Patent US20110105912 – Cerebral autoregulation indices – Google Patents brain anoxia

Cerebral autoregulation is the mechanism in humans that ensures a consistent cerebral blood flow (CBF) over a range of cerebral perfusion pressure (CPP). In a healthy subject, the cerebral arteries and arterioles constrict or dilate to maintain CBF during changes in arterial blood pressure, thereby ensuring adequate blood flow and protecting against excessive blood flow which can result in brain swelling or edema. Monitoring of CBF in the face of changing CPP can delineate the optimal range of blood pressure where autoregulation is maintained. A number of disease states, including traumatic brain injury, stroke, meningitis, cardiac arrest and other brain insults can impair cerebral autoregulation by limiting or shifting the optimal range of CPP where CBF is relatively constant.Brain anoxia


various therapies and interventions can also impair cerebral autoregulation, such as cardiopulmonary bypass and hypothermia. Continuous monitoring of the autoregulatory state is needed to protect against brain hypoxia due to hypoperfusion and cerebral edema due to over perfusion.

BRIEF DESCRIPTION OF THE DRAWINGS•

Pulse contour analysis: A cerebral oximetry sensor may be used to measure characteristics of the pulsatile component of total hemoglobin (or cerebral blood volume) as an index of cerebral blood flow resulting from arterial oscillations. (see, e.G., themelis G et al., near-infrared spectroscopy measurement of the pulsatile component of cerebral blood flow and volume from arterial oscillation, J biomed optics 2007; 12(1): 014033.) in this implementation, each heart beat causes a change in blood pressure which can be measured peripherally using a fluid-filled catheter and pressure transducer to convert pressure changes to electrical changes.Brain anoxia the slope of the change in pressure resulting from each heart beat represents the change in blood flow per time unit. Thus, the time derivative of blood pressure (or total hemoglobin) is proportional to blood flow and can be used to derive changes in flow. (see, e.G., remington J W et al., volume elasticity characteristics of the human aorta and the prediction of stroke volume from the pressure pulse, am J physiol 1948; 153: 198-308). Several products on the market today rely on the measurement of pressure change and area to derive cardiac output (flow) from the arterial pressure waveform, for example, products like lidco plus (lidco ltd.), or flotrac system (edwards lifesciences).

FIG. 1 illustrates one example of a system 100 for diagnosing cerebrovascular autoregulation of a patient 102.Brain anoxia system 100 includes a sensor 104 that is arranged proximate to an external position of the patient’s head 106. In one example, sensor 104 is a cerebral oximeter. A blood pressure monitoring device 108 is attached to the patient, for example, to a patent’s arm. Further, a pulse oximeter sensor 110 may be attached to a patent’s hand or finger, as discussed in greater detail below. A signal processing unit 112 is in communication with cerebral oximeter 104 and with blood pressure monitoring device 108. In one example, the cerebral oximeter obtains oxygen content measurements of blood within the patient’s brain. Signals from cerebral oximeter 104 may be processed internally within cerebral oximeter 104 and/or processed by signal processing unit 112.Brain anoxia for example, the oxygen content measurements of blood within the patient’s brain is taken at a plurality of times by cerebral oximeter 104 to input an oxygen content signal to signal processing unit 112.

Blood pressure monitoring device 108 obtains arterial blood pressure measurements of patient 102 at a plurality of times substantially synchronously with the oxygen content measurements and outputs an arterial blood pressure signal to signal processing unit 112. Signal processing unit 112 calculates a linear correlation coefficient based on the oxygen content signal and the arterial blood pressure signal in a time domain for a plurality of times. In one example, this linear correlation coefficient may be referred to as the cerebral oximeter index (cox).Brain anoxia the oxygen content signals transmitted from cerebral oximeter 104 to signal processor 110 can be low pass filtered by anyone of cerebral oximeter 104 itself, signal processing unit 112, or by an intermediate low pass filter in the signal line between cerebral oximeter 104 and signal processing unit 112. Blood pressure monitoring device 108, signal processing unit 112 or an intermediate device in the signal line between blood pressure monitoring device 108 and signal processor 110 can provide low pass filtering of the measured blood pressure signal.

One suitable method for creating a CBF index is to noninvasively measure red blood cell velocity in the middle cerebral artery using transcranial doppler (TCD) ultrasound. (see, e.G., czosnyka M et al., monitoring of cerebral autoregulation in head-injured patients, stroke 1996; 27(10):1829-34.) another method, which is invasive, involves using a laser-doppler probe placed on the brain parenchyma to measure red blood cell flux. (see, e.G., lam J M et al., monitoring of autoregualtion using laser doppler flowmetry in patients with head injury, J neurosurg 1997; 86(3):438-45.) both methods provide signals representative of changes in CBF for determination of an autoregulatory index but both have disadvantages.Brain anoxia TCD is technically difficult and cannot be performed in 10-20% of the population due to thick cranial bone. Laser doppler flowmetry is highly invasive and is usually reserved for only the most severely brain injured patients. A convenient, noninvasive method of measuring changes in CBF for determining an autoregulation index is needed.

