Re: Acid/base balance

shakira miles — Sun, 31 May 2009 19:01:34 GMT

You are a star thanks sooo much!!! x

Re: Acid/base balance

Juliet Drummond DAVN (med) RVN D32/33 — Sun, 31 May 2009 07:37:04 GMT

hi shakira, i have copied this from dechras fluid therapy program, i hope it helps.

juliet

Correcting acid-base and electrolyte abnormalities

Normal acid-base balance

On a daily basis the body produces large amounts of carbon dioxide (as a by product of carbohydrate and fat metabolism) and hydrogen ions (from metabolism of proteins and phospolipids).

Both carbon dioxide and hydrogen ions are acidic and therefore decrease the pH of the body. Normal cellular function requires the pH to be kept within a tight range as many intracellular enzyme processes are pH dependant.

Carbon dioxide is excreted through the lungs and hydrogen ions through the kidneys to try and maintain an appropriate pH. Carbon dioxide is present as an acid in solution in the body until it is released into the alveoli to be excreted by respiration.

It is useful to think of an excessive acid load in the body as due to excessive carbon dioxide (a respiratory acidosis) or due to other acid in the body (a metabolic acidosis).

The bicarbonate buffer system

The body has several buffer systems to deal with the daily acid load produced by metabolism. The most important of these is the bicarbonate buffer system. Bicarbonate ions bind to hydrogen ions to produce carbonic acid (H₂CO₃). This then rapidly dissociates into water and carbon dioxide, with the acidic carbon dioxide being excreted by the lungs. Bicarbonate ions are constantly regenerated and returned to the circulation by the kidneys.

Correcting acid-base and electrolyte abnormalities

Normal acid-base balance

On a daily basis the body produces large amounts of carbon dioxide (as a by product of carbohydrate and fat metabolism) and hydrogen ions (from metabolism of proteins and phospolipids).

It is useful to think of an excessive acid load in the body as due to excessive carbon dioxide (a respiratory acidosis) or due to other acid in the body (a metabolic acidosis).

The bicarbonate buffer system

Changes in pH, bicarbonate and carbon dioxide seen with acid-base abnormalities

Key: Arrows in red show the primary change and arrows in blue the expected compensatory change.

Metabolic acidosis

Many pathological processes can affect the body's ability to maintain a normal pH. Metabolic acidosis occurs either due to a loss of bicarbonate (e.g. in diarrhoea) or due to a gain in acid that has been added to or generated within the body (e.g. a diabetic ketoacidosis). As the body's pH falls bicarbonate levels also fall due to its buffering action described above which ameliorates the decrease in pH. If the pH continues to fall an increase in pulmonary ventilation occurs in an attempt to decrease the carbon dioxide levels in the blood (the pCO₂). This causes a compensatory respiratory alkalosis again ameliorating the decrease in pH.

Respiratory acidosis and alkalosis

Respiratory alkalosis and acidosis are less common in veterinary medicine than metabolic acid-base disturbances and occur due to primary abnormalities in the level of CO₂ in the body. A respiratory alkalosis occurs when an animal hyperventilates lowering their pCO₂ and a respiratory acidosis occurs when an animal hypoventilates leading to increased pCO₂.

The use of fluid therapy to correct acid-base abnormalities

Fluids can be used to manipulate metabolic, but not respiratory, acid-base derangements.

The most common acid-base abnormality is a metabolic acidosis, which occurs when the acid level in the body exceeds the available supply of bicarbonate. The serum bicarbonate level – which is normally in the low 20s – will often be down in the low 'teens.

Giving Hartmann's solution can be beneficial for these patients as it contains lactate. Lactate is metabolised by the liver and during this process it consumes hydrogen ions and generates bicarbonate therefore having an alkalinising effect on the body. Lactate produced by the body's cells during anaerobic respiration causes an acidosis and this can be confusing, but lactate in Hartmann's is bound to a sodium ion rather than a hydrogen ion and therefore will not cause an acidosis.

Conversely, normal saline may make a metabolic acidosis worse, because it has a high chloride level and this chloride competes with bicarbonate for reabsorption in the kidney leading to a decreased bicarbonate level.

The use of fluid therapy to correct electrolyte problems

Hypercalcaemia and hyperkalaemia are two specific electrolyte problems that can be manipulated with the right type of intravenous fluid.