RESUMEN
To test theoretical assumptions supporting the use of near-infrared spectroscopy (NIRS) in clinical practice, we examined the behaviour of NIR light transmission and attenuation in the human head. Sterile probes for emitting and detecting NIR light at a fixed separation of 40 mm were placed in turn on intact skin, skull, dura and cerebral cortex of 10 patients undergoing elective neurosurgery. In the first five patients, the detecting probe was moved through successive extracerebral layers with the emitter on the skin surface. In the second five patients, the process was reversed, with the emitting probe moved and the detector in the same place on the scalp. NIR intensity was measured at each tissue interface and compared with the intensity measured at the skin surface with all layers intact. Removal of bone and dura from the light path caused a significant reduction in detected intensity. The largest mean reduction in light intensity was a 14-fold decrease with removal of bone (unadjusted P < 0.0001; paired t test). The assumptions that extracerebral tissues contribute little to attenuation of NIR light in the adult head and that most of this attenuation occurs superficially in the scalp are drawn into question by this study. We postulate that the skull and/or its interface with other layers may act as an optical 'channel', distorting the behaviour of NIR light in the human head.
Asunto(s)
Encéfalo/fisiología , Cabeza/fisiología , Luz , Procedimientos Neuroquirúrgicos , Espectroscopía Infrarroja Corta , Adulto , Anciano , Femenino , Humanos , Masculino , Persona de Mediana Edad , Monitoreo Intraoperatorio/métodos , Dispersión de RadiaciónRESUMEN
We have compared the effect of increasing optode separation (range 0.7-5.5 cm) on the sensitivity of near infrared spectroscopy (NIRS) to discrete reductions in scalp and cerebral oxygenation in 10 healthy men (mean age 32, range 26-39 yr) using multichannel NIRS. During cerebral oligaemia (a mean reduction in middle cerebral artery flow velocity of 47%) induced by a mean reduction in end-tidal PCO2 of 2.4 kPa, the decrease in oxyhaemoglobin detected by NIRS became significantly greater with increasing optode separation (P < 0.0001). In response to scalp hyperaemia induced by inflation and release of a pneumatic scalp tourniquet, increases in oxyhaemoglobin became significantly smaller with increasing optode separation (P < 0.0002). These results are consistent with theoretical models of the behaviour of NIR light in the adult head and support the concept of using multi-detector NIRS to separate intra- and extracranial NIR signal changes. However, the emitter-detector separation used by currently available cerebral oximeters is not large enough to provide optimal spatial resolution.
Asunto(s)
Encéfalo/metabolismo , Arterias Cerebrales , Circulación Cerebrovascular , Hiperemia/metabolismo , Consumo de Oxígeno , Espectroscopía Infrarroja Corta/métodos , Adulto , Complejo IV de Transporte de Electrones/análisis , Hemoglobinas/análisis , Humanos , Masculino , Oxihemoglobinas/análisis , Cuero Cabelludo/irrigación sanguínea , Sensibilidad y Especificidad , Espectroscopía Infrarroja Corta/instrumentaciónRESUMEN
The ability to assess viability of tissues by monitoring changes in oxygenation and perfusion during harvesting and following transfer of free and pedicled flaps is potentially important in reconstructive surgery. Rapid detection of a critical change in tissue oxygenation could enable earlier and more successful surgical intervention when such problems arise. In this study near infra-red spectroscopy (NIRS) was used to assess changes in tissue oxygenation, haemoglobin oxygenation and blood volume in a porcine prefabricated myocutaneous flap model in response to pedicle manipulations. As far as we are aware this is the first usage of a NIRS instrument to assess changes in oxygenation in a flap model which closely simulates the clinical situation. A myocutaneous flap was raised (n = 9 pigs), tubed and the flap circulation allowed to readjust for periods between 7 and 9 days. The pedicle vessels were then subjected to arterial (n = 9), venous (n = 12) and total occlusion (n = 6). Repeatable and reproducible patterns of change were measured in each case. Comparison of mean values indicated that the differences between arterial and venous, and venous and total occlusions were significant for all NIRS parameters. The monitor was easily able to detect two additional features: (i) the presence of venous congestion indicated by raised levels of deoxygenated haemoglobin and an increase in blood volume; and (ii) the presence and magnitude of reactive hyperaemia. In two flaps release of arterial or total occlusion did not result in the expected reactive hyperaemia associated with an increase in blood volume (oxygenated haemoglobin) suggestive of possible damage to the vascular bed. NIRS proved able to detect and distinguish between microcirculatory changes occurring as a result of arterial, venous or total vascular occlusion. We believe that NIRS provides a sensitive and reliable postoperative monitor of tissue viability following transfer of free and pedicled flaps. It can accurately identify different types of problems with the pedicle vessels. In addition its predictive capabilities would allow assessment of flaps buried deep to the skin. This monitor is excellent for surgical and intensive care unit monitoring since it is unaffected by light, portable and is extremely easy to use.
