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Does earlobe cutaneous PCO2 at 37[degrees]C identify septic shock patients?

This issue's research column is based on the peer-reviewed article by Fabrice Vallee, Joaquim Mateo, Guillaume Dubreuil, Thomas Poussant, Guillaume Tachon, Ingrid Ouanounou and Didier Payen, entitled: "Cutaneous Earlobe PCO2 at 37[degrees]C to Evaluate Micro Perfusion in Septic Patients." It was prepublished online in Chest May 14, 2010 as Digital Object Identifier (DOI) 10.1378/chest.09-2690. The authors are from the Reanimation Chirurgicale et Laboratoire de Recherche du Departement d'Anesthesie-Reanimation-SMUR (Equipe d'Accueil EA 3509 Universite Paris 7 Denis Diderot), Hopital Lariboisiere, Assistance Publique- Hopitaux de Paris, Paris, France. We'll review this article by sections to teach the scientific method for a research project: Background or Introduction, Question, Hypothesis, Methods, Results, Discussion/Reflections/Future Research, Conclusions, Acknowledgements, Conflicts of Interest and Bibliography.

The Background or Introduction of a research project explains interest in the topic and why the topic is significant. Early identification of septic shock is important in order to initiate timely resuscitation. To optimize treatment during the early phase of septic shock, it is important to optimize macro-circulatory parameters such as central venous pressure and systolic blood pressure. The authors point out, however, that despite these macro-circulatory goals being met, some patients persist with compromised tissue perfusion due to micro-circulatory disturbances at the tissue capillary level. Measurement of micro-circulatory parameters can demonstrate these disturbances and a possible need for further therapy. Measurement techniques of micro-circulatory parameters include low skin laser Doppler blood flow, sublingual small vessel perfusion using Sidestream Dark-Field (SDF) imaging, post-occlusion limb tissue hemoglobin saturation using near infrared spectroscopy (NIRS), and gastric tonometry. The authors, aware that pediatric patients receive transcutaneous CO2 measurements to monitor the adequacy of ventilation, proposed clipping the CO2 sensor to the earlobe of patients in septic shock as a new indicator of skin hypoperfusion. This technique requires skin warming by a heating element in the electrode to "arterialize" capillary blood for PCO2 measurements. In this study, the authors used the sensor at 37[degrees]C, so that the measured PCO2 represented the cutaneous rather than the transcutaneous PCO2 at 42-44[degrees]C. Recent studies using transcutaneous PCO2 at 42-44[degrees]C failed to discriminate survivors and non-survivors in septic shock patients. The authors noted that cutaneous PCO2 (PcCO2) at 37[degrees]C had not been previously studied as a reflection of tissue PCO2 in septic shock patients. Their aim was to validate earlobe PcCO2 as an indicator of skin hypoperfusion in patients with septic shock.

The question is the basis for undertaking a research project. The question being asked by the authors was: Can cutaneous measurement of PCO2 (PcCO2) at 37[degrees]C using a sensor clipped to the earlobe identify septic shock patients? Note: The question asked in a research project may have the possible answers: "yes" and "no" as in this study, or may be a numerical result. The hypothesis is the preconceived answer by the researchers to the question. The authors hypothesized a yes answer to their question, i.e., cutaneous measurement of PCO2 (PcCO2) using a sensor clipped to the ear lobe will be an indicator of septic shock.


The Methods for a research project describe the study design, setting and steps to answer the Question. This project was a prospective, non-randomized, non-blinded study, using non-septic patients as controls. Each participating patient or their next of kin signed a consent form approved by the institutional committee. Patients were enrolled if they were 18 years of age or older, identified within 48 hours of onset of septic shock and had no injury or deformity to their earlobes. Control patients without septic shock were selected from admissions to the ICU in the same time period as the patients with septic shock. For both groups of patients, the following parameters were collected: temperature, systemic hemodynamic parameters, arterial blood gases (Chiron Diagnostics[R], Rapidlab 864), end-tidal CO2 (Siemens[R] ventilator capnography monitoring system) and cutaneous CO2 measurements. PcCO2 at 37[degrees]C was measured using the TOSCA 500 monitor (TOSCA[R], Radiometer Basel AG, Switzerland). The sensor was calibrated "in vitro", had one drop of contact gel applied, and was then clipped to the earlobe in a standard fashion. After a few minutes for the stabilization of the PcCO2 values, data were recorded. The sensor was removed every 12 hours, recalibrated, and then placed on the other earlobe. Because arterial PCO2 may vary and influence the PcCO2 values, the authors calculated the gradients between cutaneous and arterial values (P[c-a]CO2) and between end-tidal PCO2 (PetCO2) and PcCO2 (P[c-et]CO2) values to measure this influence. Skin micro-circulatory blood flow was measured in patients on the opposite earlobe at the same location as the previous PcCO2 measurement using a laser Doppler system (Transonic HT107/HT207). All recorded data and results were expressed as median with a p value <0.05 considered significant. Student t-test was used to compare baseline characteristics of septic shock and control patients.

The Results section displays the data compiled to answer the Question. There were 46 septic shock patients and 15 controls enrolled in this study. The baseline characteristics of the two groups were similar. At baseline measurements, both P[c-a]CO2 and P[c-et]CO2 gradients were higher in septic shock patients than in the control patients: 14.8 versus 6.0 mmHg and 25 versus 9 mmHg respectively. The micro-circulatory blood flow data suggested that the increases in both P[c-a]CO2 and P[c-et]CO2 gradients was more pronounced as micro-circulatory blood flow decreased and local PcCO2 increased.

The Discussion/Reflections/Future Research of a research project starts with a Discussion which is a summary of the research. The authors first noted that the P[c-a]CO2 and P[c-et]CO2 gradients were highly related to the micro-circulatory parameters and did not have a relation to the macro-circulatory parameters. The authors explain this result by their decision not to warm the sensor above 37[degrees]C in order to avoid conversion of cutaneous to transcutaneous PCO2. The earlobe was considered an ideal location since it was an easy site to measure PcCO2, was strictly non invasive, provided a continuous measurement and was comfortable for the patient. In Reflections, the authors comment on weaknesses and shortcomings of their work. They note the small sample size of the study and the lack of blinded investigators. In Future Research, the authors suggest studies to advance knowledge of this research topic. Their technique could be applied in the context of life threatening conditions in the ED, OR and ICU. Because the relationship between flow and volume is complex, their proposed measurement of the PCO2 gradients might be useful to detect micro-circulation changes, even in absence of macro-circulation optimization.

The Conclusion is the final summary of a research project. The authors demonstrated that cutaneous measurement of PCO2 (PcCO2) at 37[degrees]C using a sensor clipped to the earlobe is an indicator of septic shock. The answer to the question was yes; the proposed hypothesis was correct. Acknowledgements credit those who assisted a research project. This study was supported in part by the Research Grant from the University of Paris 7, Plan Quadriennal Ministere de la Recherche. Conflicts of Interest ("COI") are listed for all authors participating in a research project. Conflicts include advisory board membership, ownership of stock, receipt of services, grants, honoraria, and consulting fees or gifts from companies related to the research project. All authors had no conflict of interest to disclose. The Bibliography section includes references to support a research project as included in the manuscript by reference numbers. For this research project, there were 39 references. There is no registration number for this study.

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Herbert Patrick, M.D., M.S.E.E., is an Intensivist and member of the Active Staff at Hahnemann University Hospital, Philadelphia, PA. Dr. Patrick can be contacted at
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Author:Patrick, Herbert
Publication:FOCUS: Journal for Respiratory Care & Sleep Medicine
Date:Jul 1, 2010
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