Pre-Module Medicine Notes - Anaesthesia Module 2023 PDF

The basics of acid base analysis Blood gas analysis Blood gas analysis is useful in the diagnosis, prognosis and monitoring of several conditions in small animal clinical practice. It gives us information on the respiratory system, perfusion, acid base and electrolyte balance of the patient. There...

The basics of acid base analysis Blood gas analysis Blood gas analysis is useful in the diagnosis, prognosis and monitoring of several conditions in small animal clinical practice. It gives us information on the respiratory system, perfusion, acid base and electrolyte balance of the patient. There are two main components to acid base analysis; respiratory and metabolic. Respiratory; this looks at the efficiency of ventilation. If the ventilation is inadequate, then carbon dioxide accumulates. Extra CO2 molecules combine with water to form carbonic acid, which makes pH more acidic. Metabolic; when normal metabolism is impaired, acid will form, e.g. lactic acid from poor perfusion. The acid is not from a respiratory source so is termed metabolic. Usually patients that are clinically unwell present with acidic pH, the following is a step by step guide to help you sort of what is going on. First of all, we should look at the pH of the blood to tell whether there is acidaemia or alkalaemia. The normal blood pH is approximately 7.35-7.45. Less than 7.35 suggests acidaemia, >7.45 suggests alkalaemia, but we then need to work out why there is too much/too little acid/alkali in the blood. Next we would look at the respiratory component; indicated by the PCO2 (the partial pressure of carbon dioxide in the blood). If the pH is acidic, and the pCO 2 is raised (normal is 35-45 mmHg) then usually this suggests it is an acidaemia with a respiratory cause, therefore a respiratory acidosis. There are occasions, where there is in addition acid created by metabolic causes, and this would be an example of a mixed respiratory/metabolic acidosis. Thirdly, to evaluate the metabolic component, we would look at the Base Excess (BE) whose normal range is -5 to +5 mmol/l (this range can vary between species but this is the ball park figure). If the blood is acidaemic and the BE is more negative than -5 mmol/l, this suggests that the cause is metabolic. (Again, if the PCO 2 is also elevated, then this would be a mixed metabolic and respiratory acidosis). If the BE is more positive than +5mmol/l then this would suggest a metabolic alkalosis (either through a loss of acid or a gain of base). Usually, we will also see that the body has made some efforts to compensate for any acid base abnormalities. The compensation is made by the system not effected, so for example, we would see respiratory compensation in response to a metabolic disturbance (e.g. an animal trying to lower it’s CO 2 to shift its pH back to normal after a metabolic www.improveinternational.com | 1 acidosis). It is less common to see a metabolic compensation for a respiratory disturbance, as usually the change in ventilation is too rapid for metabolic compensation to occur. Indices of oxygenation. paO2 in mmHg is the partial pressure of oxygen dissolved in the arterial blood SaO2 (%) is the saturation of haemoglobin with oxygen as measured by arterial blood gas analysis. It is equivalent to Sp02. paO2