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Table 3 Global physiology-related variables in relation to pbtO2—linear mixed effect model analyses

From: Brain tissue oxygen monitoring in traumatic brain injury: part I—To what extent does PbtO2 reflect global cerebral physiology?

Variable

Value

SE

p

CPP model

Intercept

19.19

0.81

 < 0.001

ICP (mmHg)

0.16

0.01

 < 0.001

Intact autoregulation (yes)*

1.70

0.49

 < 0.001

CPP (mmHg)

0.10

0.01

 < 0.001

CPP if intact autoregulation (yes)**

− 0.02

0.01

 < 0.001

CPPopt model

Intercept

27.83

0.63

 < 0.001

ICP (mmHg)

0.12

0.01

 < 0.001

Intact autoregulation (yes)*

− 0.73

0.11

 < 0.001

Negative ∆CPPopt (mmHg)

− 0.14

0.01

 < 0.001

Negative ∆CPPopt if intact autoregulation (yes)**

0.07

0.01

 < 0.001

  1. Two separate models were used to predict hourly values of pbtO2 based on the global perfusion-related variables, one with absolute CPP and another with ∆CPPopt (mean hourly positive ∆CPPopt > 0 and negative ∆CPPopt < 0). The model based on absolute CPP-values also included ICP, PRx (above/below 0.30), and the interaction between ICP and CPP with PRx, and time from start of monitoring as fixed effects. Random effects were time from monitoring (slope) and patients and sequence (nested intercepts). Stepwise backward model reduction was done to exclude fixed effects variables that were not significant (p > 0.05) and the table shows the final model after these procedures. AIC was 327,026, conditional R squared was 0.467 and marginal R squared was 0.035. When the random slope (time) was added to the fixed effects for appropriate assessment of the conditional random effect variance, the conditional R squared was 0.996 and the marginal R squared was 0. A similar model was done with negative/positive ∆CPPopt instead of CPP as an independent variable and the table shows the final model after these procedures. Negative ∆CPPopt was reported as an absolute value, i.e. a higher negative ∆CPPopt was associated with a lower pbtO2 in the model above. Positive ∆CPPopt was not significant and therefore removed from the model. AIC was 284,519, conditional R squared was 0.457 and marginal R squared was 0.015. When the random slope (time) was added to the fixed effects for appropriate assessment of the conditional random effect variance, the conditional R squared was 0.996 and the marginal R squared was 0. The models were explored with and without standardization of the data, achieving the same results.”
  2. AIC akaike information criterion, CPP cerebral perfusion pressure, CPPopt optimal CPP, ICP intracranial pressure, PbtO2 partial brain tissue oxygenation, PRx pressure reactivity index, SE Standard error
  3. *Intact autoregulation; PRx < 0.30
  4. **Interactions
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Critical Care

ISSN: 1364-8535