During intensive care monitoring of patients with head-injury both arterial blood pressure (ABP) and intracranial pressure (ICP) monitoring is accomplished with a bedside physiologic monitor. The ICP recording may contain low frequency oscillations termed B-wave. The purpose of this paper is to describe a technique to classify the spectral characteristics of B-waves in the ICP recording. A linear modulation model of arterial pressure modulated by ventilator induced pleural pressure changes was devised to describe the spectral characteristics of ICP and ABP laboratory and clinical recordings that did not contain B-waves. ICP recordings that contained B-waves were classified according to whether or not their spectral characteristics were sub-harmonically related to the ventilation frequency. It is suggested that patient-ventilator interaction cause low-frequency oscillations that are sub-harmonically related to the ventilator-frequency. In contrast, the presence of low frequency spectra not related to the ventilation-frequency or evident as side bands at cardiac frequencies likely indicates the occurrence of oscillations of the cerebral vasculature associated with loss of autoregulation of cerebral blood flow.