GE Cardiocap 5 Service Manual

							
Measurement Parameters 
6.2 Measurement principles for hemodynamic parameters 
6.2.1 NIBP 
NIBP (Non-Invasive Blood Pressure) is an indirect method for measuring blood pressure. 
The NIBP measurement is performed according to the oscillometric measuring principle. The cuff is 
inflated with a pressure slightly higher than the presumed systolic pressure and deflated at a speed 
based on the patients pulse. Data are collected from the oscillations caused by the pulsating artery. 
Based on these oscillations, values for systolic, mean, and diastolic pressures are calculated. 
6.2.2 ECG 
Electrocardiography analyzes the electrical activity of the heart by measuring the electrical potential 
produced with electrodes placed on the surface of the body. 
ECG reflects 
• Electrical activity of the heart. 
• Normal/abnormal function of the heart. 
• Effects of anesthesia on heart function. 
• Effects of surgery on heart function. 
See the Users Reference Manual for electrode positions and other information. 
6.2.3 Impedance respiration 
Impedance respiration is measured across the thorax between three ECG electrodes. The respiration 
signal is made by supplying current between two electrodes and by measuring the differential current 
from the third electrode. The input current is 200 µA (31 kHz).  The impedance measured is the 
impedance change caused by breathing. When the patient is breathing or is ventilated, the volume of 
air in the lungs changes, resulting in impedance between the electrodes. From these impedance 
changes, the respiration rate is calculated and the respiration waveform is displayed on the screen.  
6.2.4 Temperature 
Temperature is measured by a probe whose resistance varies when the temperature changes, called 
NTC (Negative Temperature Coefficient) resistor. 
The resistance can be measured by two complementary methods: 
• Applying a constant voltage across the resistor and measuring the current that flows through it. 
• Applying a constant current through the resistor and measuring the voltage that is generated 
across it. 
The two methods are combined in the form of a voltage divider. The NTC resistor is connected in series 
with a normal resistor and a constant voltage is applied across them. The temperature dependent 
voltage can be detected at the junction of the resistors, thus producing the temperature signal from 
the patient. The signal is amplified by analog amplifiers and further processed by digital electronics. 
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6.2.5 Invasive blood pressure (N-XP option) 
A catheter is inserted into an artery or vein to measure invasive blood pressure. The invasive pressure 
setup—consisting of tubing, a pressure transducer, and an intravenous bag of normal saline all 
connected together by stopcocks—is attached to the catheter. The transducer is placed level with the 
heart and electrically zeroed. 
The transducer is a piezo-resistive device that converts the pressure signal to a voltage. The monitor 
interprets the voltage signal so that pressure data and pressure waveforms can be displayed. 
6.2.6 Pulse oximetry, standard 
NOTE: Only one pulse oximetry source at a time is allowed by the Cardiocap/5. When the N-XOSAT or  
N-XNSAT option is installed in the monitor, standard Cardiocap/5 pulse oximetry is not available. 
A pulse oximeter measures the light absorption of blood at two wavelengths, one in the near infrared 
(about 900 nm) region and the other in the red region (about 660 nm) of the light spectrum. These 
wavelengths are emitted by LEDs in the SpO2 sensor. The light is transmitted through peripheral tissue 
and is detected by a PIN diode opposite to the LEDs in the sensor. The pulse oximeter derives the 
oxygen saturation (SpO2) using an empirically-determined relationship between the relative absorption 
at the two wavelengths and the arterial oxygen saturation, SaO2. 
To measure the arterial saturation accurately, pulse oximeters use the component of light absorption 
giving variations synchronous with heart beat as primary information on the arterial saturation.  
 
