Physics and Monitoring in ICU – Part 2

MEASUREMENT AND EQUIPMENT 2

Written by –  Dr Madhuri Anupindi

Describe how the values for pao2, paco2, ph and bicarbonate are determined on a blood gas sample

PaO2

• Pa02 is measured using a Clark electrode which consists of a silver anode, platinum cathode, electrolyte solution, an oxygen-permeable membrane and a voltage source. The silver anode is submersed in an electrolyte solution, usually, potassium chloride, and a voltage of about 0.6V is applied across the electrodes. This causes the silver to react with the chloride producing electrons. The platinum cathode utilises these electrons produced to interact with the oxygen from the blood gas sample, where O2 + 4e + 2H20  4OH- (hydroxyl). This current flow is then measured. The more oxygen available, the more electrons which can be taken up by the cathode and the greater the current flow. The measured current flow is therefore dependent on the oxygen tension in the blood.

pH

• pH is measured with a glass electrode and uses the fact that if a glass membrane separates two solutions of different hydrogen ion concentrations, a potential difference results which are proportional to the hydrogen ion gradient between the two. A measuring silver or silver chloride electrode is encased in hydrogen sensitive glass and contains a buffer solution which maintains a constant pH. This glass electrode is placed in the blood sample and hydrogen ions pass through the glass and are buffered in the measuring chamber. There is also a reference electrode which is separated from the blood sample by the hydrogen-permeable glass and contains potassium chloride solution with no buffering properties. Once, hydrogen has equilibrated between the blood and the KCL solution, the potential difference between the measuring and reference electrodes is proportional to the hydrogen concentration.

PaCO2

• PaCO2 is measured using a Severinghaus modified glass electrode which uses the same underlying concept as the pH glass electrode. The electrode is bathed in bicarbonate containing solution and separated from the blood sample by a carbon dioxide permeable membrane. Carbon dioxide diffuses from the blood sample, through the co2 permeable membrane and into the bicarbonate chamber. It then reacts with water to produce hydrogen ions and a change in pH. The amount of hydrogen ions produced is proportional to the pCO2 and this is measured by the pH-sensitive glass electrode.

All three of these electrodes must be kept at 37 degrees.

Bicarbonate

• Is not measured directly by the ABG machine but rather is a calculated value. The Henderson- Hasselbach equation uses the measured pH and paco2 values to calculate bicarbonate.

Define and explain damping, resonance, critical damping, optimum damping and natural frequency.

Damping:

• The process by which some of the energy is absorbed in an oscillating system and thus the amplitude of some of the oscillations are reduced.
• Damping is required in all systems but both over and under damping can be a source of error.
  • Causes of overdamping include three-way taps, bubbles, clots, vasospasm, narrow, long or compliant tubing, kinks in the cannula or tubing. Overdamping will result in an under-reading of the systolic blood pressure and an over-reading of the diastolic blood pressure but minimal change in MAP.
  • Underdamping will cause a falsely elevated systolic blood pressure and lower diastolic pressure.

Critical damping:

• This is the minimum amount of damping required to prevent any overshoot. The damping coefficient in a critically damped system is 1 however this makes the system very slow to respond.

Optimum damping:

• The optimum damping provides the best compromise between responsiveness and accuracy and occurs at a damping coefficient of 0.64.
• Optimum damping can be assessed by using the fast flush test where the system is flushed with high-pressure saline which causes oscillating waves that resonate at the natural frequency of the system. Optimal damping occurs when there should be at least 1, but no more than 2 oscillations following the release of the flush valve. The time between these oscillations should be <20 – 30msec and the amplitude of each oscillation should be no greater than 1/3 of the previous oscillation.

Natural frequency

• The natural frequency is the frequency at which a material oscillates freely.  An arterial pressure waveform is made up of many different sine waves which are determined by Fourier Analysis. The arterial pressure wave consists of a fundamental wave which is the pulse rate and a series of harmonic waves whose frequencies are multiples of the fundamental wave frequency.

