Blood Gas Analysis and Osmometry - Lab Report Example

SESSION 2 BLOOD GAS ANALYSIS AND OSMOMETRY Aim: To conduct a blood gas analysis using optical technology for the measurement of blood gas parameters.To find out the osmolar properties of urine using various phenomenon related to the temperature and time. PART A: BLOOD GAS ANALYSIS Introduction: The Osmotech OPTI TM CCA blood gas analyzer uses optical technology for the measurement of blood gas parameters. The sensors are enclosed in a single use cassette. Principle: The optical sensors in the OPTI cassette are designed in a way that the analytes bind with the fluorescent sensor molecule. The sensor molecule is selective for the specific analyte is the pH sensor molecule will only react with the H+ molecules, the O2 molecules etc. the intensity of the emitted fluorescence light varies with the concentration of the ions (H+, Na +, K +) or the partial pressure of the gas molecules (O2 and CO2 ) in the sample in a predictable fashion. The relationship is specific for each analyte. Before the analyte can bind to the fluorescent molecule, it has to travel through an optical isolator( black coating) to prevent interference by unspecific light with the light detection system. PART B: OSMOMETRY Introduction: Osmolality measures the total number of osmotically active particles in a solution and is equal to the sum of the molalities of all the solutes present in that solution. The four colligative properties are changed by the dissolving of the solute in solvent. Freezing point depression Boiling point elevation Vapour pressure lowering Osmotic pressure increase Osmolarity refers to osmoles per litre, whilst osmolality refers to osmoles per kg water( There is a slight difference between plasma molality and molarity because of the non-acqueous components present such as proteins and lipids that make up about 60% of the total volume. Thus serum is only 94% water and the molality of a substance in serum is about 6% higher than its molarity. Molality is preferred because it is independent of temperature). Calculated osmolarity: The osmolality of physiological fluids is mainly determined by small molecules that are present in high concentrations. For example in serum, sodium, potassium, chloride, bicarbonate, urea and glucose are the only components present in high enough concentrations to individually affect the osmolality. Together these make up over 95% of total osmolality of serum. Larger serum molecules contribute little to the overall osmolality. A useful estimate of the osmolality is calculated from: 2 x [Na] + [Gluc] + [Urea] This uses 2 x the [Na] to take into account the matching anions and leaves out [K] to allow for the partial ionization of the electrolytes. Osmolar gap: The difference between the measured and the calculated osmolality is known as the osmolar gap. If there are unmeasured osmotically active species present then these will increase the gap. The normal osmolar gap is up to 10mmol/L and values in excess of this usually indicate the presence of an exogenous agent. The most common is ethanol, but methanol, the ethylene glycol, acetone and isopropyl alcohol will also contribute if present. Clinical significance: Different causes of hyponatraemia can be distinguished by serum osmolality measurement. It is increased in hyperosmolar hyronatraemia(eg: hyperglycaemia), normal in psuedohyponatraemia and decreased in “true” hyponatraemia. Urine(and serum) osmolality are used for the evaluation of concentrating ability of kidney. Urine osmolality varies between 50 and 1200mmol/kg in a healthy individual depending on the state of hydration. Note that there is no reference interval (“normal range”)for urine osmolality as the interpretation depends on the clinical condition of the patient who is assessed against the appropriateness of the renal response. Method: The sample of choice for blood gas analysis is arterial blood. This is usually collected from the radial artery in the wrist, but in cases where no radial