Figure 6-3. Absorption of infrared light in the finger 
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Measurement Parameters 
A general limitation of the above pulse oximetry principle is that due to the use of only two 
wavelengths, only two hemoglobin species can be discriminated by the measurement.  
Modern pulse oximeters are empirically calibrated against fractional saturation (SaO2frac) or against 
functional saturation (SaO2func) as shown below: 
SaO2frac = HbO2/(HbO2 + Hb + Dyshemoglobin) 
SaO2func = HbO2/(HbO2 + Hb). 
Functional saturation (SaO2func) is less sensitive to changes of carboxyhemoglobin and 
methemoglobin concentrations in blood. 
The oxygen saturation percentage SpO2 is calibrated against the functional saturation (SaO2func). The 
advantage of this method is that the accuracy of SpO2 measurement relative to SaO2func can be 
maintained even at rather high concentrations of carboxyhemoglobin in blood. Independent of the 
calibration method, a pulse oximeter is not able to correctly measure oxygen content of the arterial 
blood at elevated carboxyhemoglobin or methemoglobin levels. 
Plethysmographic pulse wave 
The plethysmographic waveform is derived from the IR signal and reflects the blood pulsation at the 
measuring site. Thus the amplitude of the waveform represents the perfusion. 
Sensor 
SpO2 sensors contain the light source LEDs, which are located opposite the photodiode detector. 
Different kinds of sensors are available from GE Healthcare, including clip-on and wrap styles. 
Pulse rate 
The pulse rate calculation is done by peak detection of the plethysmographic pulse wave. The signals 
are filtered to reduce noise and checked to separate artifacts. 
6.2.7 Pulse oximetry, Datex-Ohmeda enhanced (N-XOSAT option) 
NOTE: Only one pulse oximetry source at a time is allowed by the Cardiocap/5. When the N-XOSAT 
option is installed in the monitor, standard pulse oximetry and N-XNSAT are not available. 
The Datex-Ohmeda enhanced pulse oximetry option uses a two-wavelength pulsatile system—red and 
infrared light—to distinguish between oxygenated (O2Hb) and reduced (HHb) hemoglobin, each of 
which absorbs different amounts of light emitted from the oximeter sensor. The SpO2 and pulse rate 
are determined by the oximeter through sensor signal processing and microprocessor calculations. 
The SpO2 sensor contains a light source and a photodetector: 
• The light source consists of red and infrared light-emitting diodes (LEDs). 
• The photodetector is an electronic device that produces an electrical current proportional to 
incident light intensity. 
The two light wavelengths generated by the sensor light source (the red and infrared LEDs) pass 
through the tissue at the sensor site. The light is partially absorbed and modulated as it passes 
through the tissue. 
Arterial blood pulsation at the sensor site modulates transmission of the sensor’s light. Since other 
fluids and tissues present generally don’t pulsate, they don’t modulate the light passing through that 
location. The pulsatile portion of the incoming signal is used to detect and isolate the attenuation of 
light energy due to arterial blood flow.   
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Figure 6-4. Comparative light absorption 
The sensor’s photodetector collects and converts the light into an electronic signal. Since O2Hb and 
HHb allow different amounts of light to reach the photodetector at the selected wavelengths, the 
electronic signal varies according to which light source is “on” (red or infrared) and the oxygenation of 
the arterial hemoglobin. The oximeter uses this information to calculate the relative percentage of 
O2Hb and HHb. 
 
Figure 6-5. Extinction versus wavelength graph 
The photodetector sends the electronic signal, which contains the light intensity information, to the 
oximeter. The oximeter’s electronic circuitry processes the electronic signal, calculates the SpO2 and 
pulse rate values, and displays them on the screen. 
Calibration 
Datex-Ohmeda enhanced pulse oximetry uses two wavelength ranges, 650 nm to 665 nm and 930 
nm to 950 nm, both with an average power of less than 1 mW. These wavelengths are used to 
calculate the presence of oxyhemoglobin (O2Hb) and reduced hemoglobin (HHb). 
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Measurement Parameters 
A CO-oximeter typically uses four or more wavelengths of light and calculates reduced hemoglobin 
(HHb), oxyhemoglobin (O2Hb), carboxyhemoglobin (COHb), and methemoglobin (MetHb). Therefore, 
pulse oximetry readings and CO-oximetry readings will differ in situations where a patient’s COHb or 
MetHb are increased. Increased patient COHb leads to falsely increased SpO2 in all pulse oximeters. 
Datex-Ohmeda enhanced pulse oximetry uses functional calibration, which is represented 
mathematically as the percentage of hemoglobin capable of carrying oxygen that is carrying oxygen. 
 
