Interpretation Compensated and Uncompensated Blood Gas Analysis James

Interpretation: Compensated and Uncompensated Blood Gas Analysis James Barnett, RN, MSN Clinical Educator – Neuroscience PCC Vanderbilt University Medical Center May 2007

Compensatory Mechanisms n Compensation is the body’s way of restoring a normal blood p. H n Remember: Acid + Base Neutrality n Compensation DOES NOT treat the root of the problem – the reason for the acid-base imbalance is STILL THERE!!!

Compensatory Mechanisms n The body has three means to try to compensate for an acid-base imbalance n Chemical n Respiratory n Renal

Chemical Compensation Chemicals within the blood act within seconds to correct respiratory or metabolic imbalances n Used up quickly – not effective long-term n Chemical buffers in the blood include n Bicarbonate n Phosphate n Proteins n

Respiratory Compensation n Used to compensate for metabolic imbalances only n Chemoreceptors respond to changes in H+ concentrations alters respiratory rate and depth n Remember CO 2 is an acid

Respiratory Compensation n Respiratory Rate will… n Increase when blood H+ is increased (acidic p. H) n CO 2 is “blown off” n Amount of acid in blood is decreased n Decrease when H+ is decreased (alkaline p. H) n CO 2 is retained n Amount of acid in blood is increased

Respiratory Compensation n This means n Metabolic acidosis causes an increase in rate and depth of ventilation as the body attempts to get rid of acid (CO 2) n Metabolic alkalosis causes a decrease in rate and depth of ventilation as the body attempts to retain acid (CO 2)

Renal Compensation Used to compensate for respiratory imbalances n Remember: HCO 3 - is a base n Kidneys respond to changes in blood p. H n Excrete H+ and retain HCO 3 - when acidemia is present (1: 1 ratio) n Retain H+ and excrete HCO 3 - when alkalemia is present (1: 1 ratio) n

Renal Compensation n This means n A respiratory acidosis will make the kidneys excrete acid (H+) and retain base (HCO 3 -) n A respiratory alkalosis will make the kidneys excrete base (HCO 3 -) and retain acid (H+)

Renal Compensation n This is the slowest compensation n May take hours to days n Most powerful method of compensation n Ineffective in patients with renal failure

Note on Compensation The body is very smart and will not overcompensate for an imbalance

Degrees of Compensation n An acid-base imbalance will be compensated for in one of three ways n Uncompensated n Partially compensated n Fully compensated

Degrees of Compensation n Uncompensated n n Body has made no attempt to correct the acid-base imbalance Partially compensated Body is attempting to correct the imbalance n Blood p. H remains abnormal in spite of the attempt n

Degrees of Compensation n Fully compensated n The body is correcting the imbalance n Blood p. H is normal n Other blood gas values remain abnormal until the root cause is treated and corrected

Uncompensated Acid-Base Imbalance

Uncompensated Imbalance p. H abnormal n Either Pa. CO 2 OR HCO 3 - abnormal n All other values normal n n If Pa. CO 2 is abnormal n n Problem is respiratory If HCO 3 - is abnormal n Problem is metabolic

Uncompensated Imbalance n Uncompensated respiratory acidosis n Uncompensated respiratory alkalosis n p. H Pa. CO 2 HCO 3 - n n < 7. 35 > 45 WNL n n > 7. 45 < 35 WNL Remember that CO 2 is an acid and that the more of it there is the worse is the acidemia. Notice that with uncompensated respiratory, the HCO 3 is normal – this is because the body has not began to compensate for the alterations in CO 2

Uncompensated Imbalance n Uncompensated metabolic acidosis n Uncompensated metabolic alkalosis n p. H Pa. CO 2 HCO 3 - n n < 7. 35 WNL < 22 n n > 7. 45 WNL > 26 Remember that HCO 3 is a base and that the more of it there is the more alkalotic you will be. Notice that in the case of uncompensated metabolic the Pa. CO 2 is normal indicating that the body has not began to compensate.

