Besides helping infants’ lungs mature, sighs may “reset” breathing regulation mechanism

As if new parents don’t have enough to worry about getting in synch with their infants, many of them wonder “what’s with these sighs?”

Of course adults sigh; not as often as infants, though sometimes often enough to annoy others. But with an adult, you can ask if there’s a problem. With newborns, it’s hard to know if it’s good or bad, partly because how often they occur can vary from once every 50 breaths to every 100 or more. The good news is, sighs are good. The issue is, how good, and exactly what purpose do they serve?

It’s been pretty well documented by researchers that deep inspirations, the technical phrase for sighs, play a significant role in helping the mechanical function of healthy lungs. Infants actively regulate their lung function, elevating lung volume above what would be expected by their relatively stiff lung tissue and floppy chest wall compared to adults.

These seemingly “spontaneous” deep breaths often help re-open parts of the lung, especially very small airways, or alveoli, which are prone to collapse. Thus sighs help keep the lungs inflated making them more efficient and possibly also improving the exchange of gases.

On the clinical side, abnormalities of breathing control are occasionally seen in adults and infants, especially during illness. Premature babies often have transient pauses in their breathing pattern, called “periodic breathing.” When these pauses last for an abnormally long period, they are known as “apneas.” It is these excessively long pauses that may reflect some degree of abnormal function in the system that controls breathing. In its most severe form, this may be seen in conditions such as sudden infant death syndrome (SIDS).

Researchers at the University Children’s Hospital in Bern, Switzerland, and in Perth, Australia, wanted to know if in addition to mechanical benefits, sighs also helped development of communications between the respiratory control center, a specialized group of cells in the brainstem, and the lungs themselves.

They studied 25 healthy one-month-old infants during quiet sleep, while they were either lying in a crib or in a parent’s arms. The researchers hypothesized that sighs may play a crucial role in resetting mechanisms for the negative feedback system responsible for controlling breathing, somewhat like hitting the CTRL+ALT+DEL sequence on a computer. While constantly monitoring heart rate and oxygen saturation during the cross-sectional observational study, the researchers measured the regulation of breathing, including breath-to-breath tidal volume, minimum exhaled oxygen and end-tidal carbon dioxide.

The study, entitled “Effects of sighs on breathing memory and dynamics in healthy infants,” is published online in the Journal of Applied Physiology, one of 14 peer-reviewed journals published by the American Physiological Society.

Lead author David N. Baldwin works at the Department of Pediatric Respiratory Medicine, University Children’s Hospital, Bern, Switzerland and the Institute for Child Health and Centre for Child Health Research, Perth, Western Australia, Australia; J. Jane Pillow is also at Australia’s Institute for Child Health, Perth; Bela Suki is at the Department of Biomedical Engineering, Boston University, Boston, Mass.; and Hanna L. Roiha, Stefan Minocchieri and Urs Frey are at University Children’s Hospital, Bern.

The research in this study was supported in part by Swiss National Fund Grant (No. 32-68025.02). Partial support to Dr. Baldwin was provided by Swiss Government Grant (No. 2002-0041) and European Respiratory Society Long-Term Research Fellowship (LTRF 2002-032).

The data showed that a “highly regulated and stable negative feedback loop system exists in healthy infants and that sighs represent a mechanism for improving the memory associated with neuro-respiratory control of breathing….” They explain that prior to a sigh, infant breathing seems to become more regular, which may not be the best pattern. However “the sigh itself introduces a significant increase in variability of both tidal volume and minute ventilation,” which measures both respiratory rate and tidal volume. This variability lasts for about 15 breaths, they found, “after which time the system returns rapidly to a baseline level of fluctuation.”

The researchers say that the breathing pattern in young infants, a mathematical rhythm of sorts, is controlled by a long-term memory, which is necessary for a kind of homeokinesis that leads to a breathing rate of about 40 breaths a minute. Initially, they were somewhat surprised to find that this long-term memory is not affected by a sigh, though since its role is to assure long-term stability of breathing control, this makes sense. But the short-term memory, which controls breath-to-breath variability, is affected by the sigh, and seems to prepare the breathing “system” for any changes in the environment, such as noise, movement, or waking up.

Despite the recent increase in the study of infant breathing, the exact factors that cause sighing, and the total affect they have on infant development is still unclear. One intriguing area of research is to see whether sighs serve some sort of function to link the breathing control mechanism and mechanical maturation.

Also they note that sick infants and premature infants seem to sigh more often than normal babies. Thus another research area could look into how the control mechanisms and communication functions as well as short- and long-term memories may differ in these circumstances. The very long-term goal then might be to see if it’s possible to identify those premature infants who are most at risk for problems of abnormal breathing control, including SIDS.