Defense Date


Document Type


Degree Name

Doctor of Philosophy



First Advisor

Dr. Harry P. Dalton


Nosocomial (hospital-acquired) pneumonia (HAP) continues to be an important cause of morbidity and mortality in the hospital. HAP is the third most common nosocomial infection after urinary tract and surgical wound infections. In addition, HAP is the nosocomial infection with the highest mortality rate. These infections are often difficult to treat, because most are caused by Gram-negative bacilli (GNB) that may be highly resistant to antimicrobial agents. HAPs frequently occur in intensive care patients with underlying lung and/or systemic diseases. Many patients are intubated and are on assisted ventilation. Several sources of infection associated with ventilators or respirators have been described in the past. Most of these sources have been eliminated by improvement in techniques used in the disinfection and cleaning of ventilator equipment. Today, the focus of concern is microbial contamination of the breathing circuit of the ventilator. The Centers for Disease Control (CDC) recommend that the ventilator breathing circuits be changed every 24 hours. The very limited epidemiological and microbiological data from one medical center demonstrate that it may not be necessary to change these circuits as often as every 24 hours. However, before changing this conservative recommendation, more data are needed to establish the safety of changing circuits at longer intervals. The approximate cost of the ventilator circuit is $15. It is estimated that changing ventilator breathing circuits at 48 hours rather than 24 hours would amount to $50,000 per year in savings at the Medical College of Virginia. On a national scale the savings would amount to millions of dollars. Most patients who are placed on ventilatory assistance are supported by continuous volume respirators. Air is humidified when it is passed through a cascade, or wick humidifier. After passage through the humidifier, the gases are delivered to the patient by the inspiratory tubing in the breathing circuit. The inspiratory tubing is connected to the endotracheal tube of the patient by a Yconnector and swivel adaptor. Expired gases from the patient are conducted away by the expiratory tubing which connects to the other limb of the Y-connector. Condensate frequently collects in the respiratory breathing circuit. The warm moist environment of the respiratory circuit is conducive to growth of any microorganisms that may enter the circuit. When the respiratory circuit is contaminated with microorganisms, there is the potential for delivery of bacteria or fungi to the patient's lower respiratory tract. Whether or not infection takes place is determined by one or a combination of several factors including the virulence of the organisms, the size of the inoculum, the presence of foreign bodies in the respiratory tract and the status of host defenses.


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