II.  The Big Four

A.  Respiratory Acidosis     pH < 7.35 (Normal = 7.35-7.45)   CO₂ > 45 (Normal = 35-45)

 1. Causes:

     --Hypoventilation      

• Depression of the Respiratory Center (sedatives, narcotics, drug overdose, CVA, cardiac arrest, MI)

• Respiratory muscle paralysis (spinal cord injury, Guillian-Barre, paralytics)

• Chest wall disorders (flail chest, pneumothorax)

• Disorders of the lung parenchyma (CHF, COPD, pneumonia, aspiration, ARDS)

• Alteration in the function of the abdominal system (distention)

 2. Signs and Symptoms:

• CNS depression (decreased LOC)

• Muscle twitching which can progress to convulsions

• Dysrhythmias, tachycardia, diaphoresis (related to hypoxia secondary to hypoventilation)

• Palpitations

• Flushed skin

• Serum electrolyte abnormalities including elevated K+ (K+ leaves the cell to replace the H+ buffers leaving the cell)

 3. Treatment:   

• Physically stimulate the pt to improve ventilation

• Vigorous pulmonary toilet (chest PT, coughing and deep breathing, spirometer, respiratory treatments with Bronchodilators)

• Mechanical Ventilation (to increase the respiratory rate and tidal volume

• Reversal of sedatives and narcotics

• Antibiotics for infections

• Diuretics for fluid overload
  (Note: beware of NAHCO₃--Sodium Bicarbonate--can overcompensate and cause Metabolic Alkalosis. Also, if pt has been hypoxic and this is a Lactic Acidosis--NAHCO₃ can be dangerous)

B. Respiratory Alkalosis        pH > 7.45     CO₂  < 35

 1. Causes:

    --Alveolar Hyperventilation     

• Psychogenic (fear, pain, anxiety)

• CNS stimulation (brain injury, ETOH, early salicylate poisoning, brain tumor)

• Hypermetabolic states (fever, thyrotoxicosis)

• Hypoxia (high altitude, pneumonia, heart failure, pulmonary embolism)

• Mechanical hyperventilation (ventilator rate too fast)

 2. Signs and Symptoms:    

• Headache

• Vertigo

• Paresthesias (numb fingers/toes/circumoral, carpal pedal spasms and tetany)

• Tinnitus (ringing in the ears)

• Electrolyte abnormalities (decreased Ca+, K+)

 3. Treatment:  (treat the underlying cause) 

• Sedatives or analgesics

• Correction of hypoxia (possible diuretics, mechanical ventilation to also decrease respiratory rate and decrease the tidal volume)
  (Note: Brain Injury pt. may need hyperventilation)

• Antipyretics for fever

• Treat hyperthyroidism

• Breathe into a paper bag for hyperventilation

C. Metabolic Acidosis        pH < 7.35     HCO₃ < 22  (normal = 22 – 26)

 1. Causes:

    --Increased H+, excess loss of HCO₃

• Overproduction of organic acids (starvation, ketoacidosis, increased catabolism)

• Impaired renal excretion of acid (Renal Failure)

• Abnormal loss of HCO₃ (diarrhea, biliary fistula, Diamox)

• Ingestion of acid (salicylate overdose, oral anti-freeze)

 2. Signs and Symptoms:

• CNS depression (confusion to coma)

• Cardiac Dysrhythmias (elevated T wave, wide QRS to Ventricular Standstill)

• Electrolyte abnormalities (elevated K+, Cl-, Ca+)

• Flushed skin (arteriolar dilitation)

• Nausea

 3. Treatment:  (treat the underlying cause)

• NAHCO₃ (Sodium Bicarbonate) based on ABGs only and with caution

• IV fluids and Insulin for DKA

• Dialysis for Renal Failure

• Antibiotics, increased nutrition for tissue catabolism

• Increase cardiac output and tissue perfusion for low C.O. states

• Rehydrate, monitor I & O

• Treat Dysrhythmias, support hemodynamic and respiratory status

D. Metabolic Alkalosis            pH > 7.45     HCO₃ > 26

 1. Causes:

     --Loss of H+ or increased HCO₃   

• Large losses of gastric contents (vomiting, NG suction)

• Loss of K+ (diarrhea, vomiting)

• Ingestion of large amounts of bicarbonate (antacids, resuscitation)

• Prolonged use of diuretics (distal tubule lose ability to reabsorb Na+ and Cl-  therefore Na+, Cl-, K+, Ammonia are lost in the urine and these bind with H+)
  (Note: al-K+-low-sis means K+ value is low when pt is alkalotic)

 2. Signs and Symptoms:  (similar to the associated disease process)

• Diaphoresis

• Nausea and Vomiting

• Increase neuromuscular excitability (Ca+ binds with protein)

• Shallow breathing (Respiratory Compensation)

• EKG changes (increased QT, Sinus Tachycardia)

• May also see confusion progressing to lethargy to coma

• Electrolyte abnormality (decreased Ca+, normal or decreased K+, increased Base Excess on the ABG)

 3. Treatment:  (treat the underlying cause)

• Replace KCL losses in 0.9% NaCl (rehydrates and increases HCO₃ excretion)

• Diamox (Acetazolamide) (increases HCO₃ excretion)

• Monitor neuro status, re-orient, seizure precautions, monitor I & O

III.      O₂ STANDS ALONE

   Did you notice that I haven’t mentioned O₂?  The O₂ number has nothing to do with your acid-base ABG interpretation!

A. What does the PaO₂ mean?

• The O₂ tells us if the patient has HYPOXEMIA (decreased oxygen in the blood).

• Normal PaO₂ = 80-100. (Hypoxemia = PaO₂<80)

• PaO₂ assesses Perfusion (gas exchange).

• PaCo₂ assesses the adequacy of Ventilation (breathing pattern).

• The PaO₂ is very important in determining your patient’s oxygen status and needs—but it is not necessary in determining the Big Four.

B. What is saturation?

• SaO₂ (oxygen saturation) measures the percent of oxygen bound to hemoglobin. This tells weather the patient has HYPOXIA (decreased O₂ in the tissues).

• Normal SaO₂ = Greater that 95%

• Acceptable SaO₂ will vary between MDs; but PaO₂ dramatically drops when it is less that 92%.

• This is a noninvasive measurement via pulse oximetry and can be less accurate due to hypothermia, hypotension, hypovolemia, or vasoactives.

• Note: In Carbon Monoxide Poisoning—the Hgb is saturated with Carbon Monoxide. Although the patient is hypoxemic because there is no room on the Hgb for O₂ to be carried—the Saturation looks good because it can’t distinguish between the two.

Oxyhemoglobin Dissociation Curve
 

C. What are some causes of low PaO₂?

• Hypoxic Hypoxia--there’s just not enough of a supply of O₂
  ( COPD, pneumonia, ARDS, suffocation)

• Anemic Hypoxia--There’s plenty of O₂—but not enough Hgb to carry it to the tissues

• Stagnant Hypoxia--There may be enough O₂ coming in and enough Hgb to carry it--but the circulation is stagnant due to a decreased Cardiac Output. The O₂ is not being adequately carried to the tissues.

• Histotoxic Hypoxia--Poisoning like Carbon Monoxide or Cyanide. Either the blood can’t carry the O₂ or the cells can’t receive the O₂ from the blood.

By Cyndi Cramer, BA, RN, OCN

RealNurseEd.com