Arterial Blood Gas Analysis Kathleen Guentner, PhD, RN, CNE Amber Kolesar, MSN, RN UPMC Shadyside School of Nursing

ABG Analysis Three parameters must be assessed during interpretation of an arterial blood gas result Oxygenation Acid – Base status Degree of Compensation

ABG Analysis Oxygenation Evaluate the PaO2 Evaluate the SaO2 PaO2 < 80 mmHg Consider age related alterations End result = Hypoxemia Evaluate the SaO2 SaO2 < 93% Inadequate amount of oxygen is bound to hemoglobin for transport to tissues. End result = Hypoxia Partial pressure of arterial oxygen (PaO2) declines linearly with aging (about 0.3%/yr) until age 75, at which time it stabilizes at about 80 mm Hg in healthy nonsmokers. This gradual decline is mostly attributable to ventilation/perfusion (V/Q) mismatch caused by age-related collapse of peripheral airways, leading to shunting of blood through nonventilated alveoli. PaO2 at any age can be roughly estimated by the equation PaO2 = 109 - (0.43 x age). Altitude!

ABG Analysis Acid Base Balance Evaluate the blood pH (normal, increased or decreased) Evaluate the PaCO2 (increased, decreased or normal) Evaluate the HCO3 (normal, increased, or decreased)

ABG Analysis Blood pH The optimal blood pH is 7.40 The body tolerates a narrow margin of normal pH of 7.35 – 7.45 A pH lower than 7.35 = Acidosis A pH greater than 7.45 = Alkalosis

ABG Analysis Blood pH If the pH is within normal range, it is important to assess which side of 7.40 it falls. This may indicate that the patient may be acidotic or alkalotic, but compensating. 7.35 – 7.39 = Acidosis 7.41 – 7.45 = Alkalosis

ABG Analysis PaCO2 A PaCO2 less than 35 is indicative of respiratory alkalosis A PaCO2 greater than 45 is indicative of respiratory acidosis

ABG Analysis HCO3 A HCO3 level less than 22 is indicative of metabolic acidosis A HCO3 level greater than 26 is indicative of metabolic alkalosis

The Arrow Method of ABG Analysis

“Remember ROME” Respiratory Opposite Metabolic Equal (if arrows with PH is opposite= resp. If same= metabolic)

It’s Easy! Assess the pH, PaCO2 and HCO3 If the values are elevated, place the corresponding arrow. If the values are decreased, place the corresponding arrow.

Now “Match Em” If the pH and PaCO2 arrows are moving in opposite direction, this signifies a RESPIRATORY problem. pH PaCO2 = Respiratory Acidosis pH PaCO2 = Respiratory Alkalosis Remember as the pH decreases……..

Now “Match Em” If the pH and HCO3 arrows are moving in the same direction, this signifies a METABOLIC problem. pH HCO3 = Metabolic Acidosis pH HCO3 = Metabolic Alkalosis

EXAMPLE #1 pH: 7.22 PaCO2: 55 HCO3: 25 pH PaCO2 HCO3 Normal Respiratory Acidosis Normal

EXAMPLE #2 pH: 7.31 PaCO2: 35 HCO3: 20 pH PaCO2 HCO3 Normal Metabolic Let’s follow the steps we talked about Assess the pH. Is it high or low (alkalotic or acidotic), its low, so we have acidosis (7.35-7.45) Assess the PaCo2. Is it high, low, or normal it is normal (35-45) DRAW ON SCREEN, PUT “N”, or NORMAL Assess the HCO3, is it low, high or normal, it is low (22-26) moving in the same direction of the pH Refer to the chart. Acidosis is present (decreased pH) with the PaCO2 being normal, HCO3 being low, so the ph & HCO3 are moving in the same direction, reflecting a metabolic problem. If this was a patient gas we would have to restore tissue perfusion to the hypoxic tissues pH PaCO2 HCO3 Metabolic Acidosis Normal

EXAMPLE #3 pH: 7.49 PaCO2: 30 HCO3: 23 pH PaCO2 HCO3 Normal Let’s follow the steps we talked about Assess the pH. Is it high or low (alkalotic or acidotic), its high, so we have alkalosis (7.35-7.45) Assess the PaCo2. Is it high or low, it is low and in the opposite direction of the pH (35-45) Assess the HCO3, is it low, high or normal, it remains in the normal range (22-26)……. DRAW ON SCREEN, PUT “N”, or NORMAL Refer to the chart. Alkalosis is present (increased pH) with the PaCO3 decreasing, they are changing in the opposite direction reflecting a primary respiratory problem. If this was a patient gas we would have to improve ventilation status by providing 02 therapy, mechanical ventilation, due to muscles becoming exhausted, remember respiratory failure may occur pH PaCO2 HCO3 Respiratory Alkalosis Normal

