«PEKKA TIIHONEN Novel Portable Devices for Recording Sleep Apnea and Evaluating Altered Consciousness Doctoral dissertation To be presented by ...»
- 28 Sleep Apnea In this technique, the sensor bands are attached directly to the skin. The initial raw signal needs sophisticated signal processing to provide useful method to detect respiratory efforts during sleep recordings (Yasuda 2005). With this method, the cardiac-induced variations in the impedance waveform can be misinterpreted as breathing (Brouillette 1987).
In addition, inductance plethysmography can be used for the measurement of breathing movements (Whyte 1991, Kaplan 2000). Inductance plethysmography is based on the use of an elastic belt containing a zigzagged wire. An alternating current is fed through the wire. This induces a magnetic field proportional to the alternating current. The variation in patient diameter (cross-sectional area) and composition due to breathing causes variation in the inductance, and this variation can be measured. The measurement principle does not rely on belt tension and the signal is linearly related with volume changes. Unfortunately, calibration gains depend on the position of the bands around the thorax and abdomen causing potential practical problems when the patient changes body position during the night (Farre 2004).
From a portable-monitoring point of view, the sensors for detection of respiratory movements should be: easy to set up, useable in different body positions, capable of maintaining good fixation during the night, durable for long term use and easy to connect to the ambulatory device.
Figure 2.5: Thorax and abdominal geometry and the transducers set up at the end of exhalation and at the end of inhalation.
Both transducers bend due to underlying tissue movement and generate a signal which describes respiratory efforts.
Although the breathing movements are complicated (Figure 2.5), a simple cylindrical physical model may be used to obtain a rough estimate of the lung volume change (Konno 1967). Suppose that we have a cylinder (residual air left in the lungs), which has a radius R1 and a height h. Then the volume V of the cylinder is SR12h. When the cylinder is full (at the end of inhalation), the volume is SR22h. By calculating the change of the volume, we obtain
By replacing the term R2h with a constant H and noting that the 2S(R2 - R1) is the
change of circumference 'C, Equation (11) can be further simplified as follows:
Thus, the change in V is linearly related to the change in the circumference 'C or a segment of it. The change in the circumference of the thorax and abdomen segments can be very different, and in the recording of respiratory movements during sleep, these segments must be measured separately.
Recording of peripheral blood oxygen saturation with a pulse oxymeter The principle of pulse oxymetry is based on different red and infrared light absorption of oxygenated and deoxygenated hemoglobin (Figure 2.6) (Yoshiya 1980). Oxygenated hemoglobin absorbs more infrared than red light and deoxygenated hemoglobin absorbs more red than infrared light. Light absorption in blood can be measured with transmission or reflectance techniques. The transmission technique, which is the most common method, requires a reasonably translucent site with good blood flow. Typical adult and pediatric sites are the finger, toe or ear lobe. The foot or palm of hand may be used with infants.
Figure 2.6: Typical transducer setup of a pulse oxymeter and the light absorption spectra of oxy- and deoxy hemoglobin (R = red, IR = infrared).
A typical oxymeter measures the red-infrared light intensity ratio, compares it to values of an internal look-up table and then provides the SpO2 value. Most manufacturers have their own empirical device and transducer specific look-up tables.
Modern pulse oxymeters can detect variation in the light absorption due to arterial pulsation (Figure 2.7). This information can be used to calculate heart rate. The heart rate value is usually averaged and does not represent an exact beat-to-beat rate.
Although pulse oxymeters are widely used, they have certain limitations that must be recognized. The fixing of the transducer emitter and detector head can be erroneous.
For example, the fingernail may have coloured polish or plastic coating, the bilirubin level of the patient can be high, the finger can have low perfusion or the patient may suffer from the sickle cell anemia. Furthermore, patient movements can distort pulse oxymetry. All these factors can make the measurement either unreliable or even impossible. However, according to the AASM manual (AASM 2007), the sensor of choice for detection of blood oxygen saturation is the pulse oxymeter with a maximum acceptable signal averaging time of three seconds.
Physical requirements for a pulse oxymeter for portable home monitoring are ease of use, small size, low power consumption and good technical reliability. In addition, the device should have a numerical display to confirm the correct fixing of the transducer at the start of the recording.
Figure 2.7: Schematic of light absorption in a pulse oxymetry sensor.
Variable absorption of arterial blood causes variation in transmitted radiation which can be measured. Inspired by Yoshiya (1980) and Ferrera (2006).
Body position recording It has been reported that collapsibility of the upper airways is strongly influenced by body position (Penzel 2001) and in a number of patients the sleep-related breathing problems occur in one body position only (Penzel 2006). Recording of body position is important, since certain patients can be treated by avoiding provocative (usually supine) positions (Cartwright 1991, Oksenberg 2000). Modern electronic acceleration sensors often used for detection of leg movements (Itoh 2007) can be used to detect body position as well. If one wishes to record all three dimensions, then three acceleration sensors will be needed. However, one dimension can be omitted if only supine, prone, left side and right side positions are recorded. Furthermore, dynamic body movements can be recorded with accelerometers. However, if only the body position is of interest, simple gravitation sensitive switches can be used as position sensors.
