Table 3.

Ventilatory Oscillations

Characteristic	Heart Failure (n = 25)	Controls (n = 25)
Power spectral analysis of feedback amplification*
Oscillatory strength, T
Median (IQR)	1.7 (1.2)	1.4 (0.2)†
Range	1.2–11.3	1.1–2.4
Estimated loop gain, 1 − 1/T	0.46 ± 0.19	0.29 ± 0.11†
Estimated natural frequency, cycles/min	1.7 ± 0.5	2.5 ± 0.6†
Significant resonance detected‡, yes:no, n	24:1	18:7§
Time-domain analysis
Amplitude, % of mean	47 (44)	34 (23)‖
Interpeak interval SD, % of mean	26 ± 8	33 ± 6¶

Definition of abbreviation: IQR = interquartile range (75th percentile − 25th percentile).

Values are mean ± SD or median (IQR) unless otherwise indicated.

^*A resonance model was fit to the ventilation power spectrum to summarize the data. The general model is given by y = S_d(f)/|1 − LG(f)|², where the noise component S_d(f) is assumed to conform to a power law [S_d(f) = βf^−α, where α = exponent, β = offset, and f = frequency], and the chemoreflex influence is described by the simplest possible model [LG(f) = −ke^−i2πfδ/(1 + i2πfτ), where k = gain,τ = time constant, and δ = delay] (41, 50).

^†P < 0.001.

^‡Fisher’s F test compared the resonance model (feedback stimulated by biological noise) to noise (without feedback) for each individual.

^§P < 0.05, Fisher’s exact test.

^‖P < 0.05.

^¶P < 0.01.