EXHIBIT 4.1 (b)


                        AMENDMENT NO. 2 AND WAIVER TO THE
                                CREDIT AGREEMENT


Dated as of April 1, 1997

AMENDMENT NO. 2 AND WAIVER TO THE CREDIT AGREEMENT, among MEDIQ/PRN LIFE SUPPORT
SERVICES, INC., a Delaware corporation (the "Borrower"), MEDIQ INCORPORATED, a
Delaware corporation ("MEDIQ"), PRN HOLDINGS, INC., a Delaware corporation
(together with MEDIQ, the "Parent Guarantors"), the banks, financial
institutions and other institutional lenders parties to the Credit Agreement
referred to below (collectively, the "Lenders") and Banque Nationale de Paris as
administrative agent (the "Administrative Agent") for the Lenders and
NationsBank N.A., as documentation agent (the "Documentation Agent").

PRELIMINARY STATEMENTS:

(1) The Borrower, the Parent Guarantors, the Lenders, the Administrative Agent
and the Documentation Agent have entered into a Credit Agreement dated as of
October 1, 1996, as amended by Amendment No. 1 dated as of January 24, 1997 (as
so amended, the "Credit Agreement"). Capitalized terms not otherwise defined in
this Amendment and Waiver have the same meanings as specified in the Credit
Agreement.

(2) The Borrower seeks to acquire (the "UHS Acquisition") Universal Hospital
Services, Inc., a Minnesota corporation ("UHS") and has requested that the
Required Lenders amend and waive certain provisions of the Credit Agreement in
order to permit the Borrower to acquire UHS for a purchase price greater than
$140,000,000 and incur certain one time extraordinary charges associated with
the acquisition and the operations of UHS.

(3) The Required Lenders are, on the terms and conditions stated below, willing
to grant the request of the Borrower and the Borrower and such Lenders have
agreed to amend and waive the Credit Agreement as hereinafter set forth.


<PAGE>


SECTION 1. Amendments to the Credit Agreement. Section 1.01 of the Credit
Agreement is, effective as of the date hereof and subject to the satisfaction of
the conditions precedent set forth in Section 3(a) hereof, hereby amended as
follows:

(1) The definition of "EBITDA" is amended by deleting clause (g) thereof in its
    entirety and substituting therefor the following:

         "(g) all one-time expenses of the Borrower and its Affiliates incurred
              in connection with the UHS Acquisition for (i) acquisition
              expenses, lease related expenses, facility closure expenses and
              professional and other fees, and (ii) the write-down of UHS's
              Demand Positive Airway Pressure Devices inventory.

SECTION 2. Waiver. The provisions of Section 5.02(f)(i)(4) are hereby waived
solely to the extent that such section limits the aggregate amount of
Investments outstanding after giving effect to the UHS Acquisition to
$140,000,000.

SECTION 3. Conditions of Effectiveness. This Amendment and Waiver shall become
effective as of the date first above written when, and only when, the
Administrative Agent shall have received counterparts of this Amendment and
Waiver executed by the Borrower and the Required Lenders, or, as to any of the
Lenders, advice satisfactory to the Administrative Agent that such Lender has
executed this Amendment and Waiver. The effectiveness of this Amendment and
Waiver is conditioned upon the accuracy of the factual matters described herein.
This Amendment and Waiver is subject to the provisions of Section 9.01 of the
Credit Agreement.

SECTION 4. Representations and Warranties of the Borrower. The Borrower
represents and warrants as follows:

(a)  The Borrower is a corporation duly organized, validly existing and in good
     standing under the laws of the jurisdiction indicated in the recital of the
     parties to this Amendment and Waiver.

