K-Complexes: Interaction between the Central and Autonomic Nervous Systems during Sleep

Abstract

Study objectives: To investigate the relationship between K-complexes (KCs) and cardiac functioning.

Methods: Forty healthy adolescents aged 16-22 y (19 females) participated in the study. Heart rate (HR) fluctuations associated with spontaneous and evoked KCs were investigated on two nights, one with (event-related potential night) and one without auditory tones presented across the night.

Results: There was a clear biphasic cardiac response to evoked and spontaneous KCs, with an initial acceleration in HR followed by a deceleration (P < 0.001). HR acceleration occurred immediately to KCs in response to tones presented in the first third of the interbeat interval, but was delayed a beat when the tone occurred later in the cardiac cycle (P < 0.05). Sex differences were also evident. Pretone baseline HR was higher, and the magnitude of the HR response was blunted and delayed, in female compared to male adolescents (P < 0.001). Also, pretone baseline HR was lower when a tone elicited a KC compared to when it did not (P < 0.001), suggesting that KCs are possibly more likely to be elicited by external stimuli in states of reduced cardiac activation.

Conclusions: The strict dependency observed between KCs and cardiac control indicates a potential role of KCs in modulating the cardiovascular system during sleep. Sex differences in the KC-cardiac response indicate the sensitivity of this measure in capturing sex differences in cardiac regulatory physiology.

Keywords: K-complex; autonomic nervous system; cardiovascular; heart rate; sex differences; sleep.

Figure 1

Representation of the interbeat intervals (IBIs) analyzed for each of the K-complex (KC) conditions (KC+, KC-, and KCSP). For the KCSP condition, the estimated time of tone presentation (dummy tone) was calculated based on the averaged tone-to-N550 peak latency of KC+ trials on the event-relate potential nights. Electroencephalographic (EEG) traces for KC+, KC- and KCSP conditions are displayed with the simultaneous registration of the electrocardiogram (ECG). Pretone (IBI-5-to-IBI-1), tone (IBI0), and posttone (IBIs 1-to-12) IBIs are marked. IBI-4-to-IBI-2 were used as a baseline.

Figure 2

Representation of the beat-to-beat dynamics of the biphasic cardiac response to evoked KCs occurring in artifact-free N2 sleep in males (N = 18) and females (N = 19). Each point reflects the percentage change in heart rate from the pretone baseline level. Vertical bars represent standard errors. IBI = interbeat interval.

Figure 3

Magnitude of the peak in cardiac acceleration (baseline to peak, %) in response to K-complexes evoked by tones (KC+), tones that did not elicit a K-complex (KC-), and spontaneous K-complexes (KCSP). Vertical bars represent standard errors. ***P < 0.001 compared to KC+ and KCSP conditions.

Figure 4

IBI position in which the peak in acceleration occurred as a function of the period within the cardiac cycle in which the tone was presented. Vertical bars represent standard errors. *P < 0.05; ***P < 0.001. IBI = interbeat interval.

Figure 5

Baseline pretone cardiac activity (heart rate, bpm) when K-complexes are elicited (KC+) or not (KC-) by tones in both males and females. Vertical bars represent standard errors. Asterisk represent significant differences (P < 0.001) between KC+ and KC- conditions, and between sexes.