J. E. Backofen

The Relationship Between Severity of Illness and Hospital Length of Stay and Mortality

To address the question of quantification of severity of illness on a wide scale, the Computerized Severity Index (CSI) was developed by a research team at the Johns Hopkins University. This article describes an initial assessment of some aspects of the validity and reliability of the CSI on a sample of 2,378 patients within 27 high-volume DRGs from five teaching hospitals. The 27 DRGs predicted 27% of the variation in LOS, while DRGs adjusted for Admission CSI scores predicted 38% and DRGs adjusted for Maximum CSI scores throughout the hospital stay predicted 54% of this variation. Thus, the Maximum CSI score increased the predictability of DRGs by 100%. We explored the impact of including a 7-day cutoff criterion along with the Maximum CSI score similar to a criterion used in an alternative severity of illness measure. The DRG/Maximum CSI scores predictive power increased to 63% when the 7-day cutoff was added to the CSI definition. The Admission CSI score was used to predict in-hospital mortality and correlated R = 0.603 with mortality. The reliability of Admission and Maximum CSI data collection was high, with agreement of 95% and kappa statistics of 0.88 and 0.90, respectively.

Relationships of Regional Cerebral Blood Flow, Evoked Potential Responses, and Systemic Hemodynamics During Intracranial Hypertension

Several studies have described changes in somatosensory evoked potentials occurring when cerebral blood flow is reduced below 15 to 20 ml·min-l·100g-1 (Branston et al. 1984; Gregory et al. 1979; Lesnick et al. 1984). These studies generally manipulated arterial perfusion pressure to change regional blood flow. Changes in evoked potentials have also been demonstrated in models of intracranial hypertension employing an expanding mass (Nagao et al. 1979) or fluid infusion into the cerebrospinal fluid (CSF) space (Sohmer et al. 1982). However, the relationship of evoked potential alterations to cerebral blood flow and to cerebral oxygen uptake has not been well-described in these latter models, although regional blood flow is undoubtedly decreased (Nagao et al. 1984). The objective of the present study was to determine the relationships of changes in somatosensory evoked potentials (SEP) and brainstem auditory evoked responses (BAER) to regional cerebral blood flow (CBF) in a model of spatially diffuse intracranial hypertension produced by infusion of artificial CSF. Because the decrease in regional CBF would be expected to be reasonably uniform, one advantage of this model is that changes in global O2 uptake can be calculated from mixed cerebral venous O2 content. Thus, functional changes in SEP can be related to cerebral O2 uptake.

Peripheral Organ Blood Flow Distribution During Raised Intracranial Pressure in Young Lambs

Elevation of intracranial pressure (ICP) is capable of producing a rise in arterial blood pressure as a component of the classic Cushing response. The end-organ response responsible for the rise of arterial blood pressure is thought to involve sympathetically-mediated peripheral vasoconstriction and increased cardiac output. Studies by Brashear and Ross (1970) have shown that beta adrenergic blockage prevents a rise in cardiac output when ICP is raised, and that after alpha adrenergic blockade, increasing ICP produces a fall in systemic vascular resistance.

Autoregulation of Cerebral Blood Flow with Intracranial Hypertension: Effect of Hypoxia

Several studies have demonstrated that hypercapnia interferes with the cerebrovascular autoregulatory response to arterial hypotension (Harper and Glass 1965; Tuor and Farrar 1984). Although arterial hypoxia is also thought to interfere with autoregulation, the limited data on this question is less clear (Haggandal and Johansson 1965).

