Fig. 2.

Comparative montages of systolic upstrokes in central and non-central apnea. Upper trace—a montage of superimposed pulses during a central apnea illustrating a low level of variability of the normalized systolic upstroke waveforms. Lower trace—the contrasting pattern of systolic upstrokes during non-central apneas illustrating a high level of variability. In both cases, pulses are synchronized at the foot of the upstroke signals