Basic principles of acid base balance

I.  Basic principles of acid base balance - A. Introduction - ABG: Misinterpretation of ABG result can be followed by inadequate treatment which can be detrimental to a patient. - The initial changes form the hallmark of metabolic or respiratory acid/base balance disorders. - Acid base disorders broadly can be : - uncompensated, - partially compensated: pH is out of range, but there is increase or decrease of the other value outside of its normal range. For example, Metabolic acidosis is characterized by HCO3 levels that are lower than 22. To compensate for this the pCO2 levels also go down (by hyperventilation). The levels should be lower than 35. (Range is 35 to 45) - completely compensated ( pH is normal ) - Sometimes the reported results is very complex. But not all the values are necessary to ascertain the acid- base balance disorders. ABG is not a screening test and is ordered usually in severe ill patients such as kidney insufficiency, heart failure, lung failure and even P. embolism. We often need the results very fast and therefore it is reported within a few minutes and is always performed first amongst all other tests that are ordered such as CBC. In severe clinical conditions, acid/base assessment is not the only lab test we need. Others needed are bilirubin for liver, creatinine for kidney , blood glucose, urea etc. Critical metabolic parameters. The result is also reported very fast and is therefore a very useful test. - When the pH is high or low, the first thing to do is to classify it as respiratory or metabolic. - Compensation is a normal response of acid base imbalance. When compensation is present, it can be partial which means that the compensatory reaction is present but does not cause restoration of the normal pH. - Partial compensation (I already know this): This is more common: An example is hyperventilation in patients with metabolic acidosis. - Complete/fully compensation : This does not mean that the compensation cannot be complete. In this case, the pH will be normal. When ABG shows normal pH with the other factors decreased/ increased , this is not fully compensated acid base disorder but is something else. Most often, it is a co existing problem with alkalosis and acidosis. Therefore the complete normalization of the pH is really rare. - This means that if you see that that pH is within normal range, it can mean the following : - The pH is actually normal with not acid base disturbances. - Complete compensation : Quite rare but mostly exists in respiratory alkalosis. - Mixed disorders - There are three possible answers to the questions: - B. Acid base disorders arranged in order of increasing complexity - How do we identify the results? - The ABG is simply not enough to understand the disorder.Most often the acidosis or alkalosis is a consequence of an underlying condition. Acidemia and alkalemia is a metabolic consequence of that underlying problem. - We should always look for the cause/ etiology. Therefore on top of this ABG we should always base our diagnosis on clinical picture. This will address the origin of abnormal pCO2 and HCO3-. - Complete compensation is rare. - if there is complete compensation, it means that there is normal pH. The compensation whether partial or complete is to restore the normal pH of the body. - Only respiratory alkalosis is the most common disorder that is completely compensated to restore the pH. - Respiratory alkalosis is characterised by low levels of pCO2 which means that there is a drop in H+. This will cause a left shift in the Hb O2 binding curve which means it binds O2 even more strongly. Therefore it will be harder for it to lose O2 and therefore the cells start making ATP with only glycolysis which will result in lactate production. The higher H+ levels will try to neutralize the alkalosis that exists. - Combination of first two tables. - 1. The metabolic acidosis is shown by low HCO3- and the respiratory alkalosis is shown by low PCO2. Therefore the pH remains the normal. - 2. Metabolic alkalosis is shown by high HCO3 and respiratory acidosis is shown by high pCO2. Therefore the pH remains the normal. -------------------------------------- ---------------------------------------------- - In conclusion, if the pH is normal, it can mean: - (i) Acid base balance is normal i.e. normal