AMENDMENT NO. 1 TO TENTH AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT

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                                                                     Exhibit 4.3

                               AMENDMENT NO. 1 TO
                           TENTH AMENDED AND RESTATED
                            INVESTOR RIGHTS AGREEMENT

      THIS AMENDMENT NO. 1 TO TENTH AMENDED AND RESTATED INVESTOR RIGHTS
AGREEMENT (this "Amendment"), entered into on April 13, 2006, among ALSIUS
CORPORATION, a California corporation (the "Company"), and the undersigned
investors in the Company (the "Investors").

                                    RECITALS:

      A. Certain of the Investors hold shares of the Company's Common Stock,
Series A, Series B, Series C-D, Series E and/or Series F Preferred Stock and
possess registration rights, information rights, rights of first refusal, and
other rights pursuant to that certain Tenth Amended and Restated Investor Rights
Agreement, dated as of September 2, 2004, among the Company, such Investors and
certain other shareholders of the Company (the "Tenth Restated Agreement").

      B. On or about the date hereof, in connection with a bridge financing (the
"Bridge Financing"), the Company is issuing a series of 8% Unsecured Convertible
Promissory Notes (the "Bridge Notes") and warrants to purchase Common Stock of
the Company (the "Warrants").

      C. It is a condition to the closing of the Bridge Financing that this
Amendment be executed by the parties hereto, and the parties are willing to
execute, and to be bound by the provisions of, this Amendment.

      D. The undersigned Investors desire to amend the definition of
"Registrable Securities" under Section 1.1 of the Tenth Restated Agreement to
include the shares of stock issuable upon conversion of the Bridge Notes and
exercise of the Warrants.

      E. The undersigned Investors desire to amend Section 1.2(a) of the Tenth
Restated Agreement to amend the date upon which a demand registration may be
requested.

      F. The Investors executing this Amendment hold a majority of the shares
held by all Investors who are a party to the Tenth Restated Agreement and a
majority of the shares of Series C-D, Series E and Series F Preferred Stock or
Common Stock issuable upon conversion thereof, and therefore, have the power
under Section 3.6 of the Tenth Restated Agreement to amend such agreement.

      NOW, THEREFORE, in consideration of the mutual promises and covenants set
forth herein, the parties to the Tenth Restated Agreement hereby agree that the
Tenth Restated Agreement shall be amended as follows:

      1. Added Definition of 2006 Bridge Notes in Section 1.1. The undersigned
Investors hereby agree that a new definition shall be added to Section 1.1 of
the Tenth Restated Agreement to read as follows:

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            "2006 Bridge Notes" means the convertible promissory notes made by
      the Company, dated April 13, 2006, in connection with a bridge financing
      of the Company of approximately $6,000,000."

      2. Revised Definition of "Warrants." The undersigned Investors hereby
agree that the definition of "Warrants" in Section 1.1 of the Tenth Restated
Agreement shall be deleted in its entirety and replaced with the following:

            ""Warrants" means those certain warrants to purchase Common Stock of
      the Company (i) dated April 30, 2003, issued to the initial purchasers of
      the Bridge Notes and (ii) dated April 13, 2006, issued to the initial
      purchasers of the 2006 Bridge Notes."

      3. Revised Definition of "Registrable Securities." The undersigned
Investors hereby agree that the definition of "Registrable Securities" in
Section 1.1 of the Tenth Restated Agreement shall be deleted in its entirety and
replaced with the following:

