(Reuters) - Shares of Envision Healthcare Holdings Inc (>> Envision Healthcare Holdings Inc), the biggest operator of U.S. ambulance services, rose 10 percent in their market debut, giving the firm a market valuation of about $4.24 billion.

Envision, formerly known as Emergency Medical Services Corp, priced its initial public offering of 42 million shares at $23 per share - at the high end of the expected range of $20 to $23.

The company raised $966 million from the IPO, making it the biggest healthcare offering since Zoetis Inc's (>> Zoetis Inc) IPO in February. Zoetis, Pfizer Inc's (>> Pfizer Inc.) former animal health unit, raised $2.2 billion.

Envision was founded in 2005 after Canadian private equity firm Onex Corp (>> ONEX Corporation) bought medical transportation provider American Medical Response.

Apart from medical transportation, the Greenwood Village, Colorado company that was taken private for nearly $3 billion by Clayton, Dubilier & Rice in 2011, also has a presence in hospital/inpatient care, radiology surgery and tele-medicine.

Envision, which is banking on growing demand for outsourced medical services amid soaring healthcare costs and an aging U.S. population, reported 2012 net revenue of $3.3 billion and net income of $41.19 million.

As more and more people gain health insurance under President Barack Obama's Affordable Care Act, Envision expects its customer base to grow. Uninsured patients accounted for about 18.3 percent of the company's total patient volume in 2012, Envision said in a regulatory filing.

While the company holds 8 percent of the market for outsourced medical services, it sees plenty of opportunity to expand through partnerships and acquisitions.

Returns on healthcare IPOs this year are at an average of 43.9 percent since their debut date, far exceeding the Nasdaq Composite year-to-date return of 18.4 percent, and the S&P's 18.6 percent, according to accounting and actuarial firm PricewaterhouseCoopers.

Goldman Sachs, Barclays Capital, Merrill Lynch, Pierce, Fenner & Smith and Citigroup were lead underwriters to Envision's offering.

Envision shares touched a high of $25.64 after opening at $25.14 on the New York Stock Exchange.

(Reporting by Neha Dimri and Zeba Siddiqui in Bangalore; Editing by Maju Samuel)