individual. - (ii) Simple acid base disorder with complete compensation. - (iii) Mixed disorder of opposite type. - Example: Normal pH , and low levels of HCO3- and PCO2 - Metabolic acidosis with complete compensation - Respiratory alkalosis with complete compensation - Mixed disorder of opposite type. - Example : Low pH, High pCO2 and low HCO3- - Low pH : Acidosis - High pCO2 : Respiratory acidosis - Low HCO3- : Metabolic acidosis - Shows mixed disorder of same type. - In V2, The order of the disorders are also identified. Lets use the 1st row as an example: Metabolic acidosis with respiratory acidosis - Metabolic acidosis is decrease in HCO3, so the normal compensatory response is decrease in pCO2 as well but - Respiratory acidosis is an increase in pCO2. In this case, the the increase in pCO2 due to respiratory acidosis can be neutralized by the compensation of metabolic acidosis giving a normal pCO2 value. - If there is no compensation, it can mean that maybe we took the ABG too early because compensation takes time. For example, renal compensation for respiratory acidosis/ alkalosis takes 3 to 4 days. Whereas lung compensation for metabolic acidosis / alkalosis ( lesser extent) sets is within 3-4 hours. - Another reason, which is not normal, is that there is no compensation even after time passes. - Qn. If the compensation is not there, there is a problem. - Simple disorders without compensation - In general compensation is always expected to be present especially the partial. - If there is no compensation, it means - that there is lack of time, you measured it too early, - that there is kidney or lung failure - Respiratory compensation of metabolic disorders develops during 3-4 hours. - Metabolic compensation of respiratory disorders develops during 3-4 days. Conclusion - In conclusion, we can have: - Simple acid base disorders with partial compensation - Simple acid base disorders with complete compensation ( pH in normal range) - Mixed disorder of opposite type ( pH is in normal range) - Mixed disorder of the same type - Simple acid base disorders with no compensation. - C. Compensation of acid base balance disorders - D. How do we manage this acid base balance problem ? - In most patients, the normalization results from the adequate management of the underlying etiology. - So we are not administering acid or alkali to reverse the effects. - When in the patient, has two independent problem and only one is identified, then the treatment will be futile as it is not addressing the other. - E. The rules of acid base balance disorders : - The appropriate determination of acid-base balance disorders always requires the assessment of three parameters: pH, HCO3- , pCO2 - The alterations of HCO3- (Metabolic) and pCO2   are characteristic for metabolic and respiratory acid-base balance disorders respectively. - The development of an acid-base balance disorder is always followed with the physiological attempt of its correction, so called compensation. Compensatory reaction always goes in the same direction as primary alteration responsible for the generation of acid –base balance disorders. - The alteration of HCO3- and pCO2 in the same direction points to the presence of compensatory reaction - if only the pH was out of normal range or - to the mixed acid-base balance disorder if only the pH was in the normal range (coexistence of the opposite problems e.g. acidosis with alkalosis). - The exception is respiratory alkalosis. - The alteration of HCO3- and pCO2 in the opposite directions always points to the mixed acid-base balance disorder (coexistence of the disorders of the same kind e.g. acidosis with acidosis or alkalosis with alkalosis. - Normal value of HCO3- or pCO2 may suggest lack of compensatory reaction (uncompensated disorder) or, in the case when the time required for compensatory reaction has already passed, may point to the mixed acid-base balance disorder (normal value is then referred as relatively too low or too high). - The sequence of separate components of mixed disorders can not be established based on ABG result just on the base of patient’s clinical history. - The assessment of acid-base is based on ABG (arterial blood gas) sampling. - F. Indications for ABG sampling - Intro: Typically we should sample the arterial blood : This is because of the indications of checking acid base