pulse is obtained, the femoral or brachial artery may be used. The sample may also be collected from an arterial line after flushing the line to remove excess anticoagulant and fluid. In neonates and in adults when arterial puncture is contraindicated or unsuccessful, a capillary blood sample may be used. The sample is inserted into the blood analyzer that uses electrodes to measure the concentration of hydrogen ions (H+), which is reported as pH, and the partial pressures of oxygen [PO2] and carbon dioxide PO2 gases. The pH-measuring electrode consists of a special glass membrane that is selectively permeable to hydrogen ions. An electrical potential develops across the inner and outer surfaces of this membrane that is related to the log of hydrogen ion activity in the sample. A Severinghaus electrode is used to measure PCO2. The measuring principle is the same as for hydrogen ions, except that the electrode tip is covered with a gas permeable membrane, so that the pH change is proportional to carbon dioxide diffusing from the sample to the electrode surface. The PO2 is measured using a polarographic (Clark) electrode. Oxygen diffuses from the sample to the cathode, where it is reduced to peroxide ions. The electrons come from a silver anode that is oxidized, generating current in proportion to oxygen concentration at the cathode. Electrode signals are dependent upon temperature as well as concentration, and all measurements are performed at 37°C. Since the in vivo pH and levels of oxygen and carbon dioxide are temperature dependent, results may need to be adjusted for the patients actual temperature. Portable blood gas analyzers are available that can be used at the bedside. Blood gas analyzers calculate blood bicarbonate concentration using the formula: pH = 6.1 + Log bicarbonate/.0306 x PCO2. They also calculate oxygen content, total carbon dioxide, base excess, and percent oxygen saturation of hemoglobin. These values are used by physicians to assess the extent of hypoxia and acid-base imbalance. The instrument used for osmometry is freezing point depression osmometer. The sample is supercooled within a cooling fluid or using solid state cooling. A rapid stir mechanism is used to initiate crystallization. A thermister(temperature dependent resistor) reading is noted. Comparison with standards allows calculation of osmolality. Specimen: Either serum, plasma or urine samples are suitable. Procedure: 1.Make a standard solution Reagent-grade NaCl is heated to 200o C overnignt to drive off any water present in the crystals. Fill a 100 mL volumetric flask with water at 20o C to the mark. Add exactly 180 µL of additional water, and then add 0.943g salt to dissolve. Density of water is 0.9982 at 25oC Molecular weight of NaCl is 58.5 No of ions formed = 2 Osmotic coefficient is 0.93 Measure the osmolarity of this solution. Transfer 0.05ml sample to cuvette and attach to probe. Lower cuvette into cooling compartment(this initiates freezing sequence). Note reading in mmol/kg 2. Effect of fluid intake on urine osmolality Volunteers will drink either water or isotonic drink equal to 1% of body weight or consume a salt load in the form of tablets with minimal water intake.Eg a 70kg subject should drink 700 ml. Urine samples should be taken at beginning and end of a two hour period following ingestion. Measure the osmolality of the pre and post ingestion samples. 3.Measurement of osmolar gap Volunteer* will consume three standard alcoholic drinks. Serum osmolality will be both measured and calculated before and afer alcohol consumption. *NB not a student Na,urea and glucose are measured using the VITROS®DT60 || Chemistry system. Results: The result of blood gas analysis is expressed in the table below. Effect of 37oC incubation Effect of air bubble t0 t25 t50 t0 t25 t50 pH 7.47 7.45 7.45 7.44 7.47 7.46 pCO2 (mm Hg) 36 39 42 39 37 35 PO2 (mm Hg) 138 125 115 55 88 195 O2 saturation % 99 97 98 89 97 100 HCO3 (mmol/L) 25.7 26.2 24.8 25.7 26.0 24.8 Base Excess(mmol/L) 2.2 2.2 1.2 