3.2.3 Practical ConsiderationRecord

3.2.3 Practical Consideration
Recording of cardiovascular events in a laboratory (blood pressure, blood flow, electrocardiogram) is usually a time-varying periodic function.2 Furthermore, it is not surprising that each variable has its own characteristic waveform that changes repetitiously over time. In order to accurately measure such variables over time, its waveform characteristics have to be analyzed and understood. Ultimately, the main goals of recording any physiological event within the circulatory system is to obtain an accurate recording of the event that is reproducible. To this end, a clear understanding of the errors associated with the recording of the events must first be understood. Errors can arise from many sources, including the operator, the environment, and the recording modules/instruments. Within the recording system, errors can arise and be associated with components such as amplifiers, processors, recorders, transducers, catheters, and so on. Simply put, a thorough understanding of the entire setup is absolutely necessary for meaningful and accurate recordings of any cardiovascular parameter.
It must be recognized that recording blood flow or, for that matter, any cardiovascular parameter requires appropriate amplification, processing, storing, and analysis of the signal. It then follows that certain minimum criteria must be met by the recording instrument (electromagnetic flowmeters are no exception) if meaningful blood flow measurements are to be made accurately and be reproducible under a variety of conditions. These pertinent factors are phase/amplitude linearity and adequate frequency response of the recording modules and apparatus.
3.2.3.1 Phase Linearity
This is a reference to the capability of the recording module to provide a signal (i.e., rate of flow) that is as close as the measurement of the event that is being presented to it. Deviation from linearity of input and output will result in phase distortion. This will manifest itself in complex displacement of the output signal in the time axis, and thus result in erroneous measurement of blood flow.
3.2.3.2 Amplitude Response
The recording module is required to have a frequency response great enough to measure the highest harmonic of the flow variable that is being measured. There appears to be no evidence to indicate that there are significant propagated waves in terms of blood flow that exist above 30 Hz, and it seems that those above 20 Hz are fairly small.2 For example, Patel and associates13 have reported that there is little information beyond the 11th harmonic component (~30 Hz), and Bergel and Gessner14 seem to suggest that the limit is about the 8th harmonics in pulmonary artery and aortic flow pulses. It is reasonable to assume that a flowmeter that has a frequency response of >60 Hz will be capable of measuring blood flow in most species.
3.2.3.3 Amplitude Linearity
This aspect of the recording system is a little more complex and it has to do with various factors, namely, drift, noise, hysteresis, and calibration of the flowmeters. In general, the term amplitude linearity refers to the ability of the recording system to produce an output signal that is directly proportional in magnitude to the input signal amplitude. Clearly, this should apply not only to above zero baseline values but also to below the zero line, and must also include the entire range of the measurements. This is an important criterion in measuring blood flow using an electromagnetic flow probe. Therefore, it is imperative that before choosing a flowmeter the approximate range of the variable to be assessed must be known. Obviously, such values can be obtained from reference to the literature. Currently, commercially available flowmeters are capable of measuring blood flow in ranges of 5 ml/min to 20 l/min.