Resonance

• If a force with a similar frequency to the natural frequency is applied to a system it will oscillate at its maximum amplitude. This is called resonance.
• If the natural frequency of the arterial line system is close to the frequency of the sine waves of the arterial waveform, it will cause the system to resonate, distorting the signal. This will cause a widened pulse pressure and elevated systolic blood pressure. Therefore, the arterial line system needs to have a very high natural frequency of at least 8 times the fundamental frequency (the heart rate) of the arterial waveform.
• The natural frequency may be increased by reducing the length of tubing or cannula, increasing the stiffness of the cannula, increasing the diameter of the cannula or tubing and reducing the density of the fluid used.

Outline the components required to measure BP from an intra-arterial catheter. List other information (other than blood pressure) may be obtained from an arterial line trace?

The components required to measure blood pressure from an intra-arterial catheter include:

• An arterial cannula
  • Usually a short, 20gauge cannula made of polyurethane or Teflon which reduces the risk of kinking or thrombus formation. This is inserted in a peripheral artery and transmits pressure from the arterial circulation to the circuit.
• Connection tubing
  • The cannula is connected to fluid filled tubing which provides a column of non-compressible fluid between the artery and the pressure transducer.
  • The tubing is ideally short, wide and stiff and often connected to a three way tap to allow sampling and zeroing to occur. The tubing is usually labelled or colour coded (often red) to allow for easy differentiation from venous access.
• A transducer
  • The tubing is connected to a Wheatstone bridge transducer. The arterial pressure is transmitted to a flexible diaphragm by the column of fluid in the tubing and displaces a diaphragm. The amount of displacement is measured by a strain gauge which uses the principle that the electrical resistance of wire increases with stretch. The diaphragm is attached to these strain gauges and forms a Wheatstone bridge circuit. With movement of the diaphragm the gauges are stretched or compressed which alters their resistance. The circuit is constructed with four strain gauges and the diaphragm attached so that when pressure is applied, the gauges on one side are compressed which reduces their resistance and those on the other side are stretched, increasing their resistance. This potential difference generated between the two sides is proportional to the pressure applied.
  • The transducer is placed along the phlebostatic axis and is zeroed at this point
• Pressure bag
  • A 500ml bag of normal saline is pressurized to 300mmHg and attached to the tubing via a flush system. Fluid is slowly infused at around 3ml/hr and a flush system allows for a high pressure flush of the fluid to maintain patency and to check the fidelity of the system.
• Micro-processor, amplifier and display
  • This filters, amplifiers and analyses the pressure from the transducer.
  • This is then displaced on a monitor and the number and waveform can be analysed.

Other information that can be obtained from an arterial line trace includes:

• Heart rate
• Heart rhythm and presence of arrhythmias
• Mean arterial pressure
• Pulse pressure
• Pulse pressure variation: for example can be seen in hypovolemia, tamponade
• Pulsus alterans: in severe left ventricular systolic impairment
• Morphology
  • Aortic stenosis may have a slow rise, delayed systolic peak and dicrotic notch may not be discernible
  • Aortic regurgitation may have rapidly rising pressure wave, low diastolic pressure and two systolic peaks or bisferiens wave
  • HOCM may have bisferiens wave with a spike and dome configuration
  • Poor peripheral resistance may have low dicrotic notch
  • LVOT obstruction may have rapid systolic decline
• Accuracy of the trace:
  • Over or under dampened trace
• Pulse contour analysis if using PiCCO

What is the mechanism of oscillometric non-invasive BP measurement. What factors affect accuracy and what are the advantages and disadvantages?

Oscillometric devices use the fact that blood flowing through an artery between systolic and diastolic pressures causes vibrations in the arterial wall which can then be detected and transduced into electrical signals.

