MAZAR-I-SHARIF, Afghanistan (Reuters) - A Spanish physiotherapist working for the Red Cross in the northern Afghan city of Mazar-i-Sharif was shot dead on Monday, apparently by a hospital patient, officials said. Police said two arrests had been made and an investigation was underway. The International Committee of the Red Cross said Lorena Enebral Perez, 38, was killed in its orthopedic rehabilitation center in Mazar-i-Sharif.   Energetic and full of laughter, Lorena was the heart of our office in Mazar. Today, our hearts are broken,  said the ICRC s head of delegation in Afghanistan, Monica Zanarelli. Perez s work in Afghanistan involved helped people, including children, who had lost legs or arms, mostly in the war, to learn to walk again or feed themselves.    Sheer Jan Durani, a spokesman for the police chief in Balkh province, said two patients were admitted to hospital and one took out a pistol apparently concealed in a wheelchair and shot the woman. Both men were arrested, he said. Afghanistan is one of the most dangerous countries in the world for aid workers, with groups including the ICRC often facing attacks on their staff, both Afghan and foreign. Perez s death followed the killing in February of six ICRC staff in northern Afghanistan in an attack in which two other Afghan employees were abducted. The kidnapped Afghan staff were released six days ago, the ICRC statement said. The ICRC has seven rehabilitation centers in Afghanistan that manufacture more than 19,000 artificial limbs per year and treat hundreds of thousands of patients. 