Pulmonary Acid Base and ABGs Flashcards
What is pH?
- scale goes from 1-14
- water is neutral and is equal to 7
- normal blood pH: 7.35-7.45
alkalotic above 7.45, acidic below 7.35
pH is the result of what?
- result of the total amt of body acids
- carbonic acid: levels fluctuate based on concentration of CO2 and HCO3
- lactic acid, phosphoric acid, sulfuric acid and ketone bodies: levels fluctuate based on function of renal system
What are the physiologic changes that occur with pH changes (alkalosis and acidosis)?
- acidosis: decrease in force of cardiac contractions, decrease in vascular response to catecholamines, and decrease in response to the effects and actions of certain meds
- alkalosis: interferes with tissue oxygenation, normal neuro and muscular functioning
How are blood pHs measured?
- arterial sample
- typically drawn at radial artery (or femoral)
- ABG
- test collateral circulation to the hand prior to drawing a sample from the radial artery = allen’s test
What do ABG results include?
- pH
- PaO2
- PaCO2
- HCO3
- anion gap
- H+ concentration, not directly measured but can be calculated if needed
What are the normal reference ranges fo ABGs?
- pH: 7.35-7.45
- paCO2: 35-45 mm Hg
- PaO2: 80-102 mm Hg
- HCO3: 22-28 mmmol/L
- anion gap: 6-12 mmol/L
What types of pts would you order ABGs on?
- impending or current state of respiratory failure
- critically ill
- sudden unexpected deterioration
- sepsis
- multiorgan failure
- drug overdose
- assessment of pts with chronic lung disease to eval level of CO2 retention
- CO poisoning need to run a carboxyhemoglobin level
What are the main characters of acid base disturbances?
- pH
- H+
- Co2
- HCO3
What is inversely proportional to the pH?
- H+ concentration
- H+ ions are product of cellular metabolism
- CO2 is a byproduct of cellular metabolism
- most of CO2 is transported in the blood as HCO3
- converting CO2 into HCO3 frees a H+, so the more CO2 there is the more H+ that are produced
- CO2 concentration is also inversely proportional to pH. the higher the CO2 the lower (more acidic) the pH becomes
- CO2 is a weak acid that is constantly being produced through tissue metabolism which is eliminated from the body through the lungs, and increasing the minute ventilation will decrease CO2: blowing off CO2
What are the buffers in the body?
- respiratory
- renal
- carbonic acid-bicarb buffer
What buffer works the fastest?
- respiratory buffer
- the blood pH will change according to the level of carbonic acid and HCO3-
- this triggers an increase or decrease in the rate and depth of ventilation until the appropriate amt of CO2 has been reestablished
- activation of the lungs to compensate for an imbalance starts to occur within 1-3 minutes
What is the renal buffer?
bicarbonate (HCO3-)
- base
- buffer for H+
- renal system maintains the balance of HCO3- and H+
- metabolic changes that result in changes in pH take several days
How does the Carbonic acid bicarb buffer system work?
- if there is an increase in H+ concentration in the blood - the equation is driven toward the left to form carbonic acid
- if H+ concentration decreases below desired level than carbonic acid dissociates into bicarb and H+
- when CO2 levels increase the formation of more carbonic acid occurs
What are the 4 major acid base derangements?
- resp acidosis
- resp alkalosis
- metabolic acidosis
- metabolic alkalosis
What are the causes of resp. acidosis?
- pH less than 7.35 and a PaCO2 greater than 45 mm Hg
- CNS depression: meds - narcotics, sedatives, or anesthesia or head injury
or - impaired respiratory muscle function; spinal cord injury, neuromuscular disease, or neuromuscular blocking drugs
or - pulm disorders: atelectasis, pneumonia, pneumothorax, pulmonary edema, bronchial obstruction, massive pulmonary embolus
or
hypoventilation due to pain, chest wall injury/deformity, abdominal distension, obesity
or trauma - CO2 is elevated because of lack of ventilation
What are the causes of resp. alkalosis? (pH greater than 7.45, and CO2 less than 35 mm Hg)
- psychological responses: anxiety or fear
- pain
- increased metabolic demands: fever, sepsis, pregnancy, or thyrotoxicosis
- meds: such as respiratory stimulants
- CNS lesions - telling you to breathe fast
- CO2 decreased because of overventilation
Causes of metabolic acidosis? (bicarb level of less than 22 mEq/L with pH less than 7.35
- renal failure
- DKA
- diarrhea
- anaerobic metabolism: from tissue hypoxia
- starvation
- ***salicylate intoxication: ASA overdose
- ** presence of metabolic acidosis should spur a surch for hypoxic tissue somewhere in body
Causes of metabolic alkalosis? bicarb greater than 28 mEq/L with pH greater than 7.45
- either an excess of base or loss of acid within body
- excess base occurs from ingestion of: antacids, excess use of bicarbonate, use of lactate in dialysis
- loss of acids can occur secondary to: protracted vomiting, gastric suction, hypochloremia, excess admin of diuretics, and high levels of aldosterone
Definitions of hypoxemia and hypoxia?