(b)  The execution, delivery and performance by each Loan Party of this
     Amendment and Waiver and the Loan Documents, as amended hereby, to which it
     is or is to be a party, and the consummation of the transactions
     contemplated hereby, are within such Loan Party's corporate powers, have
     been duly authorized by all necessary corporate action and do not (i)
     contravene each such Loan Party's charter or by-laws, (ii) violate any law
     (including, without limitation, the Securities Exchange Act of 1934, as
     amended, and the Racketeer Influenced and Corrupt Organizations Chapter of
     the Organized Crime Control Act of 1970), rule or regulation (including,
     without limitation, Regulation X of the Board of Governors of the Federal
     Reserve System), or any order, writ, judgment, injunction, decree,
     determination or award, binding on or affecting any Loan Party or any of
     its Subsidiaries or any of their properties, (iii) conflict with or result
     in the breach of, or constitute a default under, any contract, loan
     agreement, indenture, mortgage, deed of trust, lease or other instrument
     binding on or affecting any Loan Party, any of their Subsidiaries or any of
     their properties or (iv) except for the Liens created under the Collateral
     Documents, as amended hereby, or any amendments or supplements thereto
     contemplated hereby, result in or require the creation or imposition of any
     Lien upon or with respect to any of the properties of any Loan Party or any
     of its Subsidiaries.

(c)  No authorization or approval or other action by, and no notice to or filing
     with, any governmental authority or regulatory body or any other third
     party is required for the due execution, delivery, recordation, filing or
     performance by any Loan Party of this Amendment and Waiver, any of the
     Collateral Documents or any amendments or supplements thereto contemplated
     hereby to which each such Loan Party is or is to be a party, or any of the
     Loan Documents, as amended hereby, to which it is or is to be a party.


<PAGE>


(d)  This Amendment and Waiver and each of the Collateral Documents and
     amendments and supplements thereto contemplated hereby to which each Loan
     Party is a party have been duly executed and delivered by each such Loan
     Party. This Amendment and Waiver and each of the other Loan Documents, as
     amended hereby, to which each Loan Party is a party are, and each of the
     other Collateral Documents and amendments and supplements thereto
     contemplated hereby to which each such Loan Party is or is to be a party,
     when delivered hereunder, will be, legal, valid and binding obligations of
     each such Loan Party, enforceable against each such Loan Party in
     accordance with their respective terms.

(e)  There is no action, suit, investigation, litigation or proceeding affecting
     any Loan Party or any of their Subsidiaries (including, without limitation,
     any Environmental Action) pending or threatened before any court,
     governmental agency or arbitrator that (i) would be reasonably likely to
     have a Material Adverse Effect or (ii) purports to affect the legality,
     validity or enforceability of this Amendment and Waiver, the Collateral
     Documents, any amendments or supplements thereto contemplated hereby or any
     of the other Loan Documents, as amended hereby, or the consummation of any
     of the transactions contemplated hereby.

SECTION 5. Reference to and Effect on the Loan Documents. (a) On and after the
effectiveness of this Amendment and Waiver, each reference in the Credit
Agreement to "this Agreement", "hereunder", "hereof" or words of like import
referring to the Credit Agreement, and each reference in the Notes and each of
the other Loan Documents to "the Credit Agreement", "thereunder", "thereof" or
words of like import referring to the Credit Agreement, shall mean and be a
reference to the Credit Agreement, as amended by this Amendment and Waiver.

(b) The Credit Agreement, the Notes and each of the other Loan Documents, as
specifically amended by this Amendment and Waiver, are and shall continue to be
in full force and effect and are hereby in all respects ratified and confirmed.
Without limiting the generality of the foregoing, the Collateral Documents and
all of the Collateral described therein do and shall continue to secure the
payment of all Obligations of the Loan Parties under the Loan Documents, in each
case as amended by this Amendment and Waiver.

(c) The execution, delivery and effectiveness of this Amendment and Waiver shall
not, except as expressly provided herein, operate as a waiver of any right,
power or remedy of any Lender or the Agent under any of the Loan Documents, nor
constitute a waiver of any provision of any of the Loan Documents.

SECTION 6. Costs, Expenses. The Borrower agrees to pay on demand all costs and
expenses of the Agent in connection with the preparation, execution, delivery
and administration, modification and amendment of this Amendment and Waiver, and
the other instruments and documents to be delivered hereunder (including,
without limitation, the reasonable fees and expenses of counsel for the Agent)
in accordance with the terms of Section 9.04 of the Credit Agreement.

SECTION 7. Execution in Counterparts. This Amendment and Waiver may be executed
in any number of counterparts and by different parties hereto in separate
counterparts, each of which when so executed shall be deemed to be an original
and all of which taken together shall constitute but one and the same agreement.
Delivery of an executed counterpart of a signature page to this Amendment and
Waiver by telecopier shall be effective as delivery of a manually executed
counterpart of this Amendment and Waiver.

