ABG Flashcards
What are the impacts of acidic or alkaline blood?
Below 7.35 (acidic):
- decrease in the force of cardiac contractions,
- decrease in the vascular response to catecholamines, and
- diminished response to the effects and actions of certain medications.
Above 7.45 (alkalotic):
- interferes with tissue oxygenation and
- normal neurological and
- muscular functioning.
Significant changes in the blood pH above 7.8
or below 6.8 will interfere with cellular functioning, and if uncorrected, will lead to death.
Describe the respiratory buffer response
A normal by-product of cellular metabolism is carbon dioxide (CO2). CO2 is carried in the
blood to the lungs, where excess CO2 combines with water (H2O) to form carbonic acid
(H2CO3). The blood pH will change according to the level of carbonic acid present. This
triggers the lungs to either increase or decrease the rate and depth of ventilation until the
appropriate amount of CO2 has been re-established. Activation of the lungs to compensate for an imbalance starts to occur within 1 to 3 minutes
Describe the renal buffer response
In an effort to maintain the pH of the blood within its normal range, the kidneys excrete or
retain bicarbonate (HCO3-). As the blood pH decreases, the kidneys will compensate by
retaining HCO3- and as the pH rises, the kidneys excrete HCO3- through the urine. Although the
kidneys provide an excellent means of regulating acid-base balance, the system may take from
hours to days to correct the imbalance. When the respiratory and renal systems are working
together, they are able to keep the blood pH balanced by maintaining 1 part acid to 20 parts
base.
Describe respiratory acidosis
Respiratory acidosis is defined as
- pH less than 7.35
- with a PaCO2 greater than 45 mm Hg.
Caused by an accumulation of CO2 which combines with water in the body to
produce carbonic acid, thus, lowering the pH of the blood.
Causes: Any condition that results in
hypoventilation can cause respiratory acidosis.
- Central nervous system depression related to head injury
- Central nervous system depression related to medications such as narcotics, sedatives, or
anesthesia
- Impaired respiratory muscle function related to spinal cord injury, neuromuscular diseases,
or neuromuscular blocking drugs
- Pulmonary disorders such as atelectasis, pneumonia, pneumothorax, pulmonary edema, or
bronchial obstruction
- Massive pulmonary embolus
- Hypoventilation due to pain, chest wall injury/deformity, or abdominal distension
S/S:
- Pulmonary symptoms include dyspnea, respiratory distress, and/or
shallow respirations.
- Nervous system manifestations include headache, restlessness, and confusion. If CO2 levels become extremely high, drowsiness and unresponsiveness may be noted.
- Cardiovascular symptoms include tachycardia and dysrhythmias.
Tx: increase ventilation and treat underlying issue, If the cause cannot be readily resolved, the patient may require mechanical ventilation while treatment is rendered. Although patients with hypoventilation often require supplemental oxygen, it is important to remember that oxygen alone will not correct the problem
Describe respiratory alkalosis
- pH > 7.45
- with a PaCO2 less than 35 mm Hg.
Causes: Any condition that causes hyperventilation can result in respiratory alkalosis.
• Psychological responses, such as anxiety or fear
• Pain
• Increased metabolic demands, such as fever, sepsis, pregnancy, or thyrotoxicosis
• Medications, such as respiratory stimulants.
• Central nervous system lesions
S/S:
- Nervous system alterations include light-headedness, numbness and
tingling, confusion, inability to concentrate, and blurred vision.
- Cardiac symptoms include
dysrhythmias and palpitations. Additionally, the patient may experience dry mouth,
diaphoresis, and tetanic spasms of the arms and legs.
Tx: resolve underlying problem. Patients
presenting with respiratory alkalosis have dramatically increased work of breathing and must
be monitored closely for respiratory muscle fatigue. When the respiratory muscles become
exhausted, acute respiratory failure may ensue.
Describe metabolic acidosis
- bicarbonate level < 22 mEq/L
- with a pH of less than 7.35.
