Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2000-09-29
2003-02-25
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S473000, C600S472000
Reexamination Certificate
active
06526297
ABSTRACT:
BACKGROUND OF THE INVENTION
The reliable monitoring of the depth of anesthesia has remained as one of the main challenges during the past ten years or so while measurement of most of the other key physiological parameters for comprehensive patient monitoring has practically reached a level of maturity. One of the reasons is that the concept of depth of anesthesia, when applying the modern concept of balanced anesthesia, is not a one-dimensional parameter, but has no less than five components. A balanced high quality anesthesia consists of adequate hypnosis, analgesia, muscle relaxation, suppression of the autonomous nervous system and blockade of the neuromuscular junction. Hypnosis means loss of consciousness down to a level able to guarantee amnesia, i.e. that no memories about the operation appear afterwards. Analgesia means that no pain is felt during the surgery. Sufficient muscle relaxation is required to ensure optimal operating conditions for the surgeon manipulating the tissue. The autonomous nervous system, if not suppressed, causes the patient to respond to surgical activity by shock reaction which affects heavily on the hemodynamics and endocrine system. To keep the patient completely motionless, the neuromuscular junction transmitting the orders from the brain to the muscles needs to be blocked, which means complete paralysis of the body. One practical consequence of the paralysis is that the patient also needs to be connected to a mechanical ventilator because the breathing muscles also become inoperative.
To achieve the state of balanced and adequate anesthesia, several different types of drugs are needed. For hypnosis one needs a drug affecting directly on the brain. Such a drug can be either inhalational anesthetics administered as a vapor into the lungs or intravenous agents infused into the blood circulation. Many of the hypnotically acting drugs also have an useful effect on pain, autonomous nervous system response, and muscle relaxation. However, special, dedicated drugs affect best on pain and neuromuscular blockade. What makes this complex picture even more difficult is that the practices of anesthesia vary from country to country and also among individual anesthesiologists. There are also schools of scientists that emphasize the weight of the components of the anesthesia in differing ways.
The importance of reliable monitoring of the depth of anesthesia has both safety and economy related aspects which are partly coupled one to another. Too light anesthesia and especially waking up in the middle of an operation may become an extremely traumatic experience both for the patient and the anesthesiologist. Unnecessarily deep anesthesia means increased costs in use of drugs most of which are rather expensive. Too deep anesthesia usually also affects the quality of the postoperative period and may increase time required for active recovery care.
Even though the importance of taking care and need to monitor all five components of the anesthesia is widely acknowledged, hypnosis has remained as the most difficult task because it is related to the challenging measurement of the level of consciousness which on a wider context also is a philosophical problem. Technically speaking, however, a solution able to quantify the brain activity on a consistent continuous scale extending from full alertness to maximally deep, but reversible, sleep can be considered adequate for the anesthesia purpose, if it is robust enough for the use of different drug cocktails in different individuals. Traditionally the attempts to develop methods and measurements for this purpose have been based on monitoring the electrical activity of the brain based on the weak biosignals picked up with electrodes on the skull surface. This method is called electroencephalography (EEG).
The complex EEG signal having close to random nature at first sight can be analyzed by many signal processing approaches which have developed to high level sophistication since the early days when the first findings about the change of spectral contents of the EEG signal as related to depth of consciousness were published. Generally speaking the EEG signal moves to lower frequencies when the sleep gets deeper and finally reaches a state called “burst suppression” when the signal is silent most of time with short intermediate bursts of electrical activity. The latest achievement in the EEG processing is the “bispectral index”, the BIS by Aspect Medical, which in addition to some conventional spectral analysis methods pays attention to the phase coupling between various EEG frequency pairs produced by nonlinear interaction in the electrical activity. What the actual connection of this component, which might reflect the number of independent “oscillators” in the brain, is to the level of consciousness is not well understood. Therefore, the BIS number on a scale from 100 down to zero when moving from alertness to deepest possible sleep, is a semi-empirical combination of various EEG features based on profound statistical analysis of a wide data base collected during thousands of anesthesias. The BIS is based on the processing of not only the spontaneous EEG activity, but also on evoked potentials, the response of the brain to external stimuli, which have been proposed to be used to monitor the level of consciousness. These may be, for example, audible clicks to the ear, or light electrical impulses to the nerves. The former are called acoustic evoked potentials (AEP) and the latter somatosensory evoked potentials (SSEP).
The generic problem of the EEG and evoked potentials in anesthesia application is the artifacts caused by external electrical interference from the other devices, especially the electrocautery machine, which is known as the electrical knife. Additionally the attachment procedure of the EEG electrodes is often considered as consuming too much time in the streamlined fast modern anesthesia process. An additional drawback related both to raw EEG and evoked potentials is the limited speed of response, because both methods require collection of data for a certain period of time, typically at least 5 seconds, and on the top of that some additional time for computing. This can become a problem especially when trying to detect if the patient is waking up from too light an anesthesia which happens very fast because of the physiological cascade mechanisms involving positive feedback loops.
BRIEF SUMMARY OF THE INVENTION
The purpose of this invention is to present a new approach to quantify the relevant brain activity status and its connection to the level of consciousness especially related to the adequacy of the hypnotic component of the depth of anesthesia. It is based the results of many recent findings of basic brain research which has been able to confirm the close physiological links between the electrical activity of the brain and the functional control of the blood and oxygen delivery to the neurally activated part of the brain. This is also coupled with the local metabolic activity reflected as increased glucose metabolic rate. See articles 1, 2 in the list of references. The methods which have illuminated this field the most include positron emission tomography (PET), functional magnetic resonance imaging (fMRI) and near infrared spectroscopy (NIRS).
Technically the NIRS method differs from the two others mentioned above as being much more simple and less expensive. Human tissue including the brain cortex is transparent for near-infrared (NIR) light into depth of several centimeters. The optical wavelengths suitable for this purpose are typically between 700 and 900 nanometers. The NIR spectroscopy has been employed in commercial devices like the Somanetics Invos 300 and Hamamatsu NIRO 500 with a purpose to monitor oxygenation of the cerebral blood. These devices have been designed primarily to be used during carotic artery surgical procedures to give a warning of development of ischemia on the brain tissue. Recent studies (3,4) have indicated that these two devices seem not to measure the same parameter an
Andrus Sceales Starke & Sawall LLP
Instrumentarium Corp.
Kremer Matthew
Winakur Eric F.
LandOfFree
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