Why are they using a ventilator on my loved one in ICU?

There are many reasons for a person in the ICU to be placed on a ventilator. I would have to know more about your specific loved one's condition to comment on why he/she is on a ventilator but I can offer some possible reasons. Most often a patient is placed on a ventilator to assist in their breathing (oxygenation). This may be due to respiratory conditions such as Pneumonia (Infection in the lungs), or Pulmonary Edema (fluid in the lungs), or Chronic Obstructive Pulmonary Disease (Emphysema related to smoking resulting in constricted airway), or Acute Asthma. Patients who experience cardiac events such as a Heart Attack (Myocardial Infarction) or Congestive Heart Failure may also be placed on a ventilator. A patient who experienced a severe head injury or trauma may be on a ventilator and/or placed in a medically induced coma to allow for healing. A ventilator takes over the breathing for the patient. But there are different settings. Patients may be totally dependent on the ventilator or may be able to do some breathing on their own. The ventilator has different settings based on the individual patient's needs. The use of the ventilator via an endotracheal tube (breathing tube in mouth) or tracheostomy (breathing tube in trachea through a stoma in the neck) also enables the health care practitioner to suction out secretions that impeded oxygen exchange in the lungs. To prevent the patient from becoming ventilator dependent there is a weaning process that is initiated as the patient's condition improves.