- hypoxemia: insufficient oxygenation
- hypoxia: low oxygen content in tissue
What is SaO2 or SpO2?
- blood gas
- SpO2 = pulse oximeter
- arterial oxygen saturation
- % of hemoglobin that is bound to O2
- normal depends on pt, ideally should be 95% or greater
- PaO2: arterial oxygen tension in the plasma, measured by blood gas, in general less than 80 is abnormal
What is an A-a gradient?
- difference b/t the oxygen tensino in alveoli (PAo2) and the arterial oxygen tension
What can the PaO2 from the blood gas tell us?
- can aid in assessment of the function of the alveolar capillary membrane
- determination of the difference b/t alveolar oxygen partial pressure (PAO2) and arterial oxygen partial pressure (PaO2)
- measures the integrity of alveolar capillary unit
What is the normal A-a gradient?
- normal values change with age (increase), usually norm is less than 10 mm Hg
- to calculate estimated A-a gradient: age/4 + 4
What factors go into calculating A-a gradient?
- PaO2 is measure of blood gas
- PAO2 is calculated from the alveolar gas equation
- PAO2= (FIO2x (Patm -PH20))-(PaCO2/R)
FIO2: room air= 0.21 fraction of inspired oxygen
Patm= at sea level: 760
PH20= at norm bod temp = 47 mm Hg
PaCO2= CO2 tension from blood gas - R: respiratory quotient which is 0.8 at steady state
What does hypoxemia with a normal A-a gradient mean?
- hypoventilation, high altitude
What does hypoxemia with an increased A-a gradient mean?
- diffusion defect, ventilation-perfusion mismatch, right to left shunt
What are the most common causes of a V/Q mismatch?
- most common cause of hypoxemia***
- responds to O2
- increased Aa gradient
- most common causes with ventilation problems: COPD, asthma, and pneumonia
perfusion: PE, pulm HTN, and cardiac arrest
Step 1 of ABG anaylsis (PaO2)
assess the PaO2
- is the PaO2 less than 80 mm Hg?
0 if no then there is adequate oxygenation and proceed to step 2
- if yes than go to step 1A to determine the A-a gradient
Step 1A (assess ventilation)
- is pt hypoventilating:
if CO2 is high= hypoventilation go to step 2 - if CO2 normal than no hypoventilation so proceed to step 1B to calculate A-a - need to find etiology of hypoxemia to determin degree of abnormality of pulmonary capillary membrane (problem with diffusion)
step 1B (A-a gradient)
- if A-a gradient is elevated then there is a problem on either side of the alveolar capillary membrane
Step 2: calculate pH
- Acidosis if below 7.35
- alkalosis if above 7.45
Step 3: what is CO2?
- in primary respiratory disorders: pH and CO2 change in opp directions but in primary metabolic disorders: pH and Co2 change in same direction
Step 4: is there any compensation?
- if it is a respiratory problem what is happening with HCO3-?
- if metabolic problem what is happening with Co2?
Step 5: calculate anion gap
= Na- (Cl+HCO3)
- number of anions and cations should be equal in theory but they arent all measured in equation so difference is between 6-12
- this may be helpful sorting out causes of metabolic acidosis as some causes have increased anion gap and some have normal gap
When pH is low and CO2 is high what is problem?
- resp acidosis
When pH is high and CO2 is low what is the problem?
- resp alkalosis
When CO2 is normal and pH and HCO3 are low what is problem?
-metabolic acidosis
When both pH and HCO3 are elevated and CO2 is normal whats the problem?
- metabolic alkalosis
