Postural influence on intracranial and cerebral perfusion pressure in ambulatory neurosurgical patients
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Tags: physiology, haemodynamics,
Abstract
We evaluated postural effects on intracranial pressure (ICP) and cerebral perfusion pressure (CPP: mean arterial pressure (MAP) − ICP) in neurosurgical patients undergoing 24-h ICP monitoring as part of their diagnostic workup. We identified nine patients (5 women, age 44 ± 20 yr; means ± SD), who were “as normal as possible,” i.e., without indication for neurosurgical intervention (e.g., focal lesions, global edema, abnormalities in ICP-profile, or cerebrospinal fluid dynamics). ICP (tip-transducer probe; Raumedic) in the brain parenchyma (n = 7) or in the lateral ventricles (n = 2) and cardiovascular variables (Nexfin) were determined from 20° head-down tilt to standing up. Compared with the supine position, ICP increased during 10° and 20° of head-down tilt (from 9.4 ± 3.8 to 14.3 ± 4.7 and 19 ± 4.7 mmHg; P < 0.001). Conversely, 10° and 20° head-up tilt reduced ICP to 4.8 ± 3.6 and 1.3 ± 3.6 mmHg and ICP reached −2.4 ± 4.2 mmHg in the standing position (P < 0.05). Concordant changes in MAP maintained CPP at 77 ± 7 mmHg regardless of body position (P = 0.95). During head-down tilt, the increase in ICP corresponded to a hydrostatic pressure gradient with reference just below the heart, likely reflecting the venous hydrostatic indifference point. When upright, the decrease in ICP was attenuated, corresponding to formation of a separate hydrostatic gradient with reference to the base of the skull, likely reflecting the site of venous collapse. ICP therefore seems to be governed by pressure in the draining veins and collapse of neck veins may protect the brain from being exposed to a large negative pressure when upright. Despite positional changes in ICP, MAP keeps CPP tightly regulated.
Notes
Annotations
(9/4/2022, 6:58:13 PM)
“ICP depends on the volume of cerebral blood and CSF for which the vascular component is influenced by systemic blood pressure, modified by cerebral autoregulation and venous outflow resistance (12), while the CSF component is described by ../../Knowledge/Medicine/Davson’s equation (13) as a balance between
- CSF formation,
- CSF outflow resistance, and
- venous pressure in the sagittal sinus.” Go to annotation (Petersen et al., 2016, p. 100)
“During head-down tilt, the ↑ ICP was greater than the decrease caused by head-up tilt suggesting formation of a smaller hydrostatic gradient possibly caused by collapse of major neck veins. Regulation of systemic blood pressure maintained CPP regardless of body position” Go to annotation (Petersen et al., 2016, p. 102)
“During head-down tilt, ICPMidbrain increased corresponding to a hydrostatic fluid pressure from a column of some 35 cm, i.e., with a reference just below the heart and corresponding to the location of the HIPvein (31). It therefore seems that, given an open venous system, the pressure is transmitted to the brain. Conversely, the decrease in ICP during head-up tilt angles above 20° was attenuated and when upright, ICPMidbrain correlated to a fluid column of only 12–15 cm, likely reflecting the site of internal jugular venous collapse (14, 18; Fig. 2) and corresponding to where CSF pressure is considered to be zero (14). Had the veins remained open, ICP would have been expected to reach more negative values (ICPExpected; Fig. 1). In the supine position, the internal jugular veins constitute the primary route of drainage from the brain, but in upright postures these veins respond to the decreasing transmural pressure and collapse thus acting as Starling resistor (20). Cerebral drainage thereby depends increasingly on alternative pathways such as the vertebral venous system (18, 39, 43), which is believed to remain open and thus could constitute a continuous hydrostatic fluid column and support rather large negative pressures (2, 5). Furthermore, in the standing position, CSF is displaced from the skull to the spinal compartment (25). While both displacement of CSF and pressure of alternative venous systems is likely to play a role for ICP, we consider from the present data that overall positional ICP is governed predominantly by pressure in the venous sinus, which is in turn influenced by pressure in the internal jugular veins. Collapse of neck veins in upright postures, i.e., the internal jugular veins, therefore, seems to counteract development of large negative pressures within the brain” Go to annotation (Petersen et al., 2016, p. 102)
“Hydrostatic pressure gradient according to ICP at different angles of tilt. During head-up tilt, ICP corresponded to a hydrostatic gradient with reference at the base of the skull, likely reflecting the site of venous collapse, while during head-down tilt the increase in ICP corresponded to a hydrostatic pressure gradient with reference to just below the heart, likely reflecting the venous hydrostatic indifference point.” Go to annotation (Petersen et al., 2016, p. 102)
“CPP is tightly regulated within a range of some 75– 80 mmHg despite positional variation in ICP” Go to annotation (Petersen et al., 2016, p. 103)