            ""Registrable Securities" means (i) the Common Stock issued in
      exchange for the common stock issued by Retroperfusion Systems, Inc.
      ("RSI") upon conversion of the RSI Series B, Series C and Series D
      Preferred Stock and in exchange for the RSI Series E and Series F
      Preferred Stock, (ii) the Common Stock issued upon exercise of the
      warrants issued under the Loan and Warrant Subscription Agreements dated
      on or about August 17, 1993, (iii) the Common Stock issued under the Loan
      and Warrant Subscription Agreements dated October 1994, (iv) the Common
      Stock issued or issuable upon conversion of the Series A, Series B, Series
      C-D, Series E and Series F Preferred Stock, (v) the Common Stock issuable
      upon exercise of the Warrants issued in connection with the Bridge Notes,
      (vi) the Common Stock issued or issuable upon conversion of the 2006
      Bridge Notes, (vii) the Common Stock issuable upon exercise of the
      Warrants issued in connection with the 2006 Bridge Notes and (viii) any
      Common Stock of the Company issued or issuable in respect of the above
      described securities upon any stock split, stock dividend,
      recapitalization, or similar event, or any Common Stock otherwise issued
      or issuable with respect to the above described securities; provided,
      however, that shares of Common Stock or other securities shall not be
      treated as Registrable Securities for purposes of Section 1 hereof if they
      have been sold to or through a broker or dealer or underwriter in a public
      distribution or a public securities transaction."

      2. Revised Section 1.2(a). The undersigned Investors hereby agree that the
lead-in sentence of Section 1.2(a) of the Tenth Restated Agreement shall be
deleted in its entirety and replaced with the following:

            "(a) In case the Company shall receive at any time after the earlier
      of (i) June 30, 2007 and (ii) six (6) months after the effective date of
      the first registration statement filed by the Company covering an offering
      of any of its securities to the public, a written request from Holders
      ("Initiating Holders") that the Company effect any registration,
      qualification or compliance with respect to at

                                      -2-

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      least forty percent (40%) of the Registrable Securities (or any lesser
      number of Registrable Securities if the anticipated aggregate offering
      price to the public from the proposed sale of such Registrable Securities,
      net of underwriting discounts and commissions, would exceed $8,000,000),
      the Company will:"

      3. Remainder of Tenth Restated Agreement Unchanged. Except as amended by
this Amendment, the Tenth Restated Agreement shall otherwise remain in full
force and effect.

      4. Governing Law. This Amendment shall be governed by and construed under
the laws of the State of California, without regard to its conflicts of laws
provisions.

      5. Counterparts. This Amendment may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

      6. Facsimile Signatures. This Amendment may be executed and delivered by
facsimile and upon such delivery, the facsimile signature will be deemed to have
the same effect as if the original signature had been delivered to the other
parties. The original signature copy shall be delivered to the other parties by
mail. The failure to deliver the original signature copy and/or the non-receipt
of the original signature copy shall have no effect upon the binding and
enforceable nature of this Amendment.

      IN WITNESS WHEREOF, the undersigned Investors and the Company have
executed this Amendment as of the day and year first above written.

                                ALSIUS CORPORATION,
                                a California corporation

                                By  /s/ William J. Worthen
                                    -------------------------------------
                                              William J. Worthen
                                    President and Chief Executive Officer

                                Address: 15770 Laguna Canyon Road
                                         Suite 150
                                         Irvine, CA 92618-3111

                                      -3-

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                                          MAYFIELD VIII

                                          By:  /s/ Kevin Fong
                                               -------------------------------

                                          Name: Kevin Fong

                                          Title: Managing Director

                                          Address: 2800 Sand Hill Road
                                                   Menlo Park, CA 94025

                                          MAYFIELD ASSOCIATES FUND III

                                          By: /s/ Kevin Fong
                                              -------------------------------

                                          Name: Kevin Fong

                                          Title: Managing Director

                                          Address: 2800 Sand Hill Road
                                                   Menlo Park, CA 94025

                                SIGNATURE PAGE TO
     AMENDMENT NO. 1 TO TENTH AMENDED AND RESTATED INVESTOR RIGHTS AGREEMENT

                                      -4-

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                                          FEVER TRUST

                                          By: /s/ James Beck
                                              -------------------------------

                                          Name: James Beck

                                          Title: Administrative Trustee

                                          Address: 2800 Sand Hill Road
                                                   Menlo Park, CA 94025

                                          FEVER TRUST II

                                          By: /s/ James Beck
                                              -------------------------------

                                          Name