imbalance/balance. Deoxygenation of blood can be seen in arterial blood but not in venous blood. This is to check whether the respiratory system is working or not. DKA would like to periodically check pH. Sampling arterial blood complications : - Identification of respiratory, metabolic, and mixed acid-base disorders, with or without physiologic compensation. - Measurement of the partial pressures of respiratory gases involved in oxygenation and ventilation - Monitoring of acid-base status, as in patient with diabetic ketoacidosis (DKA) on insulin infusion; - Assessment of the response to therapeutic interventions such as mechanical ventilation in a patient with respiratory failure - Determination of arterial respiratory gases during diagnostic evaluations (eg, assessment of the need for home oxygen therapy in patients with advanced chronic pulmonary disease) - Quantification of the levels of dyshemoglobins (eg, carboxyhemoglobin and methemoglobin) - Procurement of a blood sample in an acute emergency setting when venous sampling is not feasible (many blood chemistry tests could be performed from an arterial sample ) - G. Absolute contraindications for arterial blood sampling - An abnormal modified Allen test, in which case consideration should be given to attempting puncture at a different site. - What is Allen's test ? - Firm occlusive pressure is held on both the radial artery and the ulnar artery. The patient is asked to clench the fist several times until the palmar skin is blanched, then to unclench the fist. Overextension of the hand or wide spreading of the fingers should be avoided, because it may cause false-normal results. The pressure on the ulnar artery is released while occlusion of the radial artery is maintained. The time required for palmar capillary refill is noted (normal 3-15 s). - Allen''s test: To see if the ulnar artery is enough to provide blood to the hand in an event that radial artery experiences spasms. - In general, the arterial blood from dominant hand is not sampled. Therefore in the event ischemia happens , it happens on the non dominant hand. - Local infection or distorted anatomy at the potential puncture site (eg, from previous surgical interventions, congenital or acquired malformations, or burns) - The presence of arteriovenous fistulas or vascular grafts, in which case arterial vascular puncture should not be attempted. - Known or suspected severe peripheral vascular disease of the limb involved. PVD is a disease of blood vessels outside of the heart and brain. In this case it can result in spasms which will result in ischemia. - Potential spasm in the injured artery , following ischemia to the region where the artery supplies. Of course this will result in ishcemic injury. Radial artery and femoral artery is used. Radial artery is more prone to spasm. - H. Relative contraindications for arterial blood sampling - Severe coagulopathy: Which will lead to thrombus formation at the site of puncture followed by ischemia of the area supplied by the artery. - Anticoagulation therapy with warfarin, heparin and derivatives, direct thrombin inhibitors, or factor X inhibitors; aspirin is not a contraindication for arterial vascular sampling in most cases. - This is important, because this can cause non stop bleeding in the puncture site. - Use of thrombolytic agents, such as streptokinase or tissue plasminogen activator. - Bleeding complication - I. Radial Artery Blood Sampling - The air bubbles have to be removed and the syringe capped. The contact of blood to atmospheric air. 100mmHg and 40 mmHg for CO2. O2 in air is 150mmHg. Because of this gradient, the Oxygen will increase. CO2 will decrease thereby altering the ABG results. - arterial blood hemorrhage is a complication but is not the most important and therefore not dangerous and it not common also. - - J. Normal values for ABG - - Base excess: 0 +- 2.5 mmol/L : It is the concentrations of all buffering bases of blood. When calculated of buffering bases = 48 the excess is 48-48 = 0. - In metabolic acidosis : BE tends to decrease as the total amount of buffering bases decreases. Base deficit . - In metabolic alkalosis : BE tends to increase as the total amount of buffering bases increases.  