PH AND CO2 MEASUREMET.MOHAMED RIFKY.ANAESTHESIA PHYSICS .SUMARRY OF A PHYSIC BOOK.SOUND WELL BE AVAIABLE ON YOU TUBE.

1. Measurement of (H+) andC02 MOHAMED ANWER RIFKY
HYDROGEN ION CONCENTRATION (H+) AND pH: 1- PH is a measure of the hydrogen ion activity( concentration ) in a
liquid. 2- pH is defined as the negative logarithm to the base 10 of (H+). 3-pH of 7.4 = 40 nmol litre-1 (H+) or 10-7 4 mol litre-1
(The normal blood (H+) 4-The relationship between(H+) and pH is approximately linear over the middle of the clinical range
{ 0.1 for each 1 nmol litre-1 rise of (H+) }.
MEASUREMENT OF (H): (H+) electrode assembly. 1-The hydrogen-ion sensitive glass shown at its tip. 2- A potential develops
across this glass which depends on the difference of (H+) across it. The(H+) within the (H+) electrode is maintained at a constant
value by a buffer solution, so that the potential across the glass is dependent on the (H+) in the blood sample in the channel
illustrated. 3-To measure the potential>>electrical contact with the blood and with a reference electrode is used which has a
membrane at its tip to avoid contamination. 4- In both the (H+)electrode and the reference electrode, stability of the metal to solution
contact is achieved through a special electrode of silver in contact with its chloride which in turn is in contact with a solution of
chloride ions. 5-the reference electrode is linked to the membrane and blood via a saturated solution of potassium chloride which
completes the electric circuit. The potential >>direct reading of (H+). 6-There is an automatic suction control to draw the blood
through the channel, systems which measures oxygen and carbon dioxide tensions in addition to (H+). 7- Normally the blood sample
is an arterial one taken anaerobically, heparinized and anlaysed promptly. 8-Temperature control is important when measuring (H ).If
the blood sample is not at 37°C,(hypothermia conditions), >>a correction factor. 8-.Before use>>calibrated with two buffer solutions
each containing a fixed concentration of two phosphate buffers whose(H+) have been decided by international agreement. 9- The
electrode system must be carefully maintained >>accuracy. 10-Any membrane must be clean and have no holes in it and electrodes
must be regularly cleaned to remove any protein material which tends to accumulate upon them. 11-it may also be used for (H+)
measurements in urine and in cerebrospinal fluid. Ion-selective electrodes other than the (H+) electrode are available, their different
properties (appropriate glass in their construction)>> K+, Na+ or Ca+.
MEASUREMENT OF PCO2 : 1-The normal PaCO2 is 5.3 kPa or 40mmHg. 2-Measuring the Pco2 of
liquids are based on (H+)measurement >>by the reversible reactions: 3- The carbon dioxide tension is
therefore
related to the hydrogen ion concentration. 4-The Severinghaus C02 electrode(Pco2 measurement from the (H+)
change). 5-A hydrogen ion sensitive glass, shown in the centre, with electrodes either side of it. This glass is in contact
with a thin film of sodium bicarbonate solution in a nylon mesh which is fixed over the glass tip by means of an O-
ring. The liquid to be tested, usually arterial blood, is separated from the nylon mesh and bicarbonate by a plastic
membrane that is permeable to carbon dioxide and which is also attached by means of an O-ring. At the tip of the
electrode, carbon dioxide diffuses through the plastic membrane into the mesh impregnated with the bicarbonate
solution and combines with the water present as described above. resulting change of (H+) is measured by the glass
electrode. If a small hole occurs in the plastic membrane>>inaccuracy, which is about 130 Pa (1 mmHg) .
The response time of the electrode is about two to three minutes, ( slow>> time for diffusing through the plastic
membrane). The cell is maintained at 37°C, and be calibrated before use(known concentrations of carbon dioxide).
Convert the concentrations of the calibration mixtures to partial pressures. The saturated vapour pressure of water
must be subtracted (barometric pressure). New techniques with intravascular catheters are being developed to
facilitate continues measurement by the use of mass spectrometry . In the same way that transcutaneous
To get DRY CO2 pp.