SECTION 8. Governing Law. This Amendment and Waiver shall be governed by, and
construed in accordance with, the laws of the State of New York.

<PAGE>


IN WITNESS WHEREOF, the parties hereto have caused this Amendment and Waiver to
be executed by their respective officers thereunto duly authorized, as of the
date first above written.


                                           MEDIQ/PRN LIFE SUPPORT 
                                           SERVICES, INC.

                                           By /s/ Jay M. Kaplan
                                              ---------------------------------
                                              Title: Sr. Vice-President


                                           BANQUE NATIONALE DE PARIS,
                                           as Administrative Agent and as Lender

                                           By /s/ Allan Mustacchi
                                              ---------------------------------
                                              Title: Vice-President

                                           By /s/ Stephen Kovacs
                                              ---------------------------------
                                              Title: Vice-President


                                           NATIONSBANK, N.A.,
                                           as Documentation Agent and as Lender

                                           By /s/ Michael S. Sylvester
                                              ---------------------------------
                                              Title: Officer


                                           THE FIRST NATIONAL BANK OF BOSTON

                                           By /s/ Kimberly Harris
                                              ---------------------------------
                                              Title:  Vice-President


                                           CAISSE NATIONALE DE CREDIT AGRICOLE

                                           By /s/ Craig Welsh
                                              ---------------------------------
                                              Title: First Vice-President


                                           CREDITANSTALT CORPORATE FINANCE, INC.

                                           By /s/ Clifford L. Wells
                                              ---------------------------------
                                              Title:  Vice-President

                                           By /s/ Stacy Harmon
                                              ---------------------------------
                                              Title: Senior Associate


<PAGE>


                                           FIRST SOURCE FINANCIAL, LLP
                                           By: First Source Financial , Inc.
                                           as Agent/Manager

                                           By /s/ James W. Wilson
                                              ---------------------------------
                                              Title: Senior Vice-President


                                           METROPOLITAN LIFE INSURANCE COMPANY

                                           By /s/ James R. Dughi
                                              ---------------------------------
                                              Title: Assistant Vice-President


                                           LASALLE NATIONAL BANK

                                           By /s/ Olga Georgien
                                              ---------------------------------
                                              Title: First Vice-President


                                           MASSACHUSETTS MUTUAL LIFE
                                           INSURANCE COMPANY

                                           By /s/ Michael P. Hermsen
                                              ---------------------------------
                                              Title: Managing Director


                                           MELLON BANK, N.A.


                                           By
                                              ---------------------------------
                                              Title:


                                           MERRILL LYNCH SENIOR FLOATING
                                           RATE FUND, INC.

                                           By
                                              ---------------------------------
                                              Title:


                                           PILGRIM AMERICA PRIME RATE TRUST

                                           By /s/ Michael J. Bacevich
                                              ---------------------------------
                                              Title: Vice-President


<PAGE>


                                           SUMMIT BANK

                                           By /s/ Gail S. Powers
                                              ---------------------------------
                                              Title: Vice-President


                                           USTRUST

                                           By /s/ Thomas Z. Macina
                                              ---------------------------------
                                              Title: Vice-President


                                           VAN KAMPEN AMERICAN CAPITAL
                                           PRIME RATE INCOME TRUST

                                           By /s/ Kathleen A. Zarn
                                              ---------------------------------
                                              Title: Vice-President


                                           RESTRUCTURED OBLIGATIONS
                                           BACKED BY SENIOR ASSETS B.V.

                                           By its Managing Director, ABN TRUST
                                           COMPANY (NEDERLAND) B.V.

                                           By /s/ Christopher E. Jansen
                                              ---------------------------------
                                              Title:  Managing Director

                                           By 
                                              ---------------------------------
                                              Title:


                                           MERRILL LYNCH PRIME RATE PORTFOLIO

                                           By 
                                              ---------------------------------
                                              Title:


                                           SENIOR DEBT PORTFOLIO

                                           By Boston Management and Research, as
                                           Investment Advisor

                                           By 
                                              ---------------------------------
                                              Title:


                                           CERES FINANCE LTD.

