Near-Infrared Spectroscopy for Suspected Sepsis in the ED
Background and aims The conventional approach to sepsis resuscitation involves early interventions targeting global oxygenation and macro-haemodynamic variables such as central venous and systemic arterial pressures. There is increasing recognition of the importance of microcirculatory changes in shock states, including sepsis, and the relationship of these to outcome. Near-infrared spectroscopy (NIRS) is a recently developed non-invasive technology that measures tissue oxygen saturations (StO2), which may be an indirect measure of the adequacy of the microcirculation. StO2 measurements, therefore, have the potential to identify patients who are at risk of progressing to organ dysfunction and could be used to guide resuscitation. This article reviews the current state of knowledge of NIRS in the setting of sepsis, examining its application, validity and prognostic value.
Methods A search of the relevant literature was performed using Medline, Embase and Cochrane databases, and a qualitative analysis was undertaken.
Results A limited number of observational studies, mostly conducted among patients with severe sepsis, have shown that NIRS may correlate with severity of illness but demonstrate a variable relationship between StO2 and outcome.
Conclusions Outstanding questions still remain as to whether NIRS can help to risk-stratify patients with suspected sepsis in the emergency department and the utility of StO2 as a resuscitation target.
Sepsis is a significant global health challenge with high social and economic costs. Incidence is increasing and mortality rates are high even with optimal critical care. Early recognition and timely resuscitation to specified endpoints have been found to improve outcomes, and these principles have been distilled into a consensus-based approach. Mortality in sepsis is related to associated organ dysfunction (severe sepsis) and impaired tissue perfusion (septic shock) that lead to the development of multiple organ failure and death if the process is not reversed. The mainstays of therapy remain early resuscitation and organ support, antimicrobial drugs and surgical source control where appropriate.
The majority of patients diagnosed with sepsis in hospital are admitted via the emergency department (ED). Early recognition of high-risk patients with actual or impending organ dysfunction is essential to ensure correct management and disposition. The signs of organ dysfunction may be subtle in the early stages; for example, BP may be preserved until late stages in shock, and serum creatinine levels may not peak for several days. The elderly may present with non-specific symptoms and may not manifest significant fever or tachycardia, and the correct diagnosis may be confounded in those with multiple comorbidities.
While the conventional diagnosis of septic shock is based upon hypotension persisting after fluid challenge, or elevated blood lactate, there is increasing recognition that changes in the microcirculation are an important factor in impaired organ perfusion. A number of studies have demonstrated a relationship between impaired microcirculatory function and poor outcome in sepsis, but until recently reliable clinical tools to measure this have been lacking. Current targets in resuscitation in septic shock focus on optimising oxygen delivery and tissue perfusion. These include measurement of oxygen saturation in either superior vena cava (ScvO2) or pulmonary artery (SvO2) blood as an indicator of global oxygen consumption, and lactate clearance. It is recognised that microcirculatory changes persist in critical illness despite the optimisation of global haemodynamics. During the early stages of shock, BP and lactate can be relatively normal, so the ability to identify occult hypoperfusion may be important in the ED setting. Clinically applicable tools to assess the microcirculation are available; however, direct evaluation remains challenging. Techniques such as sublingual video-microscopy require a skilled operator and are more suited to repeat measurements in sedated patients in the intensive care unit (ICU) rather than being a practical option in the ED.
Within the past decade, technology known as near-infrared spectroscopy (NIRS) has been developed that allows the estimation of the oxygen saturation in skeletal muscle. NIRS uses light waves to non-invasively illuminate tissues up to 15 mm below a sensor placed on the skin. This is variably absorbed by chromophores such as haemoglobin and myoglobin. Haemoglobin has an absorption wavelength in the 700–1000 nm range, depending upon the degree of oxygenation. Algorithms convert the reflectance signal from the different chromophores to yield an estimate of the ratio of oxygenated-to-deoxygenated haemoglobin in blood vessels (arterioles, capillaries and venules) in the area examined, which is expressed as mean percent tissue oxygen saturation (StO2). This is an indirect measure of the relative oxygen delivery and consumption in that area. Additionally, an estimate of the haemoglobin concentration in the tissue of interest, expressed as tissue haemoglobin index, is made. In contrast, the more familiar oxygen saturation monitor uses direct transmission between an emitter and a detector to estimate arterial oxygen saturation (SpO2).
NIRS-derived StO2, which is available clinically at the bedside using proprietary devices, is a potential surrogate marker of the adequacy of oxygen delivery and utilisation at the microcirculatory level. For example, a fall in StO2 preceded increase in lactate and base deficit during cardiopulmonary bypass. There has been significant interest in this measure as a marker of occult shock states. For example, in trauma, StO2 recorded in the resuscitation room predicts the need for massive transfusion, organ dysfunction and mortality, although what this information adds over traditional clinical parameters such as BP and base deficit is uncertain. The pathophysiology of septic shock differs from haemorrhage, being characterised by varying degrees of vasodilation, increased cardiac output and systemic shunting. Alternatively, myocardial depression and/or hypovolaemia may predominate, leading to increased systemic vascular resistance, mediated through neurohormonal homeostatic responses. These situations may have different effects on microcirculatory function and oxygen utilisation affecting the measured StO2 value. In addition, unlike trauma, the onset of illness is often not well defined in sepsis, and patients present to the ED at various stages of illness progression. Nevertheless, the ability to rapidly and non-invasively estimate the adequacy of tissue perfusion at the bedside in the ED makes NIRS-derived StO2 a potential candidate both as a tool for assessment and to guide resuscitation.
The aim of this article is to describe the current state of the literature regarding the use of NIRS in the assessment of suspected sepsis in the ED; specifically the validity of NIRS in detecting occult shock, assessing shock severity and the predictive ability of NIRS for outcome.