Importance of Cerebral O2 Extraction Reserve During Elevated Intracranial Pressure in Young Lambs

The cerebral vascular bed autoregulates its blood flow in the face of elevated intracranial pressure (ICP) (Haggendal et al. 1970). With severe increases in ICP sufficient to reduce cerebral perfusion pressure below approximately 50 mm Hg, cerebral blood flow falls. However, it has been shown that cerebral oxygen consumption is maintained in dogs and rhesus monkeys in spite of a fall in cerebral flow (Grubb et al. 1975, Haggendal et al. 1970). This suggests that the brain has an oxygen extraction reserve which plays an important role in maintaining cerebral O2 uptake. Using the sheep as an experimental animal, we evaluated the importance of the oxygen extraction reserve during elevated ICP. We studied both newborn lambs and older lambs, for two reasons. First, it is not known how well cerebral blood flow is autoregulated and how cerebral O2 consumption is affected in the newborn when ICP is elevated. Second, the brain of the newborn lamb normally extracts a smaller fraction of oxygen than the adult sheep (Jones et al. 1982); thus, the newborn may have a greater O2 extraction reserve because it has a lower fractional O2 extraction under normal conditions.

164 EFFECT OF ELEVATED INTRACRANIAL PRESSURE ON CERE BRAL BLOO D FLOW AND EVOKED POTENTIAL RESPONSES

We studied the relationship of cerebral blood flow (CBF) and cerebral O2 uptake (CMRO2) to somatosensory evoked potential (SEP) and brainstem auditory evoked response (BAER) under conditions of elevated intracranial pressure (ICP). Sheep were anesthetized with pentobarbital and pancuronium and ventilated. ICP was increased to a fixed level by infusing mock CSF into the lateral ventricle. ICP was raised to a calculated cerebral perfusion pressure of either 0, 20-25, or 50 mmHg. CBF was measured using the radiolabelled microsphere technique. CMRO2 was calculated with sagittal sinus blood samples. When CBF fell, cerebral O2 extraction increased. However, with CBF below 70% of baseline, CMRO2 was not sustained by increased extraction. BAER interwave I to V latency increased below a mid-brain blood flow of 15 ml·min−1. 100g−1. SEP central conduction time (CCT) determined from the latency differences between N1 of foreleg SEP and C2 increased below a CBF threshold of 15-20 ml·min−1·100g−1 (50-65% reduction from baseline CBF). Changes in CCT were associated with a 25% decrease in CMRO2 from baseline. Therefore, under conditions of elevated ICP, cerebral ischemia as defined by CMRO2 appears to correlate with changes in evoked potential responses. The large threshold observed when evoked potentials are related to regional CBF is probably a function of the O2 extraction reserve.

165 CEREBRAL BLOOD FLOW AUTOREGULATION WITH ELEVATED INTRACRANIAL PRESSURE DURING NORMOXIA AND HYPOXIA

We assessed the ability to maintain cerebral blood flow (CBF) when intracranial pressure (ICP) is raised under normoxic and isocapnic hypoxic conditions in pentobarbital-anesthetized neonatal lambs (3-9 days old). ICP was increased by infusion of artificial CSF in the lateral ventricle to produce 8 mmHg stepwise decrements in cerebral perfusion pressure (CPP) from baseline (&asymp 66 mmHg) down to a CPP of approximately 26 mmHg. In one group (n=6) of normoxic lambs (arterial O2 content (CaO2) = 14.8 ± .7 ml/dl), CBF (ml·min “100g−1; microspheres) was unchanged from baseline (49 ± 7) down to a CPP of 34 mmHg (47 ± 6). At a CPP of 26 mmHg, CBF was decreased to 37 ± 5. In another group (n=6) of hypoxic lambs (CaO2 = 7.6 ± .9 ml/dl; 49 ± 4% arterial O2 saturation) baseline CBF (87 ± 11) was nearly twice that of the normoxic group. CBF was not significantly changed down to a CPP of 34 mmHg (78 ± 8), but was decreased at a CPP of 26 mmHg (73 ± 8). Cerebral fractional O2 extraction (measured at sagittal sinus) was also maintained down to a CPP of 34 mmHg in both groups before it rose at 26 mmHg. Cerebral O2 uptake was not different between groups and was not diminished with elevations of ICP in either group. We conclude that a) neonatal lambs are capable of CBF autoregulation with increasing ICP, and b) hypoxia sufficient to double baseline CBF but not diminish O2 uptake does not impair CBF autoregulation.