Asunto(s)
Supervivencia de Injerto/fisiología , Colgajos Quirúrgicos , Animales , Arteriopatías Oclusivas/metabolismo , Constricción Patológica/metabolismo , Femenino , Hemoglobinas/metabolismo , Monitoreo Fisiológico/métodos , Oxihemoglobinas/metabolismo , Cuidados Posoperatorios/métodos , Espectroscopía Infrarroja Corta/métodos , Colgajos Quirúrgicos/irrigación sanguínea , Porcinos , Enfermedades Vasculares/metabolismoRESUMEN
Hypoxia plays a key role in the pathogenesis of acute mountain sickness (AMS), but individual susceptibility is variable and cerebral symptoms do not always correlate with PaO2 measurements. Cerebral hypoxia may be more relevant than PaO2. We studied trends in cerebral regional oxygen saturation by the technique of near-infrared spectroscopy in 20 subjects ascending rapidly to 4680 m. Subjects were enrolled in a placebo-controlled, double-blind trial of medroxyprogesterone for the prevention of AMS. The fall in cerebral oxygen saturation was less than in the periphery. At 4680 m, cerebral oxygenation correlated with peripheral saturation but not with PaCO2 or with cerebral symptoms scores. At 4680 m, subjects on medroxyprogesterone had higher cerebral and peripheral saturation compared with those on a placebo. We conclude that cerebral oxygenation monitored with the Critikon 2020 system provided important information on the complex relationship of hypoxia to AMS and that other factors, such as changes in blood flow or capillary permeability, may be equally important.
Asunto(s)
Mal de Altura/fisiopatología , Oxígeno/metabolismo , Telencéfalo/metabolismo , Adulto , Mal de Altura/prevención & control , Análisis de los Gases de la Sangre , Dióxido de Carbono/metabolismo , Método Doble Ciego , Femenino , Humanos , Masculino , Medroxiprogesterona/uso terapéutico , Persona de Mediana Edad , Oximetría , Congéneres de la Progesterona/uso terapéutico , Espectroscopía Infrarroja Corta , Encuestas y CuestionariosRESUMEN
OBJECTIVE: To examine the effect of two emitter-detector separations (2.7 and 5.5 cm) on the detection of changes in cerebral and extra-cerebral tissue oxygenation using near infrared spectroscopy (NIRS). METHODS: Two NIR detectors were placed on the scalp 2.7 and 5.5 cm from a single NIR emitter. Changes in deoxyhaemoglobin (HHb), oxyhaemoglobin (O2Hb),oxidised cytochrome C oxidase (Cyt) and total haemoglobin (tHb) were recorded from each detector during the induction of cerebral oligaemia (transition from hypercapnia to hypocapnia) and scalp hyperaemia (following release of a scalp tourniquet). RESULTS: Cerebral oligaemia (mean decrease in middle cerebral artery blood flow velocity of 44%) induced by a mean reduction in end tidal CO2 of 18 mmHg was accompanied by a significant increase in the spectroscopic signal for HHb and a decrease in the O2Hb signal. The signal change per unit photon path length detected at 5.5 cm was significantly greater for HHb (p = 0.007) than that detected at 2.7 cm. In contrast, the increase in all chromophores detected at 5.5 cm during scalp hyperaemia was significantly less than that detected at 2.7 cm (p<0.001). CONCLUSIONS: The differing sensitivity of the proximal and distal channels to changes in cerebral and extracerebral oxygenation is compatible with theoretical models of NIR light transmission in the adult head and may provide a basis for spatially resolving these changes. The optimal emitter-detector separation for adult NIRS requires further investigation and may differ between individuals.
