Neonatal response to control of noise inside the incubator

Pediatric Nursing, Nov-Dec, 2001 by Amy Nagorski Johnson

While the development of neonatal medicine in the area of physiologic care of the neonate has made notable progress over the past 20 years, premature infants continue to be at significant risk for long-term neurologic, cognitive, and behavioral problems (Cherkes-Julkowski, 1998). Research suggests that the stressful nature of the neonatal intensive care unit (NICU) environment may interfere with the infant's brain development, thus contributing to long-term problems directly related to difficult adaptation (Als, 1992). Because medically fragile infants have difficulty adapting to the intensive caregiving environment outside the womb, the identification of stressful stimuli in the environment and interventions to assist adaptation is a critical component of neonatal care.

Environmental noise is a major source of stressful stimulation that can agitate and further complicate medical management of the neonate (Graven, 2000). Because high volume sound levels may be related to the overall morbidity of the premature population, there are a number of sound measurement studies reported in medical and nursing literature. Initial studies by physicians date back to the early 1970s, measuring routine sound levels ranging from 70 to as high as 117 decibels (Long, Lucey, & Philip, 1980; Peltzman, Kitterman, & Ostwald, 1970; Satish & Doll-Speck, 1993). Nursing literature reports comparable environmental sound levels of 70 to 90 decibels and recommend changes in nursing practice to reduce sound levels including: restricting teaching rounds at the bedside, adjusting volume on monitors and alarms, and modifying nursing care activities (DePaul & Chambers, 1995; Gray, Dostal, Ternullo-Retta, & Armstrong, 1998; Zahr & Balian, 1995). Furthermore, findings of Thomas and Martin (2000) suggest that sound intensity not only has negative effects on the neonate, but also can interfere with caregiver communication and job performance in the NICU.

In response to the growing concerns over the intensity of the NICU sound environment and potential effects on the neonate, the American Academy of Pediatrics (AAP) Committee on Environmental Hazards initially recommended that environmental sound levels in the NICU be reduced below 58 decibels (Beckham & Mishoe, 1990). Later, the Committee on Environmental Health of the AAP recommended that NICU sound levels should be monitored by caregivers and maintained at or below 45 decibels (AAP, 1997). During this same time, a multidisciplinary group of physicians, nurses, and researchers formed the Physical and Developmental Environment of the High-Risk Infant Center Study Group on NICU Sound to develop recommendations for practice based on sound research evidence. Specific to the NICU environment, the recommendations included the development of a program of noise control to maintain nursery sound levels at or below the recommended noise criteria and care practices to allow parent-infant interaction (Graven, 2000). The expert review panel concluded that by following their guidelines, the NICU environment would protect sleep, support neonatal physiologic stability, and reduce potential adverse effects on auditory development (Graven, 2000).

The Incubator Environment

There is a common practice of placing medically stable, premature neonates inside incubator environments for extended periods of time for growth and thermoregulation. Studies that focus on sound measurement within the incubator found higher sound levels inside the incubator than in the open NICU environment (Beckham & Mishoe, 1990; Catlett & Holditch-Davis, 1990; Saunders, 1995). Higher sound levels inside the incubator are associated with four factors: noise in the NICU environment, noise from the operating motor in the incubator, noise from caregiving equipment including ventilator and suction tubing, and noise from the neonate (Saunders, 1995). Although the current design of the incubator diminishes noise from the operating motor through the use of an acoustical foam motor mount, this only reduces the sound level inside the empty incubator to 58 decibels when tested in a laboratory environment.

The incubator environment provides a thermally-controlled, external environment for growth of the neonate. Incubators are closed systems that maintain thermal conditions without outside environmental influence, while facilitating nursing care by providing complete visualization of the neonate through plexiglass walls. The NICU and incubator environments are two interacting systems that form the comprehensive sound environment for the neonate. Thus, noise from the NICU permeates the plexiglass walls of the incubator, thereby intensifying the sound environment of the neonate. Within the incubator, there are two distinct sound fields that exist simultaneously: direct and reverberant sound. Direct sound is caused by the neonate or equipment necessary to provide neonatal care. Reverberant sound increases as sound waves reflect off the surrounding surfaces inside the incubator and are dependent on the reflectiveness of materials inside the incubator (Pelton, 1993). With this in mind, the most fundamental approach to noise reduction in the incubator is to reduce the transmission of sound fields (Luce, 1993).