Transfer function between intracranial pressure and aortic blood pressure and carotid blood flow

The compliance of the intracranial space is such that the intracranial pressure (ICP) waveform is approximately the aortic blood pressure (BP) waveform minus the dominant harmonic (heart rate (HR)) of that waveform. This has been tested across species of different resting HR. Whether this filter characteristic holds true for large changes in HR, or under changed ICP, has not been examined. This study tested these changes in 11 anesthetized rats instrumented to measure and change ICP, HR, aortic BP, and carotid blood flow. The aortic BP to ICP and carotid blood flow to ICP transfer function was determined for normal ICP at paced HRs of 300, 400 and 500 bpm, and at raised ICP at HRs of 400 and 500 bpm. The aortic BP to ICP transfer function magnitude showed a dependency on HR HR (-0.15±0.04 ×10^-3 bpm^-1, p<0.001) and mean ICP (4.4±0.6 ×10^-3 mmHg^-1). The carotid blood flow to ICP transfer function magnitude showed a dependency on mean ICP (11.1±1.8 ×10^-3 mmHg/ml/min/mmHg, p<0.001) but not on HR. The changes with different HRs indicates a degree of non-linearity in the system. Though small, this may need to be accounted for in understanding the relationship between systemic BP and flow and ICP. This data is useful in understanding the relationship between cardiovascular signals and ICP, valuable in advancing the ability to estimate ICP non-invasively.