Article Text

Statistics and analysis of the Camino ICP monitor
  1. B A GREGSON
  1. Department of Neurosurgery, University of Newcastle upon Tyne, STICH Office, Ward 31 (North Wing), Newcastle General Hospital, Westgate Road, Newcastle upon Tyne NE4 6BE, UK
  2. Regional Medical Physics Department
  1. Dr B A Gregson stich{at}ncl.ac.uk
  1. K BANISTER,
  2. I R CHAMBERS
  1. Department of Neurosurgery, University of Newcastle upon Tyne, STICH Office, Ward 31 (North Wing), Newcastle General Hospital, Westgate Road, Newcastle upon Tyne NE4 6BE, UK
  2. Regional Medical Physics Department
  1. Dr B A Gregson stich{at}ncl.ac.uk

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We have concerns about the mathematics, accuracy of the data, and conclusions in the paper by Martinez-Manas et al.1 This paper reports on a prospective study of the accuracy and complications of the Camino intracranial pressure monitor.

The authors have been lax in their use of English, failing to differentiate between their use of the words “patients” and “probes”. This would not be such a problem if they had only reported on one probe per patient, which should have been part of the protocol of the study. They have also used the verbs “to calibrate” and “to zero” interchangeably when in fact they mean “to zero”; thus the devices need to be “zeroed” before insertion not “calibrated”.

The paper reports on 108 probes in 101 patients. Details of patients should relate to 101 individuals therefore; for instance, there could not be 65 males and 43 females. There are numerous mistakes throughout the paper in the basic calculation of percentages. For instance 66 cases of head injury (fig 1) out of the 108 indications for monitoring do not account for 71% of implantations and three positive cultures from 16 subdural devices do not account for 10.7%. Furthermore, the precision suggested by the use of decimal places in reporting percentage data is totally unwarranted.

There is also concern about the failure rate of probes from the authors' analysis of infection rates and zero drift, which was performed on only 63%, and 52% of the total number of inserted probes respectively. The protocol should have included procedures to minimise this. It may be that there was a high failure rate of the catheters but this is not reported. More details should be given to ascertain whether any bias is likely to have been introduced by excluding so many probes.

Figure 3 suggests huge drifts (-24 mm Hg to +35 mm Hg) that are clinically significant and unacceptable, with 39% of probes tested failing to comply with the manufacturer's specifications. The authors demonstrated that there was no correlation between duration of monitoring and zero drift which is in agreement with previous work.2 However, the authors fail to highlight the fact that regardless of the duration of monitoring, 23% of probes tested had a zero drift of ⩾±10 mm Hg, which is clearly unacceptable.

The representation of the data as mean, median, and SD in table 3 is misleading as it is clear from fig 3 that there is a wide distribution of both positive and negative offsets. Consequently, a near zero mean drift is likely to occur even though the magnitude of the zero drift in individual cases is large. Clinically, it is the zero drift from a single patient that is important and not the zero drift of a series of probes.

The recommendation to change the catheter if a long monitoring period is expected to allow for rezeroing is not held up by the data shown in fig 3, which would suggest that there is more likely to be a larger zero drift than the manufacturer's specification in the early days.

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