Abstract and Introduction
Abstract
Background and aims The conventional approach to sepsis resuscitation involves early interventions targeting global oxygenation and macro-haemodynamic variables such as central venous and systemic arterial pressures. There is increasing recognition of the importance of microcirculatory changes in shock states, including sepsis, and the relationship of these to outcome. Near-infrared spectroscopy (NIRS) is a recently developed non-invasive technology that measures tissue oxygen saturations (StO2), which may be an indirect measure of the adequacy of the microcirculation. StO2 measurements, therefore, have the potential to identify patients who are at risk of progressing to organ dysfunction and could be used to guide resuscitation. This article reviews the current state of knowledge of NIRS in the setting of sepsis, examining its application, validity and prognostic value.
Methods A search of the relevant literature was performed using Medline, Embase and Cochrane databases, and a qualitative analysis was undertaken.
Results A limited number of observational studies, mostly conducted among patients with severe sepsis, have shown that NIRS may correlate with severity of illness but demonstrate a variable relationship between StO2 and outcome.
Conclusions Outstanding questions still remain as to whether NIRS can help to risk-stratify patients with suspected sepsis in the emergency department and the utility of StO2 as a resuscitation target.
Introduction
Sepsis is a significant global health challenge with high social and economic costs. Incidence is increasing and mortality rates are high even with optimal critical care. Early recognition and timely resuscitation to specified endpoints have been found to improve outcomes, and these principles have been distilled into a consensus-based approach. Mortality in sepsis is related to associated organ dysfunction (severe sepsis) and impaired tissue perfusion (septic shock) that lead to the development of multiple organ failure and death if the process is not reversed. The mainstays of therapy remain early resuscitation and organ support, antimicrobial drugs and surgical source control where appropriate.
The majority of patients diagnosed with sepsis in hospital are admitted via the emergency department (ED). Early recognition of high-risk patients with actual or impending organ dysfunction is essential to ensure correct management and disposition. The signs of organ dysfunction may be subtle in the early stages; for example, BP may be preserved until late stages in shock, and serum creatinine levels may not peak for several days. The elderly may present with non-specific symptoms and may not manifest significant fever or tachycardia, and the correct diagnosis may be confounded in those with multiple comorbidities.
While the conventional diagnosis of septic shock is based upon hypotension persisting after fluid challenge, or elevated blood lactate, there is increasing recognition that changes in the microcirculation are an important factor in impaired organ perfusion. A number of studies have demonstrated a relationship between impaired microcirculatory function and poor outcome in sepsis, but until recently reliable clinical tools to measure this have been lacking. Current targets in resuscitation in septic shock focus on optimising oxygen delivery and tissue perfusion. These include measurement of oxygen saturation in either superior vena cava (ScvO2) or pulmonary artery (SvO2) blood as an indicator of global oxygen consumption, and lactate clearance. It is recognised that microcirculatory changes persist in critical illness despite the optimisation of global haemodynamics. During the early stages of shock, BP and lactate can be relatively normal, so the ability to identify occult hypoperfusion may be important in the ED setting. Clinically applicable tools to assess the microcirculation are available; however, direct evaluation remains challenging. Techniques such as sublingual video-microscopy require a skilled operator and are more suited to repeat measurements in sedated patients in the intensive care unit (ICU) rather than being a practical option in the ED.
Within the past decade, technology known as near-infrared spectroscopy (NIRS) has been developed that allows the estimation of the oxygen saturation in skeletal muscle. NIRS uses light waves to non-invasively illuminate tissues up to 15 mm below a sensor placed on the skin. This is variably absorbed by chromophores such as haemoglobin and myoglobin. Haemoglobin has an absorption wavelength in the 700–1000 nm range, depending upon the degree of oxygenation. Algorithms convert the reflectance signal from the different chromophores to yield an estimate of the ratio of oxygenated-to-deoxygenated haemoglobin in blood vessels (arterioles, capillaries and venules) in the area examined, which is expressed as mean percent tissue oxygen saturation (StO2). This is an indirect measure of the relative oxygen delivery and consumption in that area. Additionally, an estimate of the haemoglobin concentration in the tissue of interest, expressed as tissue haemoglobin index, is made. In contrast, the more familiar oxygen saturation monitor uses direct transmission between an emitter and a detector to estimate arterial oxygen saturation (SpO2).
NIRS-derived StO2, which is available clinically at the bedside using proprietary devices, is a potential surrogate marker of the adequacy of oxygen delivery and utilisation at the microcirculatory level. For example, a fall in StO2 preceded increase in lactate and base deficit during cardiopulmonary bypass. There has been significant interest in this measure as a marker of occult shock states. For example, in trauma, StO2 recorded in the resuscitation room predicts the need for massive transfusion, organ dysfunction and mortality, although what this information adds over traditional clinical parameters such as BP and base deficit is uncertain. The pathophysiology of septic shock differs from haemorrhage, being characterised by varying degrees of vasodilation, increased cardiac output and systemic shunting. Alternatively, myocardial depression and/or hypovolaemia may predominate, leading to increased systemic vascular resistance, mediated through neurohormonal homeostatic responses. These situations may have different effects on microcirculatory function and oxygen utilisation affecting the measured StO2 value. In addition, unlike trauma, the onset of illness is often not well defined in sepsis, and patients present to the ED at various stages of illness progression. Nevertheless, the ability to rapidly and non-invasively estimate the adequacy of tissue perfusion at the bedside in the ED makes NIRS-derived StO2 a potential candidate both as a tool for assessment and to guide resuscitation.
The aim of this article is to describe the current state of the literature regarding the use of NIRS in the assessment of suspected sepsis in the ED; specifically the validity of NIRS in detecting occult shock, assessing shock severity and the predictive ability of NIRS for outcome.
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