REVIEW Tissue oxygenation in morbid obesity e The physiological and clinical perspective Adrian Alvarez a, * , Preet Mohinder Singh b , Ashish C. Sinha c a Department of Anesthesia, Hospital Italiano de Buenos Aires, Juan D. Peron 4190, C1181ACH Buenos Aires, Buenos Aires Province, Argentina b Department of Anesthesia, Post Graduate Institute of Medical Education and Research (PGIMER), Chandigarh, India c Department of Anesthesia, Drexel University College of Medicine; Philadelphia, PA, USA Keywords: Tissue oxygenation in morbidly obese Perioperative tissue oxygenation in obese Pathophysiology of tissue oxygen monitoring in obese summary In order to reach and maintain a normal physiological performance, each cell of the human body needs an adequate quantity of oxygen. The measurement of oxygen at the cellular level is a significant early marker of local injury. Unlike global markers of oxygen deficit that often detect on-going pathology in the irreversible phase, tissue oxygenation measurement can provide an early therapeutic window for appropriate timely intervention to prevent and revert the damage. The present review describes the physiological principles guiding tissue oxygen levels in the morbidly obese. We describe how the morbidly obese are different from lean patients in terms of oxygen delivery at various tissue levels. The text highlights how pathological alterations in tissue oxygen levels during special situations like trauma, sepsis, and active bleeding can be predicted, interpreted and therapeutically targeted to improve clinical outcomes in morbidly obese patients. The utility of tissue oxygenation monitoring in relevance to morbidly obese patients during the perioperative period along with the possible clinical implications is also discussed. We present the present evidence on the topic and extrapolate the possible future role of this monitoring for various diseased states in morbidly obese patients. Ó 2013 Elsevier Ltd. All rights reserved. In order to reach and maintain a normal physiological perfor- mance, each cell of the human body needs an adequate quantity of oxygen. Thus, it would be ideal to measure the amount of oxygen at a cellular level or as close as possible to the cell. In clinical practice this usually does not happen. Commonly used monitors evaluate the upstream component of the circulation; oxygen in blood prior to it reaching the tissue (pulse oximetry, arterial blood gas sam- pling) and the perfusion pressure, again prior to reaching the tissue (invasive and non-invasive arterial blood pressure monitoring). On the other hand, monitors also measure the downstream compo- nent; for oxygenation (venous oxygen saturation, mixed venous oxygen saturation) and the post perfusion pressure (central venous pressure). In any case, the ultimate aim of these monitors is to detect the amount of oxygen effectively reaching the peripheral tissues where actual utilization/metabolism takes place. In 1956, Clark described an electrode capable of measuring ox- ygen tension in the tissue based on polarography. Eventually this finding opened up a new dimension of understanding about the oxygen cascade. 1 Subsequent developments in technology have made it possible to directly look at microcirculation; the site of actual oxygen delivery and consumption. Although conventional monitors act adequately as a surrogate marker, they cannot mea- sure the actual factors upon which tissue oxygenation depends (i.e. adequate total blood flow, even distribution of microcirculatory flow, adequate diffusion of oxygen across capillaries and the ca- pacity of tissues to take up and utilize oxygen.) 2 The idea of using such monitoring might be vital in some patient populations like the morbidly obese. In these subjects, weight gain is out of proportion to the increase in blood volume. 3 In the morbidly obese, relative blood volume is lower than in lean counterparts. Subsequently actual tissue perfusion (and eventual oxygenation) on a per weight unit basis is also lower in these individuals. Although conventional monitors fail to detect any such abnormality in the obese, this persistent tissue dysoxia is now recognized as the crux of metabolic ailments seen in the morbidly obese patients. 4 With that said, in this review first we will address the moni- toring targets for tissue perfusion and oxygenation and discuss the factors affecting tissue oxygen tension. We will also emphasize the importance of the concept “critical oxygen delivery to tissue”. Then we will analyze the implications of tissue oxygenation de- rangements seen in morbidly obese individuals mainly focusing on chronic metabolic and inflammatory diseases as well as in sepsis and shock. Finally we will discuss the possible future directions for research in this field. * Corresponding author. E-mail address: tatotatun@gmail.com (A. Alvarez). Contents lists available at ScienceDirect Trends in Anaesthesia and Critical Care journal homepage: www.elsevier.com/locate/tacc 2210-8440/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tacc.2013.09.002 Trends in Anaesthesia and Critical Care 3 (2013) 310e315