2. INFRARED ANALYSER: 1-An instrument which gives a continuous recording of carbon dioxide concentration (capnograph),and it can also
,measure many other gases.,2-Gases that have two or more different atoms in the molecule absorb infrared,(each gas absorbs radiation at
characteristic wavelengths).3-A wavelength of .
28 μm for carbon dioxide should avoid interference from the presence of nitrous oxide. 4-
Infrared analyser (a-radiation is emitted by a hot wire>>frequency required by interference filter b-versatile instruments incorporate
variable filters for different gases. c-As photometric analysis of oxygen saturation ,the filters can be mounted on rotating discs to permit
simultaneous analysis of several sample compounds (rotating disc or chopper )>>easier to amplify and less prone to drift than a steady
signal.d-as glass absorbs infrared radiation>>windows made from a material which is transparent to infrared radiation(sodium chloride, silver
bromide or sapphire).e-Then radiation is focused on a photodetector(according to absorbed radiation), or electronically. f-Infrared radiations
in the output of the detector may also be caused by changes n the output of the infrared source and the sensitivity of the detector >> second
beam of radiation from the same source which passes through a reference cell containing carbon dioxide free air>>subtraction>>double
beam instrument. g-Infrared radiation absorbed by the carbon dioxide molecules increases their rotational and vibrational energy,which may
transfer some of this absorbed energy to the nitrous oxide molecules when the two collide>>compensation for its presence (manual
adjustment or automatic).
1-There are two types of capnograph, the side stream and the main stream. 2-In the side stream capnograph a sample is
drawn, typically at 150 ml-min-1, from a patient's airway via a narrow catheter to the analyser. A moisture
trap is present and the analyser has an exhaust port which allows the gas to be returned to the patient's breathing system in
the closed breathing system .Alternatively the exhaust gases>>scavenging system. 3-In the side stream system the transit
time for the sample to move along the catheter will delay the overall response time of the analyser but with correct tubing
and flow >>time will be less than a second ( no serious incurrecy). 4-In the main stream system a special connector is needed
in the breathing system and it incorporates a channel with sapphire windows. In this way a sampling line is avoided and the
transit time is nil. The connector is however bulkier than in the side stream system.
5-Most anaesthetists now prefer to have a full capnograph trace (a rise of baseline could >>Failure of some component such
as a valve in the breathing system. A sudden change of the capnograph >>straight line>>failure of the ventilation or of the
patient's circulation>>for example due to an air embolus. A progressive raise of end tidal carbon dioxide >>
the onset of malignant hyperpyrexia). 6-f accuracy a capnograph needs a rapid response time. This time has
two components, the transit time and the rise time. The transit time is the time for the sample to be transferred to the
analyser >>in side stream analyser is normally not important which will depend upon the size of the sample chamber and the
gas flow(under 400 ms by use of a small analysing chamber, e.g. 1 ml, bu with appropriate flow). Too low a flow >>long rise
time. Too high a flow >>significant pressure drop across the sampling line and the output of the analyser depends on
pressure. 7- addition to its use for carbon dioxide measurement the infrared analyser may also be used for nitrous oxide and
other anaesthetics,and for trace concentrations in the atmosphere. >> long infrared beam (more infrared molecules lying in
the beam)>>a-longer sample cell >>low response time. b-angled mirrors to fold the beam >>number of times through the
sample>>long response time (few parts per million) 8-gas chromatography and mass spectrometry and the refractometer
may also ,be used(physical methods). 9- Chemical method may be used>>reduction in the volume or pressure of a gas
sample after the carbon dioxide in it has been absorbed by an alkaline solution.10-The colorimetrie technique >> change of
(H+) when a dye is exposed to carbon dioxide leads to a change of colour (dye metacresol purple changes colour from purple
to yellow)>>esophageal intubation.

3. GASTRIC MUCOSAL PH: 1-Adequacy of splanchnic O2 and blood flow>>prognosis,assessed by>>gastric mucosal ph (phi) by nasogastric
tonometer (nasogastric tube,with a channeled sealed ballon). 2-The silicone ballon is permeable,inflated wit milliliters of air or saline. 3-CO2
withen the ballon equlibrites with that of mucosal stomach>>gastric mucosal cells.4-Ballon sample >>Henderson-Hasselbalch equation: >>values
below 7.32 = intracellular acidosis>>decreaseO2 delivery. 5-Pumb transfer to the analyses automatically/10 min>>infrared principle.6-Double-
lumen catheter{{reciculated>>r esponse time of 5min.
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16-PH AND CO2 MEASUREMENT:
A-GIVE A FULL DETAIL ABOUT THE
HYDROGEN ELECTRODE ASSEMLY.
B-WHAT ARE THE PRINCIPLES OF THE
INFRARED ANALYSER ?
C-WHAT ARE THE TWO TYPES OF
CAPNOGRAPH ?( FULL DETAILS ARE
REQUIRED )
D-( G.R ) AUTOMATIC COMPANSATION IS
REQUIRED IN SOME INFRARED ANALYSERS
IN THE PRESENCE OF NITROUS OXIDE.