1.6 2.4 1.5 Calculations: Henderson-Hasselbach Equation: pH = pKa+ log [ [HCO3 / αpCO2 ] ] α = 0.0391 (solubility coefficient of CO2 ) pKa = 6.1 (Dissociation coefficient) Urine Osmolality: Make a standard solution. My measurement =288mosmol/kg Osmolality =Φnc where Φ =Osmotic coefficient n= number of ions c=concentration Osmolarity =0.93 x 2 x c c= moles=gram/molecular weight =0.943/58.44=0.0161362moL Concentration=mol/L =0.0161362/0.10018 = 0.1610721moL/L Converting 0.161072moL/L to moL/kg, Concentration-moL/L /Density-kg/L = moL/kg =0.161362moL/kg Osmolality =0.93 x 2 x 0.161072 =0.300 osmol/kg =300 mosmol/kg The osmolality of urine pre and post fluid intake in mmoL/L Pre-intake Post-intake Urine H2O 0.502 0.240 Urine salt 0.366 0.677 Urine isotonic 0.316 0.226 Plasma alcohol 0.287 0.297 The following table shows the ionic variation pre and post intake of fluid in mmoL/L Pre-intake Post-intake Sodium 141 143 Glucose 5.3 5.6 Urea 6.0 4.7 It is to be noted that the serum osmolality pre-intake is 0.287mmoL/L and post-intake is 0.297mmoL/L. The osmolar gap before and after alcohol ingestion is 0.010mmoL/L. The blood alcohol reading is 0.0012%. Discussion: 1) If the blood sample cannot be immediately analyzed, it is chilled in an ice bath in a glass syringe to slow metabolic processes which can cause inaccuracy. Samples drawn in plastic syringes should not be iced and should always be analyzed within 30 minutes. Contamination with room air will result in abnormally low carbon dioxide and (generally) normal oxygen levels. Delays in analysis (without chilling) may result in inaccurately low oxygen and high carbon dioxide levels as a result of ongoing cellular respiration. This is the reason for the difference between the sample values. 2) pH, PCO2, BE, and HCO3 are all significantly correlated in arterial blood gas, venal blood gas and capillary blood gas. Correlation in PO2 is also significant, but less so. Correlation between pH, PCO2, PO2, BE, and HCO3 is similar in the presence of hypothermia, hyperthermia, and prolonged capillary refilling time. In hypotension, correlation in PO2 between venal blood gas and capillary blood gas is similar but disappeared in arterial blood gas–venal blood gas and arterial blood gas–capillary blood gas. 3) The Base Excess is defined as the amount of acid (in mmol) required to restore 1 litre of blood to its normal pH, at a PCO2 of 5.3kPa (40mmHg The Standard Bicarbonate: this is similar to the base excess. It is defined as the calculated bicarbonate concentration of the sample corrected to a PCO2 of 5.3kPa (40mmHg).  4) The use of 100.18mL is standard and this standard helps in the calculation of osmolality. 5) The measured osmolality of the solution is 288mosmol/kg and the standard osmolality is 300mosmol/kg. 6) The effect of sodium and glucose on the urine osmolality is that the urine osmolality increase with salt intake.Osmolality decrease with urea salt addition. Reference: Acid Base Balance.Issue 13(2001)Article 12:Page 3 .Accessed on March 22,2009 available at http://www.nda.ox.ac.uk/wfsa/html/u13/u1312_03.htm Arterial Blood Gas cited November 2007.Accessed on March 22,2009 and available at http://en.wikipedia.org/wiki/Blood_gas Bhagat.C.I.Gracia-Webb. P.Fletcher.E, and Beilby, J.P(1984). Blood gas analysis.Accessed on March 21 2009,available at http://www.enotes.com/nursing-encyclopedia/blood-gas-analysis Blood Gas Analysis cited on 08-14-2006.Accessed on March 21,2009 available at http://www.healthatoz.com/healthatoz/Atoz/common/standard/transform.jsp?requestURI=/healthatoz/Atoz/ency/blood_gas_analysis.jsp Calculated vs measured osmolalities revisited.Clin.Chem: 30:1703-1705 Osmotech OPTI CCA-TS Product Information Book(2006) Pesce,A.J.Kaplan,L.A.et al(2003) Clinical Chemistry:theory,analysis,correlation.4th ed.Chapter 15:Electrochemistry.St.Louis, Mosby. R.C.C.Lord .Osmosis,Osmometry and Osmoregualtion accessed on March 21,2009,available at http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1741142&blobtype=pdf Read More