Acid-Base Balance

Outlines

Introduction Body acidity has to be kept at a fairly constant level. Normal pH range within body fluids 7.35 - 7.45. Normal pH is constantly being attacked by metabolic processes within the body that produces acidic metabolites.

Acid-Base Balance An acid is a proton donor and a base is a proton acceptor. Physiologically, there are two groups of important acids: Carbonic acid (H2CO2) Non carbonic acid

Carbonic acid (H2CO2) Carbonic acid comes from CHO and fat metabolism and results in 15,000 mmol of CO2/day. Carbonic acid metabolism is mostly handled by respiration. Recall: CO2 + H20   H2CO3

Non carbonic acid (non-volatile acids) comes from protein metabolism. H+ can be captured in the form of H2SO4, H2PO4, etc. excreted by the kidneys

Factors involved in gas exchange Many facotrs affects gas exchange, this process occurs between the alveoli & pulmonary capillaries & between capillaries & tissue. Its involvied : Partial pressure Diffusion Ventilation-perfusion matching Oxyhemoglobin dissociation

1. Partial pressure When assessing a patient's oxygenation, one of the first steps is to look at PaO2 is the partial pressure of oxygen dissolved in blood. Normal PaO2 is 80 to 100 mmHg; however this is normal for a healthy person, less than 60 years old, breathing room air.

Normal acid-base balance Normal plasma pH = 7.4 (Range: 7.35 - 7.45) CO2 + H20   H2CO3  HCO3- +H+

Blood Gas Norms pH pCO2 pO2 HCO3 BE Arterial 7.35-7.45 35-45 80-100 22-26 -2 to +2 Venous 7.30-7.40 43-50 ~45

Acid-Base Regulation Three mechanisms to maintain pH Respiratory (CO2) Buffer (in the blood: carbonic acid/bicarbonate, phosphate buffers, Hgb) Renal (HCO3-)

Buffers A buffer is a substance that can give or accept protons i.e. H+, in a manner that tends to minimise changes in the pH of the solution. Usually buffers are composed of a weak acid (proton donor) and a weak base (proton acceptor) as shown in the following equation.

Regulation The process of acid-base regulation involves: Chemical buffering by intracellular and extracellular buffers  Control of pCO2 by normal respiratory function  Control of HCO3- concentration and acid excretion by the kidney 

The Renal function Reclaim filtered HCO3- (therefore, avoid HCO3- loss) Regenerate HCO3- in an amount equal to that used as buffer In contrast to the respiratory system, can fully compensate without any changes in the bicarbonate pool.

Respiratory Function the respiratory system is able to compensate for changes in the acid/base balance by increasing or decreasing ventilatory rate. This would result in an increase or decrease of pCO2 in the blood. Thus changes are compensated at cost, i.e. changes in the bicarbonate pool.

Abnormal acid-base balance Acid-base imbalances can be defined as acidosis or alkalosis. Acidosis is a state of excess H+ Acidemia results when the blood pH is less than 7.35     Alkalosis is a state of excess HCO3- Alkalemia results when the blood pH is greater than 7.45

Acid-base disturbance Disorder type Primary change in blood HCO3-→Metabolic disorder Primary change in blood pCO2→Respiratory disorder

Steps for ABG interpretation Step1: Look at Pao2 (hypoxemia) Step2: look at pH (acid or alkaline) Step 3: look at Paco2 (resp. acidosis, alkalosis or normal) Step4: look at Hco3 (metabolic acidosis, alkalosis, or normal) Step5: look back at pH (compensated or uncompensated)

ROME ABG interpretation Use the acronym ROME (Respiratory opposite, Metabolic equal) to help you remember

Examples PaO2: 90 mmHg pH: 7.25 P aCO2: 50 mmHg HCO3-: 22mEq\L (uncompensated respiratory acidosis)

Examples PaO2: 90 mmHg pH: 7.25 P aCO2: 40 mmHg HCO3-: 17mEq\L (uncompensated metabolic acidosis)

Examples PaO2: 90 mmHg pH: 7.37 P aCO2: 60 mmHg HCO3-: 38mEq\L (compensated respiratory acidosis with metabolic alkalosis\ main disordered is acidosis and alkalosis is the compensated, because the pH is on the acid side of 7.4))

Examples PaO2: 90 mmHg pH: 7.42 P aCO2: 48 mmHg HCO3-: 35 mEq\L (compensated metabolic alkalosis with respiratory acidosis \ main disordered is alkalosis and acidosis is the compensated, because the pH is on the alkaline side of 7.40)