Czosnyka et al 1recently published a study investigating the clinical significance of critical closing pressure (CCP) estimates in patients with head injury. I see problems both with the theoretical foundation of their CCP concept and with the interpretation of their results.

Firstly, the physiological meaning of both formulae of CCP presented (CCP1 and CCP2, respectively) is questionable. The implication of both presented equations is that the instantaneous value of cerebral blood flow velocity (FV(t)) at a given moment t is equal to arterial blood pressure at the given time (ABP(t)) minus CCP divided by cerebrovascular resistance (CVR):

FV(t) = (ABP(t)−CCP)/CVR      (1)

At the time of systolic and diastolic pressure values (ABPs, ABPd), respectively, it follows that systolic and diastolic FV (FVs, FVd) should be equal to (ABPs−CCP)/CVR and (ABPd−CCP)/CVR, respectively. However, it is a well known fact that the vascular resistance valid for the static pressure/flow connection (CVR0, concerning mean pressures and flows) is different from and is in general much higher than resistances determining dynamic pressure/flow relations (CVR1) as in the case of pulsatile pressures.2 Therefore, equation 1 cannot be applied to describe dynamic flow. This can best be illustrated using the frequency domain approach (ABP=mean pressure; FV=mean flow velocity; A1=amplitude of the pulsatile pressure wave; F1=amplitude of the pulsatile flow wave):

FV=(ABP−CCP)/CVR0      (2)

F1=A1/CVR1      (3)

Inserting equations 2 and 3 into the frequency domain equation for CCP2 of the authors

CCP2=ABP-A1/F1×FV      (4)

leads to

CCP2=ABP−CVR1/CVR0×(ABP−CCP)

=ABP(1−CVR1/CVR0)+CVR1/ CVR0×CCP      (5)

Obviously, CCP2 is only in the case of CVR1=CVR0 equal to CCP. Under the more realistic …

Dr Marek Czosnyka emailMC141{at}MEDSCHL.CAM.AC.UK