Evaluate for Compensation When a patient develops an acid-base imbalance the body will typically attempt to compensate for the abnormality. If the buffer systems within the body are unable to maintain a normal pH, the lungs & the kidneys will attempt to compensate. (

Evaluate for Compensation If the problem is respiratory in origin, the kidneys will work to correct it. If the problem is metabolic in origin, the lungs will work to correct it. It may take as little as 5 to 15 minutes for the lungs to recognize a metabolic problem, and start to correct it. However, the kidneys take a lot longer to correct a respiratory problem, typically up to 1 day.

Evaluate for Compensation Determine the degree of compensation: Uncompensated (if PH abnormal, and one other abnormal) Partially compensated (all 3 abnormal) Fully compensated (pH normal, but others both abnormal) if PH abnormal, and one other abnormal

Consequences: Impaired Cellular and Organ Function When compensatory mechanisms fail, the following physiologic consequences occur: Altered cell function, especially in the brain when CO2 crosses the blood–brain barrier Change in intracellular enzyme activity resulting in cell dysfunction Acidosis: decreases the level of consciousness (LOC) Alkalosis: decreases the LOC and has other neurologic manifestations; may cause dysrhythmias

Evaluate for Compensation Uncompensated pH is abnormal, & either the PaCO2 or HCO3 is also abnormal There is no indication that the opposite side has tried to correct for the other.

Evaluate for Compensation Partially Compensated pH is abnormal, & both the PaCO2 and HCO3 are also abnormal This indicates that one system has attempted to correct for the other but has not been completely successful. ALL 3 Values are abnormal

Evaluate for Compensation Fully Compensated pH is normal, & both the PaCO2 & HCO3 are abnormal The normal pH indicates that one system has been able to compensate for the other.

Steps for Evaluating Compensation Assess the pH, determine acidosis or alkalosis. Remember, even if the pH is normal, determine whether the pH falls on the acidotic or alkalotic side of 7.40. Place the corresponding arrow next to the pH (2nd line) But what happens if the pH and PaCo2 are moving in the same direction, this is not what we would expect to see right?

Steps for Evaluating Compensation Assess the PaCO2 and the HCO3 Place the corresponding arrows next to the values. Continue the matching game: Which value matches with the pH? The other component should be moving in the opposite direction, which indicates compensation.

Steps for Evaluating Compensation Example: pH 7.32 ; PaCO2 56 ; HCO3 30 Which value matches with the pH? All three values are abnormal

EXAMPLE #1 (COPD PTS) pH: 7.31 PaCO2: 55 HCO3: 28 pH PaCO2 HCO3 Partially Compensated Respiratory Acidosis

EXAMPLE #2 (Kidney Issues) pH: 7.31 PaCO2: 25 HCO3: 20 pH PaCO2 HCO3 Partially Compensated Metabolic Acidosis

EXAMPLE #3 (Anxiety) pH: 7.48 PaCO2: 25 HCO3: 21 pH PaCO2 HCO3 Partially Compensated Respiratory Alkalosis

EXAMPLE #4 (NG drainage/vomiting) pH: 7.48 PaCO2: 55 HCO3: 31 pH PaCO2 HCO3 Partially Compensated Metabolic Alkalosis

EXAMPLE #5 pH: 7.39 Normal PaCO2: 60 HCO3: 30 pH PaCO2 HCO3 N Fully Compensated Respiratory Acidosis N

Case Study A 21 year old college student is admitted to the Emergency Department after taking an overdose of Oxycontin. The patient is unconscious & breathing at a rate of 6 to 7 bpm. The patient’s ABG on room air is as follows: pH: 7.23 (low); PaCO2: 71 (high); PaO2: 64; HCO3: 26 (normal) Interpretation: (resp. acidosis. Uncompensated) Possible Cause? Drug overdose Nursing Interventions: intubation (high resp above 20, high Fio2 40) NARCAN pH= Low Paco2= High HCO3= Normal Respiratory Acidosis probably caused by over sedation/overdose of Dilaudid Expected signs & symptoms: respiratory distress, drowsiness, decreased responsiveness Nursing interventions: Give Narcan & intubate ASAP