3.2.3.3.1 Drift
Drift associated with an electromagnetic flowmeter will affect accurate measurements of blood flow. In cases where there is drift of the baseline without actual changes to slope (i.e., sensitivity), measurement of blood flow will be out of phase by the amount of change associated with the baseline. This, of course, may be either negative or positive from the zero baseline. However, when there is drift associated with the slope or actual calibration of the flowmeter, it will lead to alteration of the sensitivity curve. When using an electromagnetic flowmeter for measurement of blood flow over a period of time, no matter how well the system is calibrated, changes in zero baseline or calibration can occur, and this will result in erroneous measurement of blood flow. Certainly, physiochemical changes in blood vessels can contribute to drifting, which can ultimately have an effect on zero line stability and actual flow measurements. It is recognized that zero line stability can be greatly enhanced with good contact between electrode-to-blood vessel wall. It seems that heat production in the probe is also another source of zero line instability, especially in smaller blood vessels. It has been reported that interelectrode voltages are altered if the temperature at one electrode is changed in relation to the other.15 In smaller blood vessels, thermic effects may be considerable and may produce substantive drift. Therefore, appropriate precautions need to be taken in order to avoid drift, and care must be taken in calibration of the instrument. In addition, frequent checks of zero baseline as well as calibration of sensitivity are imperative. Simply, periodic checking and readjustments are necessary for accurate and reproducible measurement of blood flow when using an electromagnetic flowmeter. Moreover, it seems that electronic zero obtained by switching off the magnet does not always coincide with the mechanical zero. Therefore, it is advisable that whenever possible, an occlusive cuff or snare placed on the artery beyond the probe be used to obtain mechanical zero. Zero can be obtained naturally when recordings are being carried out in the ascending aorta or pulmonary artery as zero occurs during the latter portion of diastole.
3.2.3.3.2 Noise
Clearly, a potential problem in recording any signal is unwanted noise. The signal-to-noise ratio can have a significant impact on the accuracy of the instrument, and the noise is most likely seen in the lowfrequency recordings. This becomes an important factor especially when measuring low velocity flow. Obviously, since the velocity of flow is a function of frequency in experiments using a flowmeter, the signal-to-noise ratio becomes smaller at higher frequencies.
In general, noise is part of the output of an instrument, and it can be generated by the instrument, caused by external interference, or both. It is useful to distinguish the noise from the actual signal being measured. This can be done by assessing signal-to-noise ratio. Essentially, in order to determine the value that signifies the noise that is either generated or is associated with an instrument, the signal-to-noise ratio at the input is divided by the signal-to-noise ratio at the output. Needless to say, the ideal ratio is unity. When noise exists, it is desirable to identify its source and eliminate it completely or reduce it as much as possible. Most often, elimination or reduction in noise levels can be achieved by proper grounding. In addition, high frequency noise may be dealt with by the introduction of low-frequency band electronic filters provided that the filter does not eliminate or reduce the signal that is being measured. In practical terms, a flowmeter with a magnetic field of about 100 G that generates a mean signal of ~5 mV should have a noise level of no more than 0.1 mV. This translates into a signal-to-noise ratio of ~50, which is acceptable.
3.2.3.3.3 Hysteresis
An important source of error that can have a direct impact on systems linearity is hysteresis. A definition of the term means a “lag of effect.” In terms of measurements, this defines the ability of the instrument to produce an output that follows the input independently. In electromagnetic flowmeters, there are several sources of error that may be ascribed to hysteresis, for example, the nonuniformity of the magnetic field. The magnetic field produced by the magnets of the probe only covers a small length of the vessel. This essentially results in the magnetic field not being uniform along the vessel axis, and more importantly, it is not uniform across the lumen of the vessel. This can result in reduction in sensitivity by as much as 20 to 50%.14 However, this reduction is constant for a given probe and will be contained in the calibration factor. Another source may be polarization effects. Polarization effects at the recording electrodes alter contact impedance between the electrode and vessel wall. These effects may not follow similar patterns at the two recording electrodes. Under such conditions, the voltage observed is less than the one that can be calculated theoretically by the given equation. Other sources include shunting by conductive vessel walls and surrounding conductive fluids, which will be discussed and dealt with in detail later on in the chapter.

