PATIENT MONITORING

Modern medicine allows for the monitoring of high-risk pa­tients so that medical treatment can be applied adequately as their condition worsens. To detect changes in the physiologi­cal condition of each patient, appropriate monitoring is ap­plied routinely according to the patient’s condition, at least in well-equipped hospitals. Patient monitoring usually means the physiological monitoring of high-risk patients using ap­propriate instruments.

In hospitals, there are many sites where patient monitor­ing is especially important. For example, in the operating room, instruments such as a pulse oximeter are used for mon­itoring anesthesia; in the intensive care unit, vital signs are monitored; in the coronary care unit, the patient’s electrocar­diogram (ECG) is routinely monitored and analyzed automat­ically; and in the incubator, the vital signs of the infant as well as the internal environment of the incubator are moni­tored. In addition, during examinations such as cardiac cathe­terization, and therapeutic procedures such as hyper — or hy­pothermia therapy, patient monitoring is required for ensuring safety. Even in the general ward, monitoring is per­formed fairly often when some risks are suspected. By using a telemetry system, the patient is not constrained to a bed. Even out of the hospital, patient monitoring is still performed in some situations. In the ambulance, postresuscitation man­agement requires the use of a cardiac monitor. In the home where medical care such as oxygen therapy and intravenous infusion therapy is carried out, monitoring instruments are helpful. A so-called Holter recorder is used in which 24-h ECG is recorded for detecting spontaneous events such as cardiac arrhythmia.

There are many parameters that are used for patient mon­itoring: Among them are heart rate, ECG, blood pressure, car­diac output, rate of respiration, tidal volume, expiratory gas content, blood gas concentrations, body temperature, metabo­lism, electroencephalogram (EEG), intracranial pressure, blood glucose levels, blood pH, electrolytes, and body motion. Many types of monitoring techniques and instruments have been developed to enable measurement of these parameters.

For high-risk patients, monitoring should be performed continuously. The real-time display of the trend or waveform of each parameter is helpful especially in a patient who is experiencing cardiopulmonary function problems, because if a sudden failure of respiration or circulation is not detected immediately it may result in the physiological state of the patient becoming critical. The reliability of monitoring is quite important. In some situations, invasive procedures for monitoring are allowed if they are considered essential. For example, an indwelling arterial catheter is used when instan­taneous blood pressure has to be monitored continuously. However, invasive methods are undesirable if the patient’s condition is less critical. In some situations, noninvasive methods are preferred. Because noninvasive methods are al­ways more difficult to perform or less accurate than invasive methods, the development of reliable noninvasive monitoring techniques is highly desirable; many smart noninvasive tech­niques have already been developed and supplied commer­cially.

Safety is an important feature of any monitoring device because monitoring is performed for a long period of time for the critically ill patient. Electric safety is strictly required es­pecially when the monitoring device has electric contacts to the patient body. Sometimes, two or more monitors are ap­plied to a patient. Leakage current should be avoided under any failure of each device. Electromagnetic compatibility is also important. Monitoring instruments should be immune to any possible electromagnetic interference from telemetering devices, mobile telephones or other noice sources such as elec­trosurgery.

Many patient monitors have an automatic alarm function. When a monitoring item is expressed as a single value such as heart rate, blood pressure, or body temperature, the alarm condition is determined by setting a level or range, and the monitor gives an alarm sign, such as warning and urgent, according to the patient’s condition. When the monitoring item is expressed in a waveform, such as the ECG, the alarm system needs to be able to perform real-time waveform analy­ses. In any alarm system, two kinds of error—false positives and false negatives—may occur. In critically ill patients, a false negative may be fatal. While false positives may be tol­erated to some extent, repeated false alarms may seriously disturb the clinical staff. In general, any alarm system re­quires some logic, and some times highly intelligent signal processing is required.

CONVENTIONAL PATIENT MONITORING TECHNIQUES

Electrocardiogram Monitoring

For sudden heart failure, urgent treatment is required. Moni­toring of heart function is therefore quite important. The ECG is the most convenient method of monitoring the electrical function of the heart, whereas the mechanical pump function of the heart is best monitored by examining the patient’s blood pressure and cardiac output. An ECG signal can be ob­tained by attaching electrodes to the body surface. For patient monitoring, electrodes are always attached to the torso as shown in Fig. 1(a), whereas the standard lead system in which electrodes are attached to the limb and chest is used in ordinary ECG examinations for diagnosis. Disposable ECG electrodes, as shown in Fig. 1(b), are commonly used for long­term monitoring. A stable ECG can be obtained using these electrodes for a day or longer.

The ECG waveform thus obtained is always displayed on a CRT monitoring screen with ordinary sweep speeds, to­gether with other parameters. Unusual waveforms such as premature ventricular contractions can be identified visually. However, it is unlikely that someone would be able to watch the monitor screen all of the time. Most ECG monitors have a built-in computer that automatically detects abnormal waveforms and triggers the alarm. To reduce as much as pos­sible the number of false alarms, both false negatives and false positives, highly intelligent algorithms for detecting ab­normal waveforms, such as arrhythmias, have been developed

Metal snap

Figure 1. Typical electrode locations for ECG monitoring (a), and a cross-section of a disposable foam electrode (b).

(b)

and installed in intensive care monitoring systems (1). Most bedside ECG monitoring systems have a real-time display and large data storage facility that allows for retrospective observation. Some of them have a memory capacity that is able to record an ECG for up to 24 h. Radiotelemetering is convenient, even in bedside monitoring. Eliminating the cable connection to the patient is advantageous not only to make the patient less restricted, but also to attain electrical safety. However, electromagnetic compatibility should be secured when it is used together with other instruments.