Case Study A woman with a history of panic disorder arrives at the Emergency Department complaining of not being able to “catch her breath”. The woman is crying, shaking and breathing rapidly. An ABG is obtained: pH: 7.52 (high); PaCO2: 28 (norm); HCO3: 22 (norm); SaO2: 90% Interpretation: Respiratory Alkalosis uncompensated, hypoxia Possible Cause? Hyperventilating Nursing Interventions: Calm down, paper bag, pain pH= High Paco2= Normal HCO3= Normal Respiratory Alkalosis Is the patient really in resp distress? NO She is hyperventilating, we need to calm her anxiety; treat the anxiety attach!! May even use a paper bag to restore PCO2.

Case Study A teenager is being evaluated at the emergency room for complaints of muscle weakness and twitching. Her mother reports “she is always on a dieting” The patient is 5’6 and weighs 102lbs. Among the teenager’s lab results are the following ABG values: pH: 7.50 (high); PaCO2: 45 (normal); HCO3: 30 (low); SaO2 94% Interpretation? Metabolic alkalosis uncompensated (difficult to treat) Possible Cause? Nursing Interventions? Deep breaths, positioning, 2L NC O2, nutrition education, replace electrolytes, MONITOR pH= High Paco2= Normal HCO3= Low Metabolic Alkalosis Very difficult to treat! The patient probably has an eating disorder, maybe Bulimia (prolonged induced emesis) Hydrogen retention and bicarbonate excretion may be promoted by administering Can give K, Ca glucanate Bicarbonate excretion through the kidneys can be stimulated with drugs such as Diamox (Acetazolamide) → but resolution will be slow A diuretic that acts as a carbonic anhydrase inhibitor

Case Study A patient has been mechanically ventilated for the past two days in the ICU. The ventilator settings are as follows: AC, rate of 16, FIO2 50%, and TV 600. The patient’s 6am ABG results are as follows: pH: 7.49 (high); PaCO2: 29 (low); PaO2: 70; HCO3: 25 (normal): Interpretation? Respiratory alkalosis uncompensated Possible Cause? Hyperventilation, slow rate/tidal volume Nursing Interventions? Pain control, increase PEEP, AC usually initiated as initial mode of ventilation Respiratory Alkalosis Ph= High PaCO2= Low HCO3= Normal Common causes: anxiety, fear, hyperventilation Nursing interventions: Pain control, sedation, decrease rate; patient breathing with vent Patient at risk for developing cardiac dysrhythmias

Case Study A 53 year old patient presents to the Emergency Department with complaints of weakness, nausea, and shortness of breath. The patient’s STAT ABG results are as follows: pH: 7.29 (low); PaCO2: 32 (normal); HCO3: 18 (normal); PaO2 80 Interpretation:partially compensated metabolic acidosis Possible Cause? Nursing Interventions? Metabolic Acidosis pH= Low PaCO2= Normal HCO3= Normal Management Life Threatening Treat underlying cause and replace bicarbonate Insulin/glucose  uncontrolled diabetes Improvement of oxygenation in lactic acidosis some aerobic metabolism can resume IV Sodium Bicarbonate administration

Case Study The nurse is caring for a patient 2 days post-abdominal surgery The nurse notes 500cc of coffee ground drainage over the last four hours. The patient is neurologically intact. ABG results are ordered with the following results. pH: 7.29 (low); PaCO2: 65(high); PaO2: 60; HCO3: 26 (normal) Interpretation? Respiratory acidosis, uncompensated Possible Cause? Nursing Interventions? Respiratory Acidosis pH= Low PaCO2= High HCO3= Normal Not text book scenario; Would expect patient to demonstrate Metabolic Alkalosis due to vomiting, NG suctioning; coffee ground emesis paCO2 74; should be 80-100 What else do we need to know here???? COPD Heart Failure Pulmonary infection Atelectasis Pulmonary Edema Pulmonary Embolus Is the patient coughing & deep breathing- Post op abdominal surgery necessity What are is the patients respiratory rate? Patient probably display dyspnea which could lead to respiratory distress What is heart rate? Patient probably tachycardic, may have dysrhythmias Give antibiotics

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