Recording of snoring sounds Snoring is basically barometric pressure changes occurring at the same audio frequencies as normal speech. Thus, the annoying sound of snoring can be recorded with the same instruments (microphones and recorders) as human speech. Snoring originates from relaxed soft tissues in the upper airways during sleep. In order to record snoring, the recorder must be able to record signals up to 100 Hz and, thus, the digital sampling rate must be at least 200 Hz (AASM 2007) but for more accurate detection of amplitude variation, a 500 Hz sampling rate is desirable. For portable monitoring this sampling rate is far too high, providing much unnecessary data. To minimize the amount of recorded data without decreasing the diagnostic value of the signal, the averaged amplitude level of the original audio signal can be full-wave rectified and filtered to produce an integrated signal. This integrated signal (intensity level) can then be sampled only a few times per second depending on the applied filters (AASM 2007).
The range of the snoring sound amplitude is so wide that its proper recording would require 16 or more bits. However, digitising and recording so many bits in portable monitors is not feasible. Fortunately, there are convenient integrated circuits, which can make an analogue conversion of input signal to a logarithm form and then integrate the result. With this method, the signal can be adequately presented with 12 bits or even less. In addition to a microphone, the nasal pressure sensor may be used to record snoring. The same signal conditioning precautions necessary for microphones are valid for pressure sensors, amplifiers and digitisers.
2.4.3 Diagnostic recommendations
The American Academy of Sleep Medicine has collected recommendations to a manual for the scoring of sleep and associated events (AASM 2007). The rules for detection of apnea and hypopnea events are summarized in Table 2.9. The AASM suggests that the sensor of choice for detection of an apnea event is an oronasal thermal sensor and for detection of a hypopnea event, the sensor of choice is a nasal air pressure transducer. To be counted as apnea or a hypopnea event, the duration of the event must be at least ten seconds. For an apnea event, air flow amplitude recorded with an oronasal thermistor must decrease t 90% for at least ten seconds. In contrast to detection of an apnea event,
several different criteria may be used to detect a hypopnea event. For example, the limit for the drop in nasal pressure signal amplitude varies between 30 and 50%.
Table 2.9: Diagnostic criteria of apnea and hypopnea events (AASM 2007).
3.1 Conditions causing altered consciousness Many medical conditions can evoke lowering of consciousness or even coma. Among the most common causes are supra- or subtentorial lesions (haemorrhage, infarction, tumours, head injury, cerebellar infarct or tumour, pontine haemorrhage or brainstem infarct) which account for one third of cases. Two thirds of cases are of a diffuse and/or metabolic origin (encephalitis, anoxia, ischemia, hypoglycemia, hepatic encephalopathy, endocrine disorders, drug poisons or ionic and acid-base disorders) (Posner 2007). In operating theatres patients are sedated intentionally by anaesthesiologists with sedative agents.
It is essential that one can monitor the level of consciousness or depth of sedation in modern intensive care units and emergency rooms (Jordan 1999). The general physical examination is not always enough and other methods are needed. Recording of spontaneous EEG can provide additional information for several reasons: EEG is tightly linked to cerebral metabolism, it is sensitive to the most common causes of cerebral injury, it can detect neuronal dysfunction and neuronal recovery, and it is the best method for detecting epileptic activity (Jordan 1999). In addition to spontaneous EEG recording, event-related potentials can provide valuable information on the level of consciousness and the cortical functions.
3.2 Monitoring of the level of consciousness
The evaluation of a patient with an altered level of consciousness consists of information on patient history, physical examination and laboratory evaluation. The physician must begin the examination and treatment simultaneously. When the usual laboratory tests have been evaluated and the situation has become stabilised, more sophisticated methods can be applied. EEG recording is not yet a common method, but it can provide valuable additional information (Jordan 1999). Techniques that use analysis of spontaneous EEG, such as bispectral monitoring and entropy analysis, have been used for detection level of consciousness during anaesthesia (Glass 1997, LeBlanc 2006, Hata 2009, von Delius 2009). These methods use only a few frontal EEG channels and evaluate mathematical and statistical relationships among various components of the EEG signal. The mathematical algorithms are an integral part of the analysis programs of anaesthesia monitors and are proprietary information of the company that developed them.
However, conventional EEG measures mainly the resting or background state of the brain. By adding exogenous stimulation (somatosensory, auditory or visual) and recording evoked potentials, one can obtain information on the reactivity of the brain. In particular, long-latency auditory evoked-potential measurements can be used to test the functioning of auditory cortical areas even during situation of reduced consciousness (Yppärilä 2002a, Yppärilä 2002b, Haenggi 2004, Yppärilä 2004a, Yppärilä 2004b).
3.2.1 General clinical examination
The initial examination of a comatose patient must be brief but thorough (Table 3.1).
This examination contains airway, breathing and circulation tests, checking of pupillary, oculomotor and motor responses, and major laboratory diagnostic tests (Posner 2007).
Laboratory diagnostics may contain blood and urine testing, computed tomography imaging and angiography, magnetic resonance imaging and angiography, magnetic resonance spectroscopy, neurosonography (Doppler sonography), lumbar puncture, ECG, EEG and evoked potential examinations. Recording of ERPs and all other evoked potential examinations can be seen as a continuum to conventional reaction tests, such as testing the reaction to a loud noise. A number of different scales (e.g., Glasgow Coma Scale) have been devised for scoring patients with coma (Posner 2007).
However, no scale is adequate for all patients and the best policy in recording the results of the coma examination is simply to describe the findings (Posner 2007).
Table 3.1: Examination of a comatose patient according to Posner (2007).