Base excess. - Respiratory : BE is not used / limited. II. Cases Case Deduction - pH : Normal - Which means that there are three possibilities 1. Normal ABG, Completely compensated ABD, Mixed with opposite type - pCO2: Low indicating Respiratory Alkalosis - HCO3 : Low indicating Metabolic acidosis - Conclusion: - Mixed :  Metabolic acidosis with respiratory alkalosis - Respiratory alkalosis with complete compensation. - Metabolic acidosis with complete compensation. - How to distinguish ? The clinical picture is the most important. - Respiratory alkalosis + metabolic acidosis : Diarrhea ( Metabolic acidosis) + respiratory alkalosis (hyperventrilation ). A kid has a lot of diarrhea and then he hyperventilates when he enters the exam room. - Full compensated metabolic acidosis - Fully compensated respiratory alkalosis : With clinical picture of a student who wants to take a stressful exam. Bohr effects with H O2 binding curve to the left which means it binds oxygen more strongly. - pH: Alkalosis - pCO2: Decreased ; Respiratory alkalosis - HCO3- Increased; Metabolic alkalosis - Conclusion:  - Respiratory alkalosis with metabolic alkalosis - pH: Normal - Which means that it can be 1. Normal; 2. Mixed disorder of opposite type 2. Simple disorder with complete compensation - pCO2 : Low ; Respiratory alkalosis - HCO3 : Low : Metabolic acidosis - BE: shows metabolic acidosis - Conclusion : - Respiratory alkalosis with metabolic acidosis - Metabolic acidosis with complete compensation - Respiratory alkalosis with complete compensation - pH: Low ; acidosis - pCO2: High: Respiratory acidosis - HCO3: High: Metabolic alkalosis - Conclusion: - Respiriatory acidosis with partial compensation. - Calculate whether the respiratory acidosis is acute or chronic ? - change in pCO2 is 40 and 50 -- 10.4 mmHg - Change in HCO3- 1.9 - Remember that for respiratory acidosis, in acute, for every 10mmHg rise in pCO2 , there will be 1 mmHg increase of HCO3-. - pH: High; Alkalosis - pCO2; low : Respiratory alkalosis - HCO3 : High: Metabolic alkalosis - Conclusion: - Respiratory alkalosis with Metabolic alkalosis - pH; low ; acidosis - pCO2 : Very high; Respiratory acidosis - HCO3- Normal - BE: Shows acidosis - Conclusion - Respiratory acidosis with no compensation. - If it was compensated , 91.9-40 = 52 /10 = 5.2 mmol/L of HCO3 has to be raised which means the HCO3 level has to be atleast 31. - Respiratory acidosis with metabolic acidosis - pH: low; acidosis - pCO2 : High; respiratory acidosis - HCO3- : High: metabolic alkalosis - Conclusion: - Respiratory acidosis with partial compensation - Compensation rate: - pH; Normal which means that i can actually be normal, or simple disorder with complete compensation or mixed disorder of opposite type. - pCO2 : High; respiratory acidosis - HCO3 : High, Metabolic alkalosis - BE shows alkalosis - Conclusion : - Mixed : Respiratory acidosis with metabolic alkalosis - Respiratory acidosis with complete compensation - Metabolic alkalosis with complete compensation - pH; High: Alkalosis - pCO2 : Low ; Respiratory alkalosis - HCO3- High; Metabolic alkalosis - Conclusion - Respiratory alkalosis with metabolic alkalosis - pH: Normal which means it can be normal, Simple disorder with complete compensation or mixed disorder of opposite type - pCO2, Low ; Respiratory alkalosis - HCO3- Low : Metabolic acidosis - BE: Acidosis - Conclusion - Respiratory Alkalosis with metabolic acidosis - Respiratory alkalosis with compelte compensation - Metabolic acidosis with complete compensation. - pH Low ; acidosis - pCO2: High; respiratory acidosis - HCO3: Low ; metabolic acidosis - BE: acidosis - Conclusion - Respiratory acidosis with metabolic acidosis - pH: Normal which means it can actually be normal, simple disorder with complete compensation , mixed disorder of opposite type - pCO2: Normal - HCO3- : Normal - BE: Normal - Conclusion : Normal ABG - pH High; alkalosis - pCO2: Normal - HCO3- : High; Metabolic alkalosis - Conclusion : - Metabolic alkalosis with no compensation or : Maybe measured too early - Metabolic alkalosis with respiratory alkalosis - pH: Low; acidosis - pCO2: High; respiratory acidosis - HCO3: High; Metabolic alkalosis - Conclusion : - Respiratory acidosis with partial compensation. - Calculate compensation rate: 83- 40= 43 = 4.3 mmol/L of HCO3 increase. - Probably chronic Respiratory acidosis. - pH: High, Alkalosis - pCO2 : Normal - HCO3- : High :metabolic alkalosis - Conclusion : - Metabolic alkalosis with no