Abstract
Respiratory sinus arrhythmia (RSA) is related to cardiac vagal outflow and the respiratory pattern. Prior infant studies have not systematically examined respiration rate and tidal volume influences on infant RSA or the extent to which infants' breathing is too fast to extract a valid RSA. We therefore monitored cardiac activity, respiration, and physical activity in 23 six-month old infants during a standardized laboratory stressor protocol. On average, 12.6% (range 0–58.2%) of analyzed breaths were too short for RSA extraction. Higher respiration rate was associated with lower RSA amplitude in most infants, and lower tidal volume was associated with lower RSA amplitude in some infants. RSA amplitude corrected for respiration rate and tidal volume influences showed theoretically expected strong reductions during stress, whereas performance of uncorrected RSA was less consistent. We conclude that stress-induced changes of peak-valley RSA and effects of variations in breathing patterns on RSA can be determined for a representative percentage of infant breaths. As expected, breathing substantially affects infant RSA and needs to be considered in studies of infant psychophysiology.
Highlights
The respiratory system modulates autonomic outflow to various organ sites, including the heart, trachea, bronchi, and blood vessels [1,2]
Note: HR = heart rate; VT = tidal volume; Respiratory sinus arrhythmia (RSA) = respiratory sinus arrhythmia; TTOT = total respiratory cycle time; HR = heart rate; RSA/VT = RSA normalized by VT; c = adjusted for TTOT; logRSA = logarithm(RSA+1); logRSA/VT = logarithm(RSA/VT)+1. doi:10.1371/journal.pone.0052729.t002
Note: P = Play episode; SF1 = Still-Face episode 1; SF2 = Still-Face episode 2; TTOT = total respiratory cycle time; VT = tidal volume; RSA = respiratory sinus arrhythmia; logRSA/VTc = logarithm of RSA normalized by VT (logarithm(RSA/ VT)+1), adjusted for TTOT; HR = heart rate. {p-level of both t-tests for each index are Bonferroni-adjusted in this subgroup. doi:10.1371/journal.pone.0052729.t003
Summary
The respiratory system modulates autonomic outflow to various organ sites, including the heart, trachea, bronchi, and blood vessels [1,2]. Respiratory sinus arrhythmia (RSA), or its frequencydomain equivalent, high-frequency heart rate variability, is often used as an index of cardiac vagal activity [3,4,5,6]. The peak-valley (or peak-to-trough) method provides a common time-domain index of RSA [4,5] which is extracted breath-by-breath by subtracting the minimum heart rate (HR) during expiration from the maximum HR during inspiration; or, alternatively, it can be calculated with the cardiac inter-beat interval (IBI), i.e., minimum IBI during inspiration subtracted from maximum IBI during expiration. Compared to frequency domain analyses, the peakvalley method has the advantage of providing an RSA index that allows for a breath-by-breath analysis and can be extracted for very short time frames. Under adequate conditions the two methods are often highly correlated [7]
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