                                           By /s/ John H. Cullinane
                                              ---------------------------------
                                              Title: Director


<PAGE>


                                           CAPTIVA FINANCE LTD.

                                           By /s/ John H. Cullinane
                                              ---------------------------------
                                              Title: Director


                                           AMARA-1 FINANCE LTD.

                                           By 
                                              ---------------------------------
                                              Title:


                                           AMARA-2 FINANCE LTD.

                                           By
                                              ---------------------------------
                                              Title:

                                           MERRILL LYNCH PRIME RATE PORTFOLIO

                                           By Merrill Lynch Asset Management,
                                           L.P., as Investment Advisor


                                           By
                                              ---------------------------------
                                              Title:


<PAGE>


                                     CONSENT

Dated as of April 1, 1997


Reference is made to Amendment No. 2 and Waiver dated as of April 1, 1997 (the
"Amendment and Waiver"), to the Credit Agreement dated as of October 1, 1996, as
amended by amendment No. 1 dated as of January 24, 1997 (as so amended, the
"Credit Agreement"; unless otherwise defined herein, capitalized terms being
used herein as therein defined) among MEDIQ/PRN Life Support Services, Inc., a
Delaware corporation, as Borrower, PRN Holdings, Inc. a Delaware corporation and
MEDIQ Incorporated, as Parent Guarantors, Banque Nationale de Paris, as
Administrative Agent, and certain other Lender Parties party thereto.

Each of the undersigned, as a Loan Party party to certain of the Loan Documents,
hereby consents to the Amendment and Waiver and hereby confirms and agrees that
(a) notwithstanding the effectiveness of such Amendment and Waiver, each Loan
Document to which it is a party is, and shall continue to be, in full force and
effect and is hereby ratified and confirmed in all respects, except that, on and
after the effectiveness of such Amendment and Waiver, each reference in such
Loan Document to the "Credit Agreement", "thereunder", "thereof" or words of
like import shall mean and be a reference to the Credit Agreement, as amended by
such Amendment and Waiver, and (b) the Collateral Documents to which such Loan
Party is a party and all of the Collateral described therein do, and shall
continue to, secure the payment of all of the Secured Obligations (in each case,
as defined therein).


                                           MEDIQ INVESTMENT SERVICES, INC.

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Vice-President & CFO


                                           MEDIQ MANAGEMENT SERVICES, INC.

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Chief Financial Officer


                                           MEDIQ SURGICAL EQUIPMENT
                                           SERVICES, INC.

                                           By /s/ Thomas E. Carroll
                                              ---------------------------------
                                              Name:  Thomas E. Carroll
                                              Title: President


                                           VALUE-MED PRODUCTS, INC.

                                           By /s/ Thomas E. Carroll
                                              ---------------------------------
                                              Name:  Thomas E. Carroll
                                              Title: President



<PAGE>



                                           MEDIQ MOBILE X-RAY SERVICES, INC.

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Chief Financial Officer


                                           HEALTH EXAMINETICS, INC.

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Vice-President


                                           THERA-KINETICS ACQUISITION
                                           CORPORATION

                                           By /s/ Thomas E. Carroll
                                              ---------------------------------
                                              Name:  Thomas E. Carroll
                                              Title: President


                                           MDTC HADDON, INC.

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Chief Financial Officer


                                           MEDIQ DIAGNOSTIC CENTERS, INC.

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Chief Financial Officer


                                           MEDIQ DIAGNOSTICS CENTERS-I INC.

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Chief Financial Officer


                                           MEDIQ IMAGING SERVICES, INC.

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Vice-President & CFO


                                           AMERICAN CARDIOVASCULAR IMAGING
                                           LABS, INC.

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Chief Financial Officer


<PAGE>



                                           ALPHA HEALTH CONSULTANTS, INC.

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Chief Financial Officer


                                           P. I. CORPORATION

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Vice-President & CFO


                                           MEDIQ SERVICES, INC.

                                           By /s/ Michael F. Sandler
                                              ---------------------------------
                                              Name:  Michael F. Sandler
                                              Title: Chief Financial Officer