EXHIBIT 4.1(b)


                  Amendment No.2 and Waiver to Credit Agreement



<PAGE>


                                 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.


         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:


<PAGE>


         (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,

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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.

         (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

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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.

         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: Senior Vice President - CFO


                                         BANQUE NATIONALE DE
                                           PARIS, as Administrative Agent and as
                                             Lender


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




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                                         By /s/ Steve Kovacs
                                            -----------------------------------
                                            Title: Assistant 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 F. 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


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<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. Dingler
                                            -----------------------------------
                                            Title: Assistant Vice President


                                         LASALLE NATIONAL BANK

                                         By /s/ Olga Georgiev
                                            -----------------------------------
                                            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:



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<PAGE>


                                         PILGRIM AMERICA PRIME RATE TRUST

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


                                         SUMMIT BANK

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


                                         USTRUST

                                         By /s/ Thomas 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
                                         TRUSTCOMPANY (NEDERLAND) B.V.


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

                                        By /s/ Chancellor LGT Senior Secured
                                               Management, Inc.
                                           ------------------------------------
                                           Title:



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<PAGE>

                                         MERRILL LYNCH PRIME RATE PORTFOLIO

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


                                         SENIOR DEBT PORTFOLIO

                                         By Boston Management and Research,
                                            as Investment Advisor

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

                                         CERES FINANCE LTD.

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


                                         CAPTIVA FINANCE LTD.

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



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<PAGE>



                                         AMARA-1 FINANCE LTD.

                                         By /s/
                                            -----------------------------------
                                            Title: Director

                                         AMARA-2 FINANCE LTD.

                                         By /s/
                                            -----------------------------------
                                            Title: Director


                                         MERRILL LYNCH PRIME RATE PORTFOLIO
                                         By Merrill Lynch Asset Management, L.P.
                                         as Investment Advisor

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


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<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: Vice President & CFO
  



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<PAGE>



                                       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


                                       MEDIQ MOBILE X-RAY SERVICES, INC.

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


                                       HEALTH EXAMINETICS, INC.

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


                                       THERA-KINETICS ACQUISITION CORPORATION

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



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<PAGE>


                                       MDTC HADDON, INC.

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


                                       MEDIQ DIAGNOSTIC CENTERS, INC.

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


                                       MEDIQ DIAGNOSTICS CENTERS-I INC.

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


                                       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: Vice President & CFO


                                       ALPHA HEALTH CONSULTANTS, INC.

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


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<PAGE>


                                       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: Vice President & CFO




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