Kết quả (Việt) 1:

3.2.3 thực tế xem xétGhi âm của các sự kiện tim mạch trong một phòng thí nghiệm (huyết áp, lưu lượng máu, electrocardiogram) thường là một thời gian thay đổi định kỳ function.2 Furthermore, nó không phải là đáng ngạc nhiên rằng mỗi biến có dạng sóng đặc trưng riêng của mình rằng những thay đổi repetitiously theo thời gian. Để chính xác đo lường các biến theo thời gian, đặc điểm dạng sóng của nó phải được phân tích và hiểu rõ. Cuối cùng, mục tiêu chính của ghi âm bất kỳ sự kiện sinh lý trong hệ thống tuần hoàn là để có được một ghi chính xác của sự kiện là thể sanh sản nhiều. Để kết thúc này, một sự hiểu biết rõ ràng của các lỗi liên quan đến với bản ghi âm của các sự kiện đầu tiên phải được hiểu. Lỗi có thể phát sinh từ nhiều nguồn, bao gồm cả các nhà điều hành, môi trường, và các mô-đun ghi âm/dụng cụ. Trong hệ thống ghi âm, lỗi có thể phát sinh và được liên kết với các thành phần như khuếch đại, bộ vi xử lý, ghi âm, cảm biến, ống thông, và như vậy. Chỉ cần đặt, một sự hiểu biết toàn diện về các thiết lập toàn bộ là hoàn toàn cần thiết cho bản thu âm có ý nghĩa và chính xác của bất kỳ tham số tim mạch.Nó phải được công nhận rằng ghi lưu lượng máu hoặc, cho rằng vấn đề, bất kỳ tham số tim mạch yêu cầu thích hợp khuếch đại, xử lý, lưu trữ, và phân tích các tín hiệu. Nó sau đó theo tiêu chí tối thiểu nhất định phải được đáp ứng bởi công cụ ghi âm (điện từ flowmeters là không có ngoại lệ) nếu có ý nghĩa máu chảy phép đo được thực hiện một cách chính xác và có thể sanh sản nhiều theo một loạt các điều kiện. Các yếu tố cần thiết là giai đoạn/biên độ linearity và đáp ứng đầy đủ tần số của các ghi âm mô-đun và bộ máy.3.2.3.1 giai đoạn LinearityĐây là một tham chiếu đến khả năng của các mô-đun ghi âm cung cấp một tín hiệu (tức là, tốc độ dòng chảy) đó là càng gần càng đo lường các sự kiện đang được trình bày với nó. Độ lệch từ linearity của đầu vào và đầu ra sẽ dẫn đến biến dạng giai đoạn. Điều này sẽ biểu hiện bản thân trong trọng lượng rẽ nước phức tạp của các tín hiệu đầu ra trong các trục thời gian, và do đó dẫn đến đo lường sai của lưu lượng máu.3.2.3.2 biên độ phản ứngThe recording module is required to have a frequency response great enough to measure the highest harmonic of the flow variable that is being measured. There appears to be no evidence to indicate that there are significant propagated waves in terms of blood flow that exist above 30 Hz, and it seems that those above 20 Hz are fairly small.2 For example, Patel and associates13 have reported that there is little information beyond the 11th harmonic component (~30 Hz), and Bergel and Gessner14 seem to suggest that the limit is about the 8th harmonics in pulmonary artery and aortic flow pulses. It is reasonable to assume that a flowmeter that has a frequency response of >60 Hz will be capable of measuring blood flow in most species.3.2.3.3 Amplitude LinearityThis aspect of the recording system is a little more complex and it has to do with various factors, namely, drift, noise, hysteresis, and calibration of the flowmeters. In general, the term amplitude linearity refers to the ability of the recording system to produce an output signal that is directly proportional in magnitude to the input signal amplitude. Clearly, this should apply not only to above zero baseline values but also to below the zero line, and must also include the entire range of the measurements. This is an important criterion in measuring blood flow using an electromagnetic flow probe. Therefore, it is imperative that before choosing a flowmeter the approximate range of the variable to be assessed must be known. Obviously, such values can be obtained from reference to the literature. Currently, commercially available flowmeters are capable of measuring blood flow in ranges of 5 ml/min to 20 l/min.3.2.3.3.1 Drift
Drift associated with an electromagnetic flowmeter will affect accurate measurements of blood flow. In cases where there is drift of the baseline without actual changes to slope (i.e., sensitivity), measurement of blood flow will be out of phase by the amount of change associated with the baseline. This, of course, may be either negative or positive from the zero baseline. However, when there is drift associated with the slope or actual calibration of the flowmeter, it will lead to alteration of the sensitivity curve. When using an electromagnetic flowmeter for measurement of blood flow over a period of time, no matter how well the system is calibrated, changes in zero baseline or calibration can occur, and this will result in erroneous measurement