For ambulatory patients, Holter ECG monitoring is per­formed in which the ECG is recorded typically for 24 h. The typical Holter recorder records the ECG on an audio cassette tape for 24 h, then the tape is brought to the hospital, the recorded ECG is played back by a scanner at 60 or 120 times the recording speed, and analyzed automatically so that dif­ferent kinds of arrhythmias and other abnormalities may be classified and counted. To detect and record only pathological waveforms, a digital recorder with solid-state memory can be used; for example, a system can detect automatically the change in ECG during transient myocardial ischemia and re­cord up to 18 episodes that are only 6 s each (2). Although longer time digital recording needs a very large memory ca­pacity, 24 h recording is realized using a small hard disk drive in a system in which the ECG data is first stored in a solid-state memory and then transferred to the disk over short periods of time (3).

Blood Pressure Monitoring

Arterial blood pressure monitoring is essential in a patient whose circulation is unstable, and is commonly performed during cardiovascular surgery and postoperative care. There are two methods of blood pressure monitoring—direct and in­direct. In the direct method, a catheter is introduced into the artery as shown in Fig. 2, and a pressure transducer is con­nected to the proximal end of the catheter. To avoid blood clotting in the catheter, a small amount of saline is supplied either continuously or intermittently. Intraarterial pressure can be measured accurately enough as long as the transducer is adequately calibrated. Either a strain-gage or capacitive type of pressure transducer is commonly used for this pur­pose. Disposable pressure transducers are convenient because sterilization of the transducer before use is troublesome. In addition, the performance of disposable pressure transducers is comparable or even better than that of expensive reusable pressure transducers (4).

The catheter-tip pressure transducer which has a pres­sure-sensing element at the tip is sometimes used for in — traarterial pressure monitoring. It has many advantages: It has no time delay and has a flat frequency response over a wider range; saline injection is unnecessary; and it is less af­fected by the mechanical motion of the catheter. However, it is fragile and expensive. Many different principles can be used in detecting pressure at the tip, such as semiconductor strain gauges, and capacitive and optical principles. Some transducers have many pressure-sensing elements near the tip. For example, a model is available that has up to six pres­sure sensing elements in an 8F size tip (outer diameter 2.67 mm) (Mikro-Tip, Millar Instruments, Inc., Houston Texas).

While the direct blood pressure measurement method is accurate and reliable, it is an invasive procedure, and, thus, an indirect noninvasive method is preferred for less critical patients. The most common method of indirect blood pressure measurement is the auscultatory method in which a pressure cuff is attached to the upper arm. The cuff is deflated from a position somewhat above the systolic pressure, and both the systolic and diastolic pressures are determined by monitoring a sphygmomanometer while listening for the Korotkoff sound using a stethoscope. While the auscultatory method is the standard method of clinical blood pressure measurement, and is actually performed for patient monitoring such as during anesthesia, it is neither automatic nor continuous. Hence, a noninvasive continuous blood pressure monitor had been in demand. Two methods have now become available: the vascu­lar unloading method and the tonometry.

Figure 2. The conventional method of direct arterial pressure moni­toring.

Finger cuff

The vascular unloading method is used to measure instan­taneous intraarterial pressure by balancing externally ap­plied pressure to the intravascular pressure using a fast pneumatic servo-controlled system (5). As shown schemati­cally in Fig. 3(a), a cuff is attached to a finger, and near-infra­red light transmittance is measured at the site where the cuff pressure is affected uniformly. Because absorption at near — infrared is mainly due to the hemoglobin in blood, the change in light absorption corresponds to the change of blood volume at optical pass, thus a pulsatile change in transmitting light intensity is observed from the pulsation of the artery. It is possible to compensate for the pulsatile change of arterial blood volume by introducing a servocontrol in which cuff pres­sure is controlled by the intensity of the transmitted light so that an increase of arterial blood increases light absorption and the signal increases cuff pressure so as to obstruct fur­ther increase of arterial flow. If such a servocontrol works fast enough and with a sufficient loop gain at an adequate level of light intensity, a condition is realized where the intraarterial and the cuff pressures are balanced. At this condition, the circumferential tension of the arterial wall is reduced to zero; such a situation is called vascular unloading. It has been shown that accurate blood pressure together with instanta­neous arterial pressure waveforms can be obtained when an adequate servocontrol system is introduced and adjusted cor­rectly (6). A commercial unit that uses this principle has been developed (Finapress, Ohmeda, Englewood, Colorado). In this system, the interface module, which has a pneumatic servo — valve is attached to the back of the hand so that the connec­tion from the valve to the finger cuff is minimized, thus reduc­ing the time delay.

Tonometry is a method of measuring internal pressure from the reaction force. When a flat plate is pressed onto a flexible deformable boundary membrane to which internal pressure is exerted, internal pressure can be measured from the outside regardless of the transverse tension developed in the membrane. This principle has been applied successfully in intraocular pressure measurement, and it is also applicable to arterial blood pressure measurement (7). As shown in Fig. 3(b), the tonometry transducer, the tonometer, is applied to the skin surface so that an artery is just beneath the sensing element, and a part of the arterial wall is flattened. To detect the pressure at the center of the arterial flattening, a multi — ple-element transducer is used, and the value at the center of the pressure distribution is detected automatically. Measure­ment is always performed on the radial artery at the wrist. A tonometer is now commercially available (Jentow, Nihon Co­lin Co., Komaki-shi Japan).

Sometimes, blood pressure is monitored in an ambulatory patient. For this purpose, a fully automated portable sphyg — momanometry system is used. A pressure cuff is attached to the upper arm, and is inflated intermittently at selected inter­vals. The Korotkoff sound is detected by a microphone, and systolic and diastolic pressures are determined and stored in a memory. Commercial models are now available (e. g., Medi — log ABP, Oxford Medical Ltd., Oxford, UK) (8).

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