Spectral characteristics of B-waves and other low-frequency activity

Low frequency oscillations of intracranial pressure (ICP) between 0.5 to 2.0 cycles/min have been termed B-waves. While such low frequency activity may be generated by cerebral vasomotor activity, activity in the B-wave band may also be related to other causes. The objective of this study is to: 1) describe the low frequency characteristics of ICP and arterial blood pressure (ABP) recordings with a mathematical model based on ventilator-induced intrathoracic pressure (ITP) modulation of arterial blood pressure; and 2) use the model to eliminate low frequency activity within the B-wave range unrelated to vasomotor activity. The model describes the frequency locations of spectra associated with ventilation about both the principal and first harmonic cardiac frequency to within 1% error. Spectra within the B-wave range harmonically related to the ventilator frequency were classified as caused by patient-ventilator interaction. Of 11 patients with severe head-injury, 64% (7/11) demonstrated B-wave activity, and 4 patients demonstrated the activity solely in the ICP recording. Variation of heart rate did not correspond to the occurrence of B-waves. The proposed model describes the low frequency spectra found within ABP and ICP recordings and can be used to eliminate spectra with the B-wave range related to ventilation.