The calculation of SpO2 assumes 1.6% carboxyhemoglobin (COHb), 0.4% methemoglobin (MetHb), 
and no other pigments. These values are based on the Datex-Ohmeda Pulse Oximeter Empirical 
Calibration Study. Appreciable variation from these values will influence SpO2 accuracy. 
6.2.8 Pulse oximetry, Nellcor compatible (N-XNSAT option) 
NOTE: Only one pulse oximetry source at a time is allowed by the Cardiocap/5. When the N-XNSAT 
option is installed in the monitor, standard pulse oximetry and N-XOSAT are not available. 
The N-XNSAT pulse oximetry option uses the Nellcor pulse oximetry algorithm and should be used with 
Nellcor pulse oximetry sensors only. Refer to the Cardiocap/5 User’s Reference Manual for a list of 
Nellcor sensors approved for use with this option. 
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6.3 Measurement principles for airway gases, spirometry, and NMT 
6.3.1 CO2, N2O, and agent measurement 
TPX is a side-stream gas analyzer that measures real-time concentrations of CO2, N2O and anesthetic 
agents (Halothane, Enflurane, Isoflurane, Desflurane, and Sevoflurane). The TPX analyzer identifies 
anesthetic agents or mixtures of two anesthetic agents and measures their concentrations. It also 
detects mixtures of more than two agents and issues an alarm. 
 
Figure 6-6. TPX sensor principle 
TPX is a nondispersive infrared analyzer that measures absorption of the gas sample at seven infrared 
wavelengths that are selected using optical narrow band filters. 
The infrared radiation detectors are thermopiles. 
Concentrations of CO2 and N2O are calculated from absorption measured at 3 to 5 µm. 
 
Figure 6-7. Absorbance of N2O and CO2 
Identification of anesthetic agents and calculation of their concentrations is performed by measuring 
absorptions at five wavelengths in the 8 to 9 µm band, then using a set of five equations to solve the 
concentrations. 
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Measurement Parameters 
 
Figure 6-8. Infrared absorbance of AAs 
To achieve measuring accuracy, numerous software compensation parameters are individually 
determined for each TPX during factory calibration. 
6.3.2 O2 measurement 
The differential oxygen measuring unit uses the paramagnetic principle in a pneumatic bridge 
configuration. The signal picked up with a differential pressure transducer is generated in a measuring 
cell with a strong magnetic field that is switched on and off at a frequency of 165 Hz. The output signal 
is a DC voltage proportional to the difference in O2 concentration between the two gases to be 
measured. 
 
Figure 6-9. O2 measurement principle 
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6.3.3 Patient spirometry measurement 
In mechanical ventilation, breaths are delivered to the patient by a ventilator with a proper tidal volume, 
respiration rate (RR), and inspiration/expiration ratio in a time determined by the ventilator settings.  
Patient Spirometry monitors patient ventilation and displays these parameters: 
• Expiratory and inspiratory tidal volume (TV) in ml 
• Expiratory and inspiratory minute volume (MV) in l/minute 
• Expiratory spontaneous minute volume in l/minute 
• Expiratory volume in first second (V1.0) in percent (%) 
• Inspiration/expiration ratio (I:E) 
Airway pressure 
• Peak pressure (Ppeak) 
• Mean airway pressure (Pmean)—Critical care software only 
• End inspiratory pressure (Pplat) 
• PEEPi, PEEPe—Critical care software only 
NOTE: PEEPi = intrinsic PEEP, PEEP-PEEPe 
• Positive end expiratory pressure (PEEP)—Anesthesia software only 
• Real time airway pressure waveform (Paw) 
PEEP, Ppeak, Pmean, and Pplat are measured by a pressure transducer on the PVX board. Atmospheric 
pressure is used as a reference. The pressure measurement is made from the airway part that is 
closest to the patient between the patient circuit and the intubation tube. 
Airway flow 
• Real time flow waveform (V) 
• Compliance (Compl) 
• Airway resistance (Raw) 
• Pressure volume loop 
• Flow volume loop 
The kinetic gas pressure is measured using the Pitot effect. A pressure transducer measures the Pitot 
pressure. The obtained pressure signal is linearized and corrected according to the density of the gas. 
Flow speed is calculated from these pressure values, then the tidal volume (TV) value is integrated. The 
minute volume (MV) value is calculated and averaged using the TV and respiratory rate values. 
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Measurement Parameters 
Compliance and airway resistance 
Compliance tells how big a pressure difference is needed to deliver a certain amount of gas into the 
patient. Compliance is calculated for each breath from the following equation: 
ComplTVexp
PPEEPiPEEPeplat
=−−
 
