Letters to the Editor

monitoring plus bedside ECHO. Such a study may provide more definitive answers to the value of ECHO in pediatric septic shock management. Except that it may not be ever possible in PICUs such as ours where focused ECHO and ultrasound are in the “DNA” of the unit, and everyday clinical decisions are dependent on standard clinical and laboratory information closely integrated with serial bedside ECHO/ultrasound findings. The authors have disclosed that they do not have any potential conflicts of interest. Suchitra Ranjit, MD, FCCM, Pediatric Intensive Care Unit, Apollo Children’s Hospital, Chennai, Tamil Nadu, India; Niranjan Kissoon, MBBS, FAAP, FCCM, Pediatric Emergency Department and Intensive Care Unit, British Columbia Children’s Hospital and the University of British Columbia, Vancouver, BC, Canada.

REFERENCES

1. Gaspar HA, Tuma PL, Brunow Carvalho W, et  al: Bedside Echocardiography for Pediatric Hemodynamic Monitoring: What Is the Impact in the Outcome? Pediatr Crit Care Med 2014; 15: 386–387 2. Ranjit S, Aram G, Kissoon N, et al: Multimodal monitoring for hemodynamic categorization and management of pediatric septic shock: A pilot observational study. Pediatr Crit Care Med 2014; 15:e17–e26 3. Breitkreutz R, Walcher F, Seeger FH: Focused echocardiographic evaluation in resuscitation management: Concept of an advanced life support-conformed algorithm. Crit Care Med 2007; 35:S150–S161 4. Vignon P, Mücke F, Bellec F, et al: Basic critical care echocardiography: Validation of a curriculum dedicated to noncardiologist residents. Crit Care Med 2011; 39:636–642 5. Arntfield RT, Millington SJ: Point of care cardiac ultrasound applications in the emergency department and intensive care unit—A review. Curr Cardiol Rev 2012; 8:98–108 6. Schefold JC, Storm C, Bercker S, et al: Inferior vena cava diameter correlates with invasive hemodynamic measures in mechanically ventilated intensive care unit patients with sepsis. J Emerg Med 2010; 38:632–637 7. Nagdev AD, Merchant RC, Tirado-Gonzalez A, et  al: Emergency department bedside ultrasonographic measurement of the caval index for noninvasive determination of low central venous pressure. Ann Emerg Med 2010; 55:290–295 8. Feissel M, Michard F, Faller JP, et al: The respiratory variation in inferior vena cava diameter as a guide to fluid therapy. Intensive Care Med 2004; 30:1834–1837 9. Wachsberg RH: Narrowing of the upper abdominal inferior vena cava in patients with elevated intraabdominal pressure: Sonographic observations. J Ultrasound Med 2000; 19:217–222 DOI: 10.1097/PCC.0000000000000093

Thresholds for Cerebral Perfusion Pressure To the Editor:

I

n a recent issue of Pediatric Critical Care Medicine, I read with great interest the article by Allen et al (1) on ­age-specific thresholds for cerebral perfusion pressure (CPP) and outcomes in pediatric traumatic brain injury (TBI). Allen et al (1) are to be commended for this work in which they sought to establish age-normative thresholds for CPP in pediatric TBI versus adults. Within the Methods section, however, it is not clear where the transducer for the

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intracranial pressure (ICP) was set (presumably at the foramen of Monroe or ear level), as well as the arterial catheter or blood pressure transducer zero point, which should also be “zeroed” at the same level as the ICP monitor if one is calculating CPP = mean arterial blood pressure – ICP (2, 3). When head of bed elevation occurs in hospitalized patients, the head is higher than the level of the right atrium, which is the common location for the zero point for blood pressure measurements. Such basic methodological differences might lead to CPP variation if the zero point is not defined a priori, theoretically affecting results. Overall, the authors are commended on the study, which is important work in the field of pediatric neurocritical care that provides more physiologically meaningful data than ICP-targeted only trials (4). The author has disclosed that he does not have any potential conflicts of interest. William D. Freeman, MD, Departments of Neurology, Neurosurgery, and Critical Care, Mayo Clinic, Jacksonville, FL

REFERENCES

1. Allen BB, Chiu YL, Gerber LM, et al: Age-Specific Cerebral Perfusion Pressure Thresholds and Survival in Children and Adolescents with Severe Traumatic Brain Injury. Pediatr Crit Care Med 2014; 15:62–70 2. Rose JC, Mayer SA: Optimizing blood pressure in neurological emergencies. Neurocrit Care 2004; 1:287–299 3. Nates JL, Niggemeyer LE, Anderson MB, et  al: Cerebral perfusion pressure monitoring alert! Crit Care Med 1997; 25:895–896 4. Chesnut RM, Temkin N, Carney N, et  al; Global Neurotrauma Research Group: A trial of intracranial-pressure monitoring in traumatic brain injury. N Engl J Med 2012; 367:2471–2481 DOI: 10.1097/PCC.0000000000000088

The authors reply:

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e thank Freeman (1) for his question and agree that his observation highlights unavoidable difficulties inherent in multi-institutional observational studies. Hospital, ICU, and nursing variances, even when subtle, can absolutely affect what we interpret and report as a “fixed variable” which then applied formulaically to produce cerebral perfusion pressure (CPP) is prone to error. As our research was an observational study, we did not control, nor honestly query, the methods in which each individual ICU collected their physiologic variables (2). As such, there was no standard transducer zero point for blood pressure in the study. We assumed, as standard of care, that intracranial pressure (ICP) was routinely based on a transducer set at the approximate level of the Foramen of Monro. Hopefully, despite these questions, our findings will remain a stepping-off point for further discussions and betterdesigned and controlled prospective studies to assess the relative importance of targeted CPP thresholds based on age and physiology and maybe, at some point, not just continuously measured brain perfusion but also brain oxygenation. May 2014 • Volume 15 • Number 4