Why does increased icp cause bradycardia

Brain edema usually results from increased capillary pressure or actual damage to the capillary walls that allows them to leak. As the brain starts to swell, two things begin to occur. First, edema begins to compress the blood vessels supplying the brain. This compression results in reduced blood flow to the brain and ultimately brain ischemia. The ischemia will then cause the arteries leading to the brain to dilate, causing an additional increase in capillary pressure and a further increase in intracranial pressure.

The increased capillary pressure worsens edema See Figure 1. Second, decreased cerebral blood flow to the brain subsequently decreases oxygen delivery to the sensitive brain tissues. This reduces the ability of the capillaries in the brain to function normally and results in increased capillary permeability and leakage.

This allows sodium to enter the cells of the brain, causing cellular edema and ultimately cell death. Blood flow to the brain is directly related to cerebral perfusion pressure CPP , which can be defined as follows:.

Thus, to perfuse the brain, the arterial blood pressure represented as mean arterial pressure must be greater than the intracranial pressure. Otherwise, the blood could not be pushed into the cranial vault and the brain would not be perfused. The hypothalamus activates the sympathetic nervous system, causing peripheral vasoconstriction and an increase in cardiac output.

The catheter was connected via a cm long 1. The heart rate was monitored continuously via the ECG. All patients remained in the supine position with the head flexed, so that the burr hole was located at the apex. Patients were kept normothermic by a forced-air warming system. Once stable profiles of capnography and blood pressure were reached, ventilatory and drug delivery settings were kept unchanged.

A rigid Caemaert endoscope Wolf, Knittlingen, Germany with an outer diameter of 6 mm was used. After positioning the patient and infiltrating with local anaesthetic, a burr hole was made at the classical point for endoscopic entry to the lateral ventricle and the standard neuroendoscopical introduction was performed.

The outlet of the endoscope flushing system was connected by a cm long pressure tube to a pressure transducer for continuous monitoring of the intracranial pressure. During the introduction of the endoscope the optical element was already inserted in the correct channel. We then began irrigation with Ringer lactate at body temperature. We made sure that the distal end of the outflow tube was fixed at the same level as the burr hole, so that there was no siphoning effect or raised intracranial pressure.

At moderate flushing rates of the endoscope, the pressure value reliably represents the intracranial pressure at the bottom of the fourth ventricle, as long as no obstruction or increased resistance occurs inside the endoscope. The inflow of the rinsing fluid is managed by the surgeon. Using the same zero reference point for both transducers allows a precise determination of the cerebral perfusion pressure, independent of patient positioning.

Both systems were calibrated against atmospheric pressure and both pressure transducers were connected to an S5-monitor Datex-Ohmeda, Helsinki, Finland. All variables were recorded numerically at 0. In addition, ECG, invasive arterial pressure and intracranial pressure waveforms were registered at Hz. The waveforms were analysed using invasive arterial pressure Art and intracranial pressure ICP signals.

The cerebral perfusion pressure was calculated as the difference between mean arterial pressure and mean intracranial pressure. High-resolution waveforms at Hz were visualized for detailed description of haemodynamic phenomena. In addition, trend curves were created at 1 Hz for whole-procedure evaluation of haemodynamic effects.

The four algorithms are given in the Appendix. All data were analysed for possible events: different classes of combined events were defined as shown in Table 2. For changes in cerebral perfusion pressure, we defined an event as a decrease in cerebral perfusion pressure lower than 50 mm Hg. Subsequent categories were defined as a cerebral perfusion pressure below 50, 40, 30, 20 and 15 mm Hg.

The baseline values were defined as the mean values in the minutes during the procedure before an increase of the intracranial pressure occurred. Since the administration of remifentanil was kept constant and tolerance 10 for its analgesic and haemodynamic effects 11 may develop, rescaling the baseline values was sometimes necessary.

In this assessment, we defined a Cushing reflex as a simultaneous occurrence of hypertension and tachycardia. The sensitivity of a Cushing reflex to detect a decrease in cerebral perfusion pressure below a certain value was determined as the ratio of the number of decreases in cerebral perfusion pressure coinciding with a Cushing reflex to the total number of decreases of cerebral perfusion pressure below the specified value.

The specificity of a Cushing reflex in detecting a decrease in cerebral perfusion pressure was defined as the ratio of the number of Cushing reflexes associated with a decrease in cerebral perfusion pressure below a certain level to the total number of observed Cushing reflexes.

For statistical analysis of the data, non-parametric correlations were determined using SPSS All patients awoke in the operating theatre and were directly transferred to the intensive care unit. For assessment of postoperative complications, intensive care files were evaluated for possible events.

As described by Buxton and colleagues, 12 possible complications in neuroendoscopy are delay in waking, pneumoencephalus, pneumoventricle, convulsions, transient anisocoria, transient hemiparesis, haemorrhage, cerebral infarction, transient fever, meningism, infection, short-term memory loss, diabetes insipidus, inappropriate antidiuretic hormone secretion, transient cerebrospinal leaks, chronic subdural haematomas, traumatic basilar aneurysm and hydrocephalus.

The data recorded total recording time The preoperative clinical characteristics of the patients are shown in Table 1. The end-tidal CO 2 concentration and body temperature stayed within target ranges in all patients. Preoperative clinical characteristics of the 17 patients studied. Intracranial HT refers to the possibility, on clinical grounds, for the patient to have pre-operative intracranial hypertension. Hb, haemoglobin; Hct, haematocrit. Table 2 shows the incidence of haemodynamic changes for each patient in the seconds following a decrease in the cerebral perfusion pressure to a certain level, together with the type of procedure performed and the age of the patient.

