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Vol. 9, No. 7
July 2004


TRANSFUSION IS A VAP RISK FACTOR

Key Point:
The risk of ventilator-associated pneumonia is significantly increased in patients who are given packed red blood cell transfusions.

WASHINGTON, DC—Now there is one more issue to consider when weighing the risks and benefits of packed red blood cell (pRBC) transfusion in mechanically ventilated patients: It appears to increase the risk of ventilator-associated pneumonia (VAP).

In a recent study of 1,518 mechanically ventilated patients, none of whom had preexisting pneumonia, pRBC transfusion was a strong independent risk factor for VAP; this held true regardless of whether the volume transfused was large or small.[1] Even more worrisome was the finding that pRBC transfusion markedly elevated the risk of late-onset VAP, a more severe form of pneumonia that develops after patients are on the ventilator for five or more days.

“There is a tendency for blood transfusion to be thought of as benign,” remarked lead investigator Andrew F. Shorr, MD, MPH, in an interview with Pulmonary Reviews. “But our data suggest that it may be harmful in a way that had not been previously considered,” said Dr. Shorr, Chief of the Pulmonary Clinic at the Walter Reed Army Medical Center in Washington, DC.

SECONDARY ANALYSIS OF CRIT DATA

Because no previous study had specifically explored the relationship between pRBC transfusion and VAP, Dr. Shorr and colleagues conducted a secondary analysis of data collected during the CRIT trial, a prospective observational investigation of current transfusion practice in US intensive care units. Of the 4,892 patients in the CRIT trial, only those who received at least 48 hours of mechanical ventilation and who did not have preexisting pneumonia were eligible for inclusion in the secondary analysis.

In the final cohort of 1,518 patients, there were 311 cases of VAP; of these cases, 40% were late-onset. The diagnosis of VAP was established based on the presence of fever, leukocytosis, purulent secretions, and a new infiltrate on the chest radiograph. The mean duration of mechanical ventilation before VAP diagnosis was 5.1 days.

Among the patients without VAP, the pRBC transfusion rate was 51.4%; that rate was significantly higher—58.2%—in the patients with VAP and dramatically higher—71.0%—in the subgroup with late-onset VAP. The difference in pneumonia risk could not be explained by the patients’ ages, admitting diagnoses, or severity of illness; after these and other confounding variables were controlled for, pRBC transfusion remained an independent risk factor for VAP.

The age of the pRBCs appeared to have no bearing on VAP risk. Whether the number of units administered affected the extent of the risk was unclear. The odds ratio (OR) for VAP was similar (about 1.9) in the patients who were given only one or two units and in those who had received more than two units of blood. However, when the number of units transfused was examined as a continuous variable, a trend toward a dose-response relationship was observed.

Among the patients with late-onset VAP, the OR for pneumonia was 1.96 for those who had been given one or two units of pRBCs and 2.37 for recipients of more than two units. In this subgroup, the incidence of late-onset VAP was significantly and directly associated with the amount of blood transfused.

Other risk factors for VAP that were identified in the study include total parenteral and enteral nutrition, trauma, continuous sedation, and the duration of mechanical ventilation. Antibiotic therapy appeared to protect against VAP, but this finding was not statistically significant.

WHAT IS THE MECHANISM?

Two pathways through which transfusions may increase the infection risk have been postulated:

  • The white cells in transfused blood may facilitate VAP by serving as a foreign antigen that alters T-cell function and induces an immunomodulatory effect.
  • Transfusion may promote the release of proinflammatory and anti-inflammatory cytokines; in addition, high levels of many cytokines may accumulate in donor blood as the blood degrades in storage over time.

 

VENTILATOR CIRCUIT DOES NOT AFFECT VAP RISK

Although packed red blood cell transfusions may need to be added to the list of risk factors for ventilator-associated pneumonia (VAP), at least one thing can be crossed off that list. There is growing evidence that the ventilator circuit is not a particularly important source of infectious microbes that cause VAP.

Thus, the American Association for Respiratory Care has altered its clinical practice guidelines on the care of the ventilator circuit.[1] Key recommendations from the new guidelines are listed below:

  • Ventilator circuits do not have to be changed routinely for infection control purposes. However, the maximum time that a ventilator circuit can safely be left in place is not known.
  • Although the available evidence suggests that the VAP rate is lower with passive humidification than with active humidification, other issues related to the use of passive humidifiers (eg, resistance, dead space volume, and airway occlusion risk) preclude a recommendation for the general use of passive humidifiers.
  • When they are included in the ventilator circuit, passive humidifiers do not need to be changed daily for reasons of infection control or technical performance. They can remain in place for at least 48 hours and, in some patients (eg, those with COPD), for up to one week.
  • The use of closed suction catheters should be considered part of a VAP prevention strategy. These catheters do not need to be changed daily for infection control purposes, but the maximum time they can safely be left in place is not known.
  • There is insufficient evidence about the VAP risk to make recommendations concerning the use of heated versus unheated circuits, the type of heated humidifier included, the method for filling the humidifier, or the technique for clearing condensate from the ventilator circuit. However, it seems prudent to suggest that the following should all be avoided: breaks in the ventilator circuit, excessive accumulation of condensate in the circuit, accidental drainage of condensate into the patient’s airway, and contamination of caregivers during ventilator disconnection or condensate disposal.

—Timothy Begany

Reference
1. Hess DR, Kallstrom TJ, Mottram CD, et al; American Association for Respiratory Care. Care of the ventilator circuit and its relation to ventilator-associated pneumonia. Respir Care. 2003;48:869-879.

 

—Timothy Begany

Reference
1. Shorr AF, Duh M-S, Kelly KM, et al. Red blood cell transfusion and ventilator-associated pneumonia: a potential link? Crit Care Med. 2004;32:666-674.

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