Mechanism

• Cuff placed around patient’s arm and is inflated with air until the external pressure exceeds the intra-arterial systolic pressure and arterial flow past the cuff ceases
• The cuff is slowly deflated and a pressure sensor inside the cuff detects arterial pulsations as oscillations
• As the cuff pressure decreases the oscillations increase in amplitude to a maximum which is the mean arterial pressure
• An algorithm is used to calculate the systolic and diastolic pressures, generally using a percentage of the maximum oscillation

Factors affecting accuracy

• Cuff size: length should be 80% of arm circumference and width should be 40%. Cuffs that are too large will under-estimate the blood pressure while cuffs that are too small will over-estimate it
• Rhythm: arrhythmias can result in inaccuracy 
• Extremes of blood pressure: underestimates high blood pressure and overestimates low blood pressure
• State of vessel: overestimates blood pressure if significant arteriosclerosis
• Level of measurement: arm should be at the level of the heart – if it is significantly above the level of the heart BP will be underestimated and if below it will be overestimated

Advantages

• Non-invasive
• Does not require calibration
• Can be used at home or in out-patient settings
• Cheap
• Familiar
• Easy to use  automatic
• Measures MAP
• Can measure BP over time and monitor trends
• Eliminates observer bias or error

Disadvantages:

• Complications include muscle tremors, pressure areas, pain, limb oedema, neuropraxia
• Not continuous
• Inaccuracies as described earlier
• Does not measure diastolic or systolic blood pressure – uses an algorithm generally only known to manufacturer and these algorithms vary with device

Describe principles and limitations of measurement of cardiac output using a thermodilution technique.

The thermodilution technique is a variation on the indicator dilution technique of cardiac output measurement, and is generally thought to be the gold standard for measurement of cardiac output. Traditionally this technique requires a pulmonary artery catheter but this technique is also now utilised by PiCCO. A known volume of cold fluid with a known temperature is injected rapidly into the right atrium port of the PAC, this should then decrease the temperature in the pulmonary artery. The temperature of the blood is measured within the pulmonary artery by the thermistor at the end of the PAC, and a temperature over time curve is produced. The area under the curve is then used to calculate the cardiac output using the modified Stewart-Hamilton equation. The change in temperature in the pulmonary artery over time should be inversely proportional to the rate of blood flow, that is the cardiac output. Thus, the mean decrease in temperature over time is inversely proportional to the cardiac output.

Limitations of measurement technique include:

• Requirements of measurement
  • Has to be taken in end-expiration and to be the average of 3 measurements as there is natural variability. Cardiac output can vary up to 10% with changes in respiration
• Volume of injectate
  • Increased volume can underestimate cardiac output while decreased volume can overestimate it
• Speed of delivery of injectate
  • Less reliable if injection is too slow
• Temperature of injectate
  • The closer the temperature is to blood the greater scope for error
  • Colder injectate is more accurate but may cause bradyarrhythmais
• Position of pulmonary artery catheter and thermistor tip
  • Pulmonary artery catheter should be in West’s Zone 3 for the temperature to be accurate
• Cardiac pathology
  • Tricuspid regurgitation results in retrograde flow of injectate
  • Arrhythmias
  • Intra-cardiac shunt
  • Low cardiac output state: may over-estimate CO in face of severe cardiogenic shock due to excessive heat exchange between blood and injectate as a result of the slow flow
• Risks of pulmonary artery catheter

Briefly outline the principles behind CO measurement using pulse wave contour analysis

Pulse wave contour analysis analyses the arterial waveform and uses the principle that aortic pulse pressure is proportional to stroke volume and inversely proportional to aortic compliance. It integrates the area under the systolic part of the arterial pressure waveform which is assumed to be proportional to stroke volume. Cardiac output is then calculated as it equals stroke volume multiplied by heart rate. PiCCO system uses a combination of pulse wave contour analysis and calibration with thermodilution to calculate cardiac output

All pulse contour analysis systems rely on an optimal arterial signal so under or over damped traces will affect the accuracy. Similarly, arrhythmias, aortic regurgitation and the use of intra-aortic balloon pumps will affect the contour of the arterial waveform and thus the calculation of cardiac output.

Why did the sceptic develop hypertension? Because he took everything with a pinch of salt.