The incidences of haemodynamic changes with a normal cerebral perfusion pressure are also given Isolated. Hypertension associated with tachycardia was prominently present when the cerebral perfusion pressure dropped below 15 mm Hg. At higher cerebral perfusion pressure levels, the changes were less prominent.

Isolated cases of haemodynamic change are also noted. In the last row, the sum of all the events in the respective category is shown.

The procedures are: revision of a ventriculoperitoneal shunt revision ; diagnostic ventriculoscopy ventr. Figure 1 shows the relation between cerebral perfusion pressure and the relative changes in heart rate and mean arterial pressure.

Figure 2 shows the relation between the intracranial pressure and the relative changes in heart rate and mean arterial pressure. The events from patient 14 are shown by open squares; many of these haemodynamic events occurred during tumour retraction. The sensitivity and specificity for the determination of decreased cerebral perfusion pressure by a Cushing reflex are shown in Table 3.

Because multiple haemodynamic events occurred during tumour retraction in patient 14, haemodynamic changes may have been induced by direct stimulation of the brainstem. However, this was the only patient where severe bradycardia and hypertension were seen during a decreased cerebral perfusion pressure.

Therefore the sensitivity analysis was performed using all patients and also with all patients except patient Because we saw multiple episodes of bradycardia during tumour retraction in patient 14, the sensitivity and specificity is also shown with the events of this patient excluded from the analysis.

We have selected three patients for a detailed description. In the first Fig. At s, the intracranial pressure increased further, resulting in a decrease in the cerebral perfusion pressure and the occurrence of tachycardia and hypertension.

After alerting the surgeon, the pressure was released, leading to normalization of the intracranial pressure, heart rate and blood pressure within s. Figure 3 b shows a slower increase in intracranial pressure accompanied by a marginal increase in mean arterial pressure and an initial moderate decline of the heart rate. At s, a sudden additional increase in intracranial pressure is seen together with abrupt severe bradycardia. After 20 s, the heart rate recovers rapidly. Figure 3 c shows a Cushing reflex in a 3-month-old baby.

The baseline cerebral perfusion pressure was 39 mm Hg since the normal mean arterial pressure is much lower in infants. In this patient, we see hypertensive adaptation at an intracranial pressure of 10 mm Hg and tachycardia at 34 mm Hg.

None of the patients suffered any of the postoperative complications described by Buxton and colleagues. During neuroendoscopic procedures, early recognition of an excessive increase in intracranial pressure, jeopardizing brain perfusion, is of major importance for preserving cerebral homeostasis. Isolated or combined bradycardia and hypertension are commonly used during neuroendoscopy to alert the surgeon to increased intracranial pressure or mechanical stimulation of the floor of the third ventricle.

They hypothesized that the tachycardia coincided mostly with an increase in intracranial pressure caused by high-speed fluid irrigation or obstruction of the outflow tube. The observed tachycardia was nearly always accompanied by systemic hypertension. These signs might be seen as an atypical Cushing response. Unfortunately, intracranial pressure was not recorded in this study because a two-channel cystoscope, which did not allow measurement of the intracranial pressure, was used.

The classic response, as described by Cushing 14 in , consists of apnoea, increased blood pressure and bradycardia. Therefore it might be interesting to perform a prospective evaluation of the adequacy of the occurrence of haemodynamic alterations for assessing the cerebral perfusion status during these procedures by simultaneously measuring heart rate and arterial and intracranial pressure.

We investigated the association between cerebral perfusion pressure and the occurrence of haemodynamic changes. In our opinion, this was defensible and desirable since we were focusing mainly on the fast haemodynamic changes following increased intracranial pressure and decreased cerebral perfusion pressure.

Hypertension accompanied by tachycardia occurred in 14 of the 15 patients; a combination of hypertension and bradycardia was observed in the remaining case. Therefore a sensitivity and specificity analysis was performed as shown in Table 3.

Furthermore, the single case where no tachycardia was observed was in patient 14 during tumour retraction, where we saw an abrupt bradycardia which may have prevented the emergence of tachycardia because of direct stimulation.

A decrease in the cerebral perfusion pressure to 15—30 mm Hg often results in a Cushing reflex, but frequently causes hypertension without tachycardia or bradycardia. A decrease in the cerebral perfusion pressure to 30—40 mm Hg almost never results in a Cushing reflex.

When herniation of the brain is imminent, loss of extraocular movement will occur, with the pupils dilating, becoming unreactive and turning outward. To summarize, with increases in intracranial pressure, the Cushing response begins with a rise in systolic blood pressure, widening pulse pressure, bradycardia and irregular breathing. Left uncorrected, the heart rate will increase, breathing will become shallow with periods of apnea, and the blood pressure will begin to fall.

Eventually the patient will develop an agonal rhythm. Brain stem activity will cease when herniated, and the patient will experience cardiac and respiratory arrest. Harvey Cushing, a pioneer of neuroanesthesia. J Anesth 22 4 : —6, Surgical stress exposes an asymptomatic sick sinus syndrome: Diagnostic and management dilemmas. Barker E. Avoiding increased intracranial pressure. Nursing 20 5 : 64Q—64RR, May Robert E.

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