Partially Compensated Imbalances n Occur when compensation mechanisms are activated, but have not had sufficient time to normalize the blood p. H n NOTE: Some people say that there is no such thing as “partially” compensated – it is kind of like being “a little pregnant” – but it is indicative of a part of the process called compensation

Partially Compensated Imbalances p. H is abnormal n Both Pa. CO 2 and HCO 3 - are abnormal in the same direction (increased or decreased from normal) n If Pa. CO 2 is high (↑ acid), HCO 3 - will also be high (↑ alkaline) to neutralize the environment n If Pa. CO 2 is low (↓ acid), HCO 3 - will also be low (↓ alkaline) to neutralize the environment n

Partially Compensated Imbalances n Partially Compensated Respiratory Acidosis n Partially Compensated Respiratory Alkalosis n p. H Pa. CO 2 HCO 3 - n n < 7. 35 > 45 > 26 n n > 7. 45 < 35 < 22 In the case of Partially Compensated Resp Acidosis, the p. H is low, indicating an acid environment…when you look at the Pa. CO 2, it too is acidic, which is how you know that you have a respiratory acidosis. With the HCO 3 being high, you can deduce that the body is raising its base to counteract the acid represented by the p. H; therefore, partially compensated respiratory acidosis.

Partially Compensated Imbalances n Partially Compensated Metabolic Acidosis n Partially Compensated Metabolic Alkalosis n p. H Pa. CO 2 HCO 3 - n n < 7. 35 < 22 n n > 7. 45 > 26 With partially compensated metabolic acidosis, you notice first that the p. H is low (acidosis). Ask yourself, which number is representative of an acid condition. In this case it is the low base (HCO 3), so you know you have a metabolic acidosis. You know it is partially compensate because the Pa. CO 2 is low indicating that CO 2 (an acid) is being lost from the body to correct for the low p. H.

Compensated Imbalances Occur when compensatory mechanisms have been able to fully normalize blood p. H

Compensatory Mechanisms n Both Pa. CO 2 and HCO 3 - are abnormal, but in the same direction n If Pa. CO 2 is high (↑ acid), HCO 3 - will also be high (↑ alkaline) n If Pa. CO 2 is low (↓ acid), HCO 3 - will also be low (↓alkaline)

Compensated Imbalances n Compensated Respiratory Acidosis n Compensated Respiratory Alkalosis n p. H n Pa. CO 2 HCO 3 - n WNL but closer to 7. 35 > 45 > 26 n WNL but closer to 7. 45 < 35 < 22 In compensated respiratory acidosis, the p. H tends to range between 7. 35 and 7. 39 – still acidic, But in the normal p. H range. When you look at the Pa. CO 2, you notice that it is high (acidic), bu The HCO 3 is also high, indicating that the body has compensated and normalized the low p. H.

Compensated Imbalances n Compensated Metabolic Acidosis n Compensated Metabolic Alkalosis n p. H n Pa. CO 2 HCO 3 - n WNL but closer to 7. 35 < 22 n WNL but closer to 7. 45 > 26

Mixed Imbalances Occur when patient has both metabolic and respiratory disorders that cause an acid-base imbalance n Examples: n Diabetic Keto. Acidosis (metabolic acidosis) with decreased respiratory drive (respiratory acidosis) n Severe vomiting (metabolic alkalosis) with high fever (respiratory alkalosis) n

Mixed Imbalances n p. H will be normal n Pa. CO 2 and HCO 3 - will be abnormal n Pa. CO 2 will be high with low HCO 3 - (both tend toward acid side) n Pa. CO 2 will be low with high HCO 3 - (both tend toward base side)

Mixed Imbalances n Mixed acidosis n Mixed alkalosis n p. H Pa. CO 2 HCO 3 - n n < 7. 35 > 45 < 22 n n > 7. 45 < 35 > 26 Notice with the mixed acidosis that you have an acidic p. H (less than 7. 35, with other Parameters indicating an acid environment. High Pa. CO 2 (too much acid). Low HCO 3 (too little base – an acidic environment). This is classic mixed acidosis.

Finished You have finished this in-service. n Please go to the next in-service titled: n n Effects of Acid Base on Oxygenation