Cause: either a deficit of base in the bloodstream or an excess of acids, other than CO2. - Diarrhea and intestinal fistulas may cause decreased levels of base. - Causes of increased acids include: • Renal failure • Diabetic ketoacidosis • Anaerobic metabolism • Starvation • Salicylate intoxication
S/S:
- Nervous system manifestations include headache, confusion, and restlessness progressing to lethargy, then stupor or coma.
- Cardiac dysrhythmias are common and
- Kussmaul respirations occur in an effort to compensate for the pH by blowing off more CO2. Warm, flushed skin, as well as
- nausea and vomiting are commonly noted.
Tx:
- dependent upon the cause.
- search for hypoxic tissue somewhere in the body. Hypoxemia can lead to anaerobic metabolism system-wide, but hypoxia of any tissue bed will produce metabolic acids as a result of anaerobic metabolism even if the PaO2 is
normal. The only appropriate way to treat this source of acidosis is to restore tissue perfusion
to the hypoxic tissues. Other causes of metabolic acidosis should be considered after the
possibility of tissue hypoxia has been addressed.
-sodium bicarbonate is indicated only for known
bicarbonate-responsive acidosis, such as that seen with renal failure. Routine use of sodium
bicarbonate to treat metabolic acidosis results in subsequent metabolic alkalosis with
hypernatremia and should be avoided.
Describe metabolic alkalosis
- bicarbonate level > 26 mEq/liter
- with a pH greater than 7.45.
Cause: excess of base or a loss of acid within the body.
-Excess base occurs from ingestion of antacids, excess use of bicarbonate, or use of
lactate in dialysis.
-Loss of acids can occur secondary to protracted vomiting, gastric suction, hypochloremia, excess administration of diuretics, or high levels of aldosterone.
S/S:
- Neurologic symptoms include dizziness, lethargy, disorientation, seizures and coma.
- Musculoskeletal symptoms include weakness, muscle twitching, muscle cramps and tetany.
- The patient may also experience nausea, vomiting, and respiratory depression.
Tx: Metabolic alkalosis is one of the most difficult acid-base imbalances to treat. Bicarbonate
excretion through the kidneys can be stimulated with drugs such as acetazolamide (Diamox™),
but resolution of the imbalance will be slow. In severe cases, IV administration of acids may
be used. It is significant to note that metabolic alkalosis in hospitalized patients is usually
iatrogenic in nature
What are normal ABG levels?
pH 7.35 to 7.45
PaO2 80 to 100 mm Hg.
SaO2 95% to 100%.
PaCO2 35 to 45 mm Hg.
HCO3 22 to 26 mEq/liter
B.E. –2 to +2 mEq/liter.
Describe how you would determine if a pt is suffering from an acidosis or alkalosis of respiratory or metabolic origin?
See
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What conditions affect the binding affinity of oxygen?
1) enhanced release of the oxygen molecule (C waveform, shift to R):
- acidosis,
- fever,
- elevated CO2 levels,
- increased 2,3-diphosphoglycerate (2,3-DPG, a by-product of glucose metabolism).
2) keep the oxygen molecule tightly attached (B waveform, shift to the L):
- hypothermia,
- alkalosis,
- low PCO2, and
- decrease in 2,3-DPG.
A shift to the L has more negative implications for the patient than a shift to the R.
Which arteries are suitable for ABG blood samples?
- needs to be close to surface with adequate redundant circulation
1) radial/ulnar artery
2) femoral artery
3) brachial artery (last resort, lacks redundant circulation, damage can result in ischemia of forearm and hand)
What are the 6 steps of ABG analysis?
A 6-step program for ABG analysis
- Analyze the pH.
- Analyze the PaCO2.
- Analyze the HCO3.
- Match either the PaCO2 or the HCO3 with the pH.
- Does either the PaCO2 or the HCO3 go in the opposite direction of the pH?
- Analyze the PaO2 and SaO2.
