to any applicable contrary rules under
Regulation G, there shall be released from this pledge a portion of the pledged
Stock held by Pledgeholder hereunder upon payments of the principal of the Note.
The number of shares of the pledged Stock which shall be released shall be that
number of shares of full Stock which bears the same proportion to the initial
number of shares of Stock pledged hereunder as the payment of principal bears to
the initial full principal amount of the Note.

        8. Withdrawal or Substitution of Collateral. Pledgor shall not sell,
withdraw, pledge, substitute or otherwise dispose of all or any part of the
Collateral without the prior written consent of Pledgee.

        9. Term. The within pledge of Stock shall continue until the payment of
all indebtedness secured hereby, at which time the remaining pledged stock shall
be promptly delivered to Pledgor, subject to the provisions for prior release of
a portion of the Collateral as provided in paragraph 7 above.

        10. Insolvency. Pledgor agrees that if a bankruptcy or insolvency
proceeding is instituted by or against him, or if a receiver is appointed for
the property of Pledgor, or if Pledgor makes an assignment for the benefit of
creditors, the entire amount unpaid on the Note shall become immediately due and
payable, and Pledgee may proceed as provided in the case of default.

        11. Pledgeholder Liability. In the absence of willful or gross
negligence, Pledgeholder shall not be liable to any party for any of his acts,
or omissions to act, as Pledgeholder.

        12. Invalidity of Particular Provisions. Pledgor and Pledgee agree that
the enforceability or invalidity of any provision or provisions of this Security
Agreement shall not render any other provision or provisions herein contained
unenforceable or invalid.

        13. Successors or Assigns. Pledgor and Pledgee agree that all of the
terms of this Security Agreement shall be binding on their respective successors
and assigns, and that the term "Pledgor" and the term "Pledgee" as used herein
shall be deemed to include, for all purposes, the respective designees,
successors, assigns, heirs, executors and administrators.

        14. Governing Law. This Security Agreement shall be interpreted and
governed under the laws of the State of California.


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        IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the day and year first above written.

"PLEDGOR"                            IRWIN & BROWNING, INC.,
                                     a Georgia corporation

                                     By: s/  Tim Irwin
                                         -----------------------------

                                     Title: President
                                            --------------------------

                                     2900 Paces Ferry Rd., Bldg. B
                                     Atlanta, GA  30339
                                     ---------------------------------
                                     Address


        "PLEDGEE"                    VITALCOM INC.,
                                     a Delaware corporation


                                     By: s/  Frank T. Sample
                                     ---------------------------------

                                     Title: CEO
                                     ---------------------------------


        "PLEDGEHOLDER"
                                      s/  Shelley B. Thunen
                                     ---------------------------------
                                     Secretary of VitalCom Inc.


<PAGE>   13




                                    Exhibit C

                      ASSIGNMENT SEPARATE FROM CERTIFICATE



        FOR VALUE RECEIVED and pursuant to that certain Security Agreement dated
as of July 14, 1998 (the "Agreement"), Irwin & Browning, Inc. ("Purchaser")
hereby sells, assigns and transfers unto __________________________________ (10,
000 ) shares of the Common Stock of VitalCom Inc., a Delaware corporation,
standing in the undersigned's name on the books of said corporation represented
by certificate no. _______ herewith, and does hereby irrevocably constitute and
appoint ___________________ attorney to transfer the said stock on the books of
the said corporation with full power of substitution in the premises. THIS
ASSIGNMENT MAY ONLY BE USED AS AUTHORIZED BY THE AGREEMENT AND THE EXHIBITS
THERETO.


Dated:  July 14, 1998
                                    PURCHASER

                                    IRWIN & BROWNING, INC.,
                                    a Georgia corporation



                                    By: s/ Tim Irwin
                                        -----------------------------


                                    Title:      President
                                          -----------------------------







INSTRUCTION: PLEASE DO NOT FILL IN ANY BLANKS OTHER THAN THE SIGNATURE LINE. THE
PURPOSE OF THIS ASSIGNMENT IS TO ENABLE THE CORPORATION TO ACQUIRE THE SHARES
UPON DEFAULT UNDER PURCHASER'S NOTE WITHOUT REQUIRING ADDITIONAL SIGNATURES ON
THE PART OF THE PURCHASER.