The airway resistance, Raw, is calculated using an equation that describes the kinetics of the gas flow 
between the lungs and the D-lite flow sensor. The equation states that the pressure at the D-lite can at 
any moment of the breath be approximated using the equation 
p(t) = Raw • V’(t) + V(t) / Compl + PEEP 
where p(t), V(t) and V(t) are the pressure, flow and volume measured at the D-lite at a time (t), 
Raw is the airway resistance, Compl is the compliance, and PEEP is the total end-expiratory 
pressure. 
D-liteTM flow sensor 
Patient Spirometry uses a D-lite flow sensor. The adult D-lite sensor measures adults; the pediatric 
measures children. Single-use and reusable versions of these sensors are available. 
The D-lite adapter measures kinetic pressure by a two-sided Pitot tube. Velocity is calculated from the 
pressure difference according to Bernoullis equation:  
 
where: v = velocity (m/second), dP = pressure difference (cmH2O), and ρ = density (kg/m3)  
Flow is then determined using the calculated velocity: 
F = v x A 
where: F = flow (l/minute), v = velocity (m/second), and A = cross area (m2) 
The volume information is obtained by integrating the flow signal. 
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6.3.4 NeuroMuscular Transmission (NMT) measurement 
The NMT feature provides peripheral nerve stimulation and response measurement that supports 
electromyography EMG. You can also use the Cardiocap/5 NMT feature as a nerve locator for regional 
nerve blocking with a regional block cable, however, in this case there is no response measurement. 
Nerve stimulation 
There are three NMT stimulus modes: Train of Four (TOF), Double Burst 3,3 (DBS) and Single Twitch (ST). 
• Train of Four stimulus generates four stimulation pulses at 0.5 second intervals. The response is 
measured after each stimulus and the ratio of the fourth and first response of the TOF sequence is 
calculated (TOF%). If the first response does not exceed a certain signal level, TOF% is not 
calculated due to poor accuracy. 
• Double burst (3,3) stimulation includes two bursts with a 750 ms interval. Both bursts consist of 
three pulses separated by 20 ms intervals. The responses of both bursts are measured, and the 
ratio of the second and first response is calculated (DBS%). EMG responses are measured 
immediately after the first stimulus pulse of both bursts. 
• Single Twitch stimulation generates one stimulation pulse. The response is measured after the 
stimulus. To prevent decurarization of the stimulated area, the measurement is automatically 
stopped after 5 minutes stimulation in 1-second cycle time. 
Tetanic/Post-Tetanic Count (PTC) 
Tetanic/PTC can measure deeper relaxation than TOF. The tetanic stimulation is produced when Start 
is chosen under Tetanic/PTC. The length of stimulation is 5 seconds. The stimulation generates 
pulses with a frequency of 50 Hz and with a selected pulse width and current. After tetanic stimulation 
and a three second delay, Single Twitch stimulation is produced to detect the post tetanic count (PTC). 
PTC describes the number of responses detected after tetanic stimulation. If there is no response, the 
measurement will be stopped. If responses do not fade away, a maximum of 20 responses will be 
calculated. If more can be detected, the PTC value is displayed only as “> 20” and measurement 
stops.  
If the TOF, DBS, or ST measurement cycle was on when tetanic stimulation started, the cycle will 
continue after the PTC. After completing the PTC measurement during 1 minute TOF, DBS, or another 
PTC measurement is not possible. This is to avoid erroneous readings due to post-tetanic potentiation. 
Response 
 
Figure 6-10. Principle of response measurement 
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