Asunto(s)
Oximetría/instrumentación , Espectroscopía Infrarroja Corta/métodos , Adulto , Circulación Cerebrovascular , Humanos , Masculino , Monitoreo Fisiológico/instrumentación , Espectroscopía Infrarroja Corta/instrumentaciónAsunto(s)
Hemodinámica , Oximetría/métodos , Oxígeno/sangre , Oxígeno/metabolismo , Espectroscopía Infrarroja Corta/métodos , Animales , Encéfalo/metabolismo , Modelos Animales de Enfermedad , Femenino , Humanos , Trasplante de Hígado/efectos adversos , Trasplante de Hígado/fisiología , Masculino , Enfermedades Metabólicas/sangre , Enfermedades Metabólicas/metabolismo , Monitoreo Fisiológico/métodos , Hemorragia Posoperatoria/sangre , Hemorragia Posoperatoria/diagnóstico , Hemorragia Posoperatoria/metabolismo , Ratas , Ratas Endogámicas Lew , Colgajos Quirúrgicos/efectos adversos , Colgajos Quirúrgicos/fisiología , Procedimientos Quirúrgicos Operativos , PorcinosRESUMEN
OBJECTIVE: To develop an in vivo model for testing spatially resolved spectroscopy and quantified near infrared spectroscopy (NIRS) cerebral blood flow measurements. METHOD: Multiple detector NIRS has been used to study changes in tissue oxyhaemoglobin (O2Hb) concentration during selective internal carotid angiography. A significant reduction in O2Hb occurred in tissue interrogated by detectors situated between 0.7 and 4.1 cm from the NIRS light source. RESULTS: The time course of O2Hb concentration change was consistent with displacement of oxygenated blood by the radiocontrast medium from vascular beds of differing flow and NIR light attenuation. Increasing changes in O2Hb concentration per unit photon path length--predicted to occur at greater emitter-detector separations if those changes had occurred predominantly in cerebral tissue--were found in the first four seconds after injection of radiocontrast medium. However, later changes (6-10 s) were larger and were not proportional to emitter-detector separation. CONCLUSION: The findings indicate that simple assumptions regarding the distribution of the internal carotid artery blood supply to cerebral and extracerebral tissues, the photon path length through those tissues, and their relative contributions to attenuation of NIR light may not be justified.
Asunto(s)
Arteria Carótida Interna/diagnóstico por imagen , Angiografía Cerebral , Oxihemoglobinas/análisis , Espectroscopía Infrarroja Corta , Encéfalo/irrigación sanguínea , Encéfalo/diagnóstico por imagen , HumanosRESUMEN
A prototype instrument has been developed to combine the monitoring techniques of laser Doppler flowmetry and reflection pulse oximetry, both of which depend on laser light backscattered from the skin tissue. Simultaneous and continuous measurements of both microvascular blood flow and blood oxygen saturation were obtained from the same measurement site. Oximetry readings from the combined instrument were calibrated using a model system, and compared with those determined using a Nellcor N-100 transmission pulse oximeter in a limited clinical study: they showed a variability of +4% and read low by 5-10%.
Asunto(s)
Rayos Láser , Oximetría/instrumentación , Ultrasonido , HumanosRESUMEN
A new infrared laser Doppler blood flow instrument (Moor MBF3D) was evaluated using an in vitro model allowing measurements over a range of flow velocities and concentrations. The responses correlated well (r = 0.96, p less than 0.01) with those obtained simultaneously using a Perimed PF3 laser Doppler instrument. The different processing bandwidths of the instruments were investigated and the wideband mode of operation is recommended for flow measurements where there may be fast moving red blood cells (rbcs). The infrared instrument is capable of dual-channel operation, and the two channels are shown to respond almost identically for similar changes in blood flow through the in vitro model (r = 0.999, p less than 0.01). The main advantage of the dual-channel instrument is that continuous measurements may be made simultaneously at two different skin sites allowing dynamic flow responses to be compared.
Asunto(s)
Reología , Diseño de EquipoRESUMEN
Laser Doppler flowmetry (LDF) is now a well-established, non-invasive technique for measuring microvascular blood perfusion. However, there are a number of factors which can seriously affect the interpretation of the laser Doppler signal which are often not considered during routine use. These include: consideration of signal processing limitations, choice of processing bandwidth, problems with motion artefact and instrument calibration, the effect of probe pressure on the skin, and the type of laser used. This paper reviews many of the problems and limitations frequently encountered using the laser Doppler technique.