of blood flow. Certainly, physiochemical changes in blood vessels can contribute to drifting, which can ultimately have an effect on zero line stability and actual flow measurements. It is recognized that zero line stability can be greatly enhanced with good contact between electrode-to-blood vessel wall. It seems that heat production in the probe is also another source of zero line instability, especially in smaller blood vessels. It has been reported that interelectrode voltages are altered if the temperature at one electrode is changed in relation to the other.15 In smaller blood vessels, thermic effects may be considerable and may produce substantive drift. Therefore, appropriate precautions need to be taken in order to avoid drift, and care must be taken in calibration of the instrument. In addition, frequent checks of zero baseline as well as calibration of sensitivity are imperative. Simply, periodic checking and readjustments are necessary for accurate and reproducible measurement of blood flow when using an electromagnetic flowmeter. Moreover, it seems that electronic zero obtained by switching off the magnet does not always coincide with the mechanical zero. Therefore, it is advisable that whenever possible, an occlusive cuff or snare placed on the artery beyond the probe be used to obtain mechanical zero. Zero can be obtained naturally when recordings are being carried out in the ascending aorta or pulmonary artery as zero occurs during the latter portion of diastole.
3.2.3.3.2 Noise
Clearly, a potential problem in recording any signal is unwanted noise. The signal-to-noise ratio can have a significant impact on the accuracy of the instrument, and the noise is most likely seen in the lowfrequency recordings. This becomes an important factor especially when measuring low velocity flow. Obviously, since the velocity of flow is a function of frequency in experiments using a flowmeter, the signal-to-noise ratio becomes smaller at higher frequencies.
In general, noise is part of the output of an instrument, and it can be generated by the instrument, caused by external interference, or both. It is useful to distinguish the noise from the actual signal being measured. This can be done by assessing signal-to-noise ratio. Essentially, in order to determine the value that signifies the noise that is either generated or is associated with an instrument, the signal-to-noise ratio at the input is divided by the signal-to-noise ratio at the output. Needless to say, the ideal ratio is unity. When noise exists, it is desirable to identify its source and eliminate it completely or reduce it as much as possible. Most often, elimination or reduction in noise levels can be achieved by proper grounding. In addition, high frequency noise may be dealt with by the introduction of low-frequency band electronic filters provided that the filter does not eliminate or reduce the signal that is being measured. In practical terms, a flowmeter with a magnetic field of about 100 G that generates a mean signal of ~5 mV should have a noise level of no more than 0.1 mV. This translates into a signal-to-noise ratio of ~50, which is acceptable.
3.2.3.3.3 Hysteresis
An important source of error that can have a direct impact on systems linearity is hysteresis. A definition of the term means a “lag of effect.” In terms of measurements, this defines the ability of the instrument to produce an output that follows the input independently. In electromagnetic flowmeters, there are several sources of error that may be ascribed to hysteresis, for example, the nonuniformity of the magnetic field. The magnetic field produced by the magnets of the probe only covers a small length of the vessel. This essentially results in the magnetic field not being uniform along the vessel axis, and more importantly, it is not uniform across the lumen of the vessel. This can result in reduction in sensitivity by as much as 20 to 50%.14 However, this reduction is constant for a given probe and will be contained in the calibration factor. Another source may be polarization effects. Polarization effects at the recording electrodes alter contact impedance between the electrode and vessel wall. These effects may not follow similar patterns at the two recording electrodes. Under such conditions, the voltage observed is less than the one that can be calculated theoretically by the given equation. Other sources include shunting by conductive vessel walls and surrounding conductive fluids, which will be discussed and dealt with in detail later on in the chapter.