compensation (Measured too early) - Metabolic alkalosis with respiratory alkalosis - pH: Normal which means that it can be normal, or there can be a simple disorder with complete compensation or mixed disorder of opposite type. - pCO2: Normal - HCO3: Normal - Conclusion Normal ABG but it is taken from venous blood. - Sometimes, venous blood is obtained by mistake especially when obtaining blood from femoral artery. - pH: low : acidosis - pCO2: Normal which means it can be simple disorder with no compensation, or mixed disorder of the same type. - HCO3: Low : Metabolic acidosis - Conclusion : - Metabolic acidosis with no compensation. - Metabolic acidosis with respiratory acidosis - Differentiation between the two depends on time. - For final exam, the ABG results only show in one possible way. Interpretation depends on time. - pH: High: alkalosis - pCO2: Low ; Respiratory alkalosis - HCO3 : Normal  which means that it can be an simple disorder with no compensation or mixed disorder of the same type. - Conclusion : - Respiratory alkalosis with no compensation ; maybe measured too early. - Respiratory alkalosis with metabolic alkalosis. - pH: Low : Acidosis - pCO2: Low : Respiratory alkalosis - HCO3: Low : Metabolic acidosis - Conclusion : - Metabolic acidosis with partial compensation. - pH: Normal which means that it can be actually normal, or simple disorder that is completely compensated or mixed disorder of opposite type. - pCO2: Normal - HCO3: Normal - Conclusion ; Normal - Venous gasometry - pH: Low : Acidosis - pCO2: Normal which means that it can be a simple disorder with no compensation or a mixed disorder of the same type. - HCO3: Low : metabolic acidosis - Conclusion : - Metabolic acidosis with no compensation or - Metabolic acidosis with respiratory acidosis - pH : low: acidosis - pCO2 : high: Respiratory acidosis - HCO3- Normal which means that there is a simple disorder with no compensation or mixed disorder of the same type. - Conclusion : - Respiratory acidosis with no compensation - Respiratory acidosis with metabolic acidosis - pH: Low : Acidosis - pCO2 : Low : Respiratory alkalosis - HCO3- : Low : Metabolic acidosis - Conclusion : - Metabolic acidosis with partial compensation - pH: high; alkalosis - pCO2: Low : Respiratory alkalosis - HCO3- ; low : Metabolic acidosis - Conclusion : Respiratory alkalosis with partial compensation. - pH: low : acidosis - pCO2: High : respiratory acidosis - HCO3- : Normal which means that there is simple disorder with no compensation or mixed disorder of same type. - Conclusion : - Respiratory acidosis with no compensation - Respiratory acidosis with metabolic acidosis - pH: Normal which means it can be normal, or simple disorder with complete compensation, or mixed disorder of opposite type. - pCO2 High: Respiratory acidosis - HCO3 : High : Metabolic alkalosis - Conclusion: - Respiaratory acidosis with complete compensation - Metabolic alkalosis with complete compensation - Respiratory acidosis + metabolic alkalosis - Fully compensated R acidosis for few days will result in complete normalization of the pH value. - pH: Low: Acidosis - pCO2: high: respiratory acidosis - HCO3- : high: Metabolic alkalosis - Conclusion : - Respiratory acidosis with partial compensation. - pH: Low : acidosis - pCO2: Low : Respiratory alkalosis - HCO3- Low : Metabolic acidosis - Conclusion : Metabolic acidosis with partial compensation - pH: Low :acidosis - pCO2: High: respiratory acidosis - HCO3: Low : Metabolic acidosis - Conclusion : - Metabolic acidosis with respiratory acidosis - Remember that severe vomiting can cause metabolic alkalosis due to loss of acids.  - - a. - pH: low : acidosis - pCO2 : Normal which means that it can be a simple disorder with no compensation or - HCO3: Low : Metabolic acidosis. - Conclusion: - Metabolic acidosis with no compensation - Metabolic acidosis with respiratory acidosis - b. Probably due to kidney failure. - pH: low : acidosis - pCO2:low  Respiratory alkalosis - HCO3: low : Metabolic acidosis - Metabolic acidosis with partial compensation - pH:low : acidosis - pCO2: High: respiratory acidosis - HCO3- : High metabolic alkalosis - Conclusion : - Respiratory acidosis with partial compensation - pH: acidosis - pCO2: high: Respiratory acidosis - HCO3: High: Metabolic alkalosis - Conclusion: Respiratory acidosis with partial compensation