An improved implementation includes a second cerebral oximetry sensor 110 placed in a periphery location of the body to acquire a pleth waveform by continuously measuring optical absorption changes in the near-infrared range. Second sensor 110 is placed ideally in an area where pressure changes are maximal such as the palm of the hand or the volar aspect of the forearm.Brain anoxia this additional sensor can be used to derive a continuous signal representing peripheral blood vessel distention from which variations in blood pressure caused by slow wave activity can be derived. The variations in vessel distention caused by slow wave activity are extracted from the signal using filtering as previously described and are correlated with cerebral oximetry variations in the time domain, thus deriving a continuously updating correlation coefficient representing the autoregulation state of the patient. This sensor can also be configured to measure peripheral tissue oxygen saturation in addition to vessel distention using the same method as is used for cerebral oximetry. This implementation is an improvement over other methods because both of the measurements are completely noninvasive, both can be performed by a single device, and a measurement of continuous somatic tissue oxygen saturation can be derived.Brain anoxia

In many cases where an invasive means of blood pressure is considered unsuitable due to the lack of an invasive arterial catheter or when the risks outweigh the benefits of placing a catheter, a proxy for changes in pressure may be used to calculate autoregulation indices (such as changes in heart beat intervals described previously). In these cases a noninvasive sensor 108 and/or 110 for measuring blood pressure may be used, such as by using an occlusive cuff. Most automated cuff pressure devices also have a means to communicate a time-stamped value for blood pressure that can be used to help determine the range of blood pressure where autoregulation is intact by associating past autoregulation indices with previously obtained pressures.Brain anoxia these intermittent values can be used to automatically plot correlation coefficients as a function of pressure, enabling the caregiver to determine at a glance whether the blood pressure is too high or too low to support intact autoregulation. Alternatively, if an automated system is not used, the noninvasive autoregulation monitor can alert the staff when autoregulation is impaired, prompting a cuff pressure measurement or invasive pressure monitoring to better understand if the pressure is above or below the accepted normal range.

Variations in cerebral oximetry measured oxygen saturation may be caused by changes in arterial oxygen saturation. In some patients arterial oxygen saturation may be below the accepted normal range of 90-100% and/or may vary significantly over time.Brain anoxia this is typically true for infants and children with congenital heart defects such as septal defects, persistent patent ductus arteriosus, or other right-to-left shunts where deoxygenated venous blood mixes with oxygenated arterial blood as it is pumped into the systemic circulation. The reduction in arterial oxygen saturation is sometimes referred to as cyanosis as it can impart a bluish tinge to the skin. When arterial saturation is lower than normal, it tends to vary more often and to a greater extent because the arterial saturation range is located on the steeper part of the oxyhemoglobin dissociation curve and small changes in po2, ph and pco2 have a greater effect on arterial oxygen saturation.Brain anoxia lower and/or varying arterial saturation levels may also be present in adult patients who have acute respiratory distress syndrome (ARDS), chronic obstructive pulmonary disease (COPD), patients receiving mechanical ventilation or in patients receiving supplemental oxygen therapy. Variations in arterial oxygen saturation can cause parallel changes in cerebral oxygen saturation that can interfere with the measurement of autoregulation.

In one example, a NIRS system is described that can continuously measure blood volume pulsations for calculation of cbfi (as described above) using a near-infrared wavelength close to or at the isobestic point for hemoglobin (805 nm) where oxyhemoglobin and unbound hemoglobin absorb equally.Brain anoxia this ensures that the measurement of cbfi remains accurate during periods where arterial saturation may be below normal, for example, in patients with cyanosis due to left-right cardiac shunts or other pathology. This system also employs one or more additional wavelengths which are used to measure cerebral oxygen saturation (rso2). The system is designed to import an invasive blood pressure signal from a primary physiological monitor or can be designed to accept a blood pressure transducer to directly measure blood pressure. The monitor has the capability to display rso2, systolic, diastolic and mean blood pressure, cbfi and a representation of autoregulation index at multiple blood pressure levels.Brain anoxia this display consists of a graph where blood pressure is plotted on the x-axis and the correlation coefficient between blood pressure and cbfi are plotted on the y-axis. This display allows the user to immediately determine the optimal blood pressure range to assure the lowest correlation coefficient and therefore the optimal range to assure autoregulation is intact.

CONCLUSION•

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