Cardiac output in idiopathic normal pressure hydrocephalus: association with arterial blood pressure and intracranial pressure wave amplitudes and outcome of shunt surgery

Abstract

Background: In patients with idiopathic normal pressure hydrocephalus (iNPH) responding to shunt surgery, we have consistently found elevated intracranial pressure (ICP) wave amplitudes during diagnostic ICP monitoring prior to surgery. It remains unknown why ICP wave amplitudes are increased in these patients. Since iNPH is accompanied by a high incidence of vascular co-morbidity, a possible explanation is that there is reduced vascular compliance accompanied by elevated arterial blood pressure (ABP) wave amplitudes and even altered cardiac output (CO). To investigate this possibility, the present study was undertaken to continuously monitor CO to determine if it is correlated to ABP and ICP wave amplitudes and the outcome of shunting in iNPH patients. It was specifically addressed whether the increased ICP wave amplitudes seen in iNPH shunt responders were accompanied by elevated CO and/or ABP wave amplitude levels.

Methods: Prospective iNPH patients (29) were clinically graded using an NPH grading scale. Continuous overnight minimally-invasive monitoring of CO and ABP was done simultaneously with ICP monitoring; the CO, ABP, and ICP parameters were parsed into 6-second time windows. Patients were assessed for shunt surgery on clinical grade, Evan's index, and ICP wave amplitude. Follow-up clinical grading was performed 12 months after surgery.

Results: ICP wave amplitudes but not CO or ABP wave amplitude, showed good correlation with the response to shunt treatment. The patients with high ICP wave amplitude did not have accompanying high levels of CO or ABP wave amplitude. Correlation analysis between CO and ICP wave amplitudes in individual patients showed different profiles [significantly positive in 10 (35%) and significantly negative in 16 (55%) of 29 recordings]. This depended on whether there was also a correlation between ABP and ICP wave amplitudes and on the average level of ICP wave amplitude.

Conclusions: These results gave no evidence that the increased levels of ICP wave amplitudes seen in iNPH shunt responders prior to surgery were accompanied by elevated levels of ABP wave amplitudes or elevated CO. In the individual patients the correlation between CO and ICP wave amplitude was partly related to an association between ABP and ICP wave amplitudes which can be indicative of the state of cerebrovascular pressure regulation, and partly related to the ICP wave amplitude which can be indicative of the intracranial compliance.

Figure 1

Illustration of continuous monitoring in patient #5. Recordings of three consecutive 6-s time windows (time windows 364-366) from a patient, showing (a) the averaged cardiac output (CO) over each 6-s time window, (b) the arterial blood pressure (ABP) signal, and (c) the intracranial pressure (ICP) signal for the same time period. For each of the 6-s time windows the calculated CO, mean ABP wave amplitude (MWA_ABP), and mean ICP wave amplitude (MWA_ICP) is presented in (d).

Figure 2

Cardiac output (CO) arterial blood pressure (ABP) and intracranial pressure (ICP) wave amplitudes plotted against post-shunt changes in NPH score. The association between change in NPH score 12 months after shunting and the pre-operative overnight average measurements of (a) CO (Pearson correlation coefficient 0.24; ns), (b) ABP wave amplitude (Pearson correlation coefficient 0.21; ns), and (c) ICP wave amplitude (Pearson correlation coefficient 0.61; P = 0.003) are presented for the 22 shunted iNPH patients.