Pulmonary Acid Base and Alterations in Gas Exchange Flashcards
What is the normal blood pH?
What is the equation to find pH?
Normal blood pH is 7.35 to 7.45
pH=6.1 + log10 ([HCO3-]/0.03 x PaCO2)
pH is the total amount of what in the body?
Levels flucuate based on what? 2
What other acids are we including in pH measurements? 4
All these acids flucuate based on what?
acid
Levels fluctuate based on the concentration of CO2 and HCO3-
- Lactic acid,
- phosphoric acid,
- sulfuric acid and
- ketone bodies
Levels fluctuate based on the function of the renal system
Physiologic changes that occur with pH changes
Acidosis? 3
Alkalosis? 2
↓force of cardiac contractions
↓ vascular response to catecholamines
↓ response to the effects and actions of certain medications
- interferes with tissue oxygenation
- normal neurological and muscular functioning
How is blood pH measured?
The blood pH is measured with an arterial sample
What should we do before we draw a blood sample from the radial artery?
Test collateral circulation to the hand prior to drawing a sample from the radial artery = Allen’s Test
ABG results include the following 5
Is H+ concentration measured?
pH PaCO2 PaO2 HCO3 Anion gap
Not directly measured but can be calculated if needed
Normal reference ranges: pH PaCO2 PaO2 HCO3 Anion gap
pH 7.35-7.45 PaCO2 35-45 mmHg PaO2 80-102 mmHg HCO3 22-28 mmol/L Anion gap 6-12 mmol/L
Types of patients to order blood gases on include:
8
- Impending or current state of respiratory failure
- Critically ill
- Sudden unexpected deterioration
- Sepsis
- Multiorgan failure
- Drug overdose
- Assessment of patients with chronic lung disease to evaluate level of CO2 retention
- Carbon monoxide poisoning need to run a carboxyhemoglobin level
What are the main players in acid base disturbances?
4
pH
H+
CO2
HCO3-
- Hydrogen ion concentration is inversely proportional to what?
- What are hydrogen ions a product of?
- CO2?
4, Most of the CO2 transported in the blood is what? - Converting CO2 into HCO3- frees a ______ ion
- The more CO2 there is the ______hydrogen ions are produced
- What is the equation for this reaction?
- pH
- cellular metabolism
- cellular metabolism
- HCO3-
- hydrogen
- more
- H2O + CO2 ↔ H2CO3 ↔ HCO3- + H+
CO2 is proportional how to pH?
inversely
The higher the CO2, the ________ the pH becomes
CO2 is a ____ ___ that is constantly being produced through ____ _______?
Increase the minute ventilation will do what to CO2?
lower (more acidic)
weak acid
tissue metabolism
Increasing the minute ventilation will decrease CO2
What are the three different kinds of buffers we talked about in the lecture?
Respiratory
Renal
Carbonic acid-bicarbonate buffer
How would you describe the repsponse of the respiratory buffer?
The blood pH will change according to the what? 2
This triggers an increase or decrease in the ____ and ____ of _______ until the appropriate amount of CO2 has been re-established
Activation of the lungs to compensate for an imbalance starts to occur within ______?
Fast!
level of carbonic acid and HCO3-
rate and depth of ventilation
1-3 min
What is the renal buffer?
What is it a buffer for?
Renal system maintains the balance of what? 2
Metabolic changes that result in changes in the pH take how long?
Bicarbonate (HCO3-)- base
Buffer for hydrogen ions
Renal system maintains the balance of HCO3- and H+
Metabolic changes that result in changes in the pH take several days
If there is an increase in the H+ concentration in the blood what happens?
If H+ concentrations in the blood drop below the desired level what happens?
What happens when CO2 levels increase?
Formation of carbonic acid (drives equation to the left)
Carbonic acid dissociates (drives the equation to the right)
Formation of more carbonic acid (drives the equation to the right)
The 4 major acid base derangements
Respiratory acidosis
Respiratory alkalosis
Metabolic acidosis
Metabolic alkalosis
What is respiratory acidosis defined as?
What causes it?
5
pH less than 7.35 with a PaCO2 > than than 45 mm Hg
- Central nervous system depression
- Impaired respiratory muscle function
- Pulmonary disorders
- Hypoventilation
- Trauma
For respiratory acidosis what things could depress the central nervous system?
2
What could impair repsiratory muscle function? 3
What pulmonary disorders could cause this? 6
What would the hypoventilation be due to?
5
- medications (narcotics, sedatives, or anesthesia)
- head injury
- spinal cord injury,
- neuromuscular diseases
- neuromuscular blocking drugs
- Atelectasis
- Pneumonia
- Pneumothorax
- Pulmonary edema
- Bronchial obstruction
- Massive pulmonary embolus
- Pain
- Chest wall injury/deformity
- Abdominal distension
- Obesity
- Trauma
What is respiratory alkalosis defined as?
What causes it?
5
pH >7.45 with a PaCO2 less than 35 mm Hg.
- Psychological responses
Anxiety or fear - Pain
- Increased metabolic demands
Fever, sepsis, pregnancy, or thyrotoxicosis - Medications, such as respiratory stimulants
- Central nervous system lesions
What are things that could increase metabolic demands?
4
Fever,
sepsis,
pregnancy, or
thyrotoxicosis
Defintion and Causes (6)of Metabolic Acidosis
bicarbonate level of less than 22 mEq/L with a pH less than 7.35
- Renal failure
- Diabetic ketoacidosis
- Diarrhea
- Anaerobic metabolism
from tissue hypoxia - Starvation
- Salicylate intoxication
Pearl: The presence of metabolic acidosis should spur a search what??
for hypoxic tissue somewhere in the body (need to save dying tissue)
Defintion and causes (8) of Metabolic Alkalosis
3 and 5
bicarbonate level > 28 mEq/L with a pH > 7.45
1. Either an excess of base or a loss of acid within the body Excess base occurs from ingestion of: 2. antacids 3. excess use of bicarbonate 4. use of lactate in dialysis Loss of acids can occur secondary to: 5. protracted vomiting 6. gastric suction 7. Hypochloremia 8. Excess administration of diuretics 9. High levels of aldosterone
What is hypoxemia?
What is hypoxia?
Insufficient oxygenation = hypoxemia
Low oxygen content in tissue = hypoxia
SaO2 (blood gas) or SpO2 (pulse oximeter) is defined as what?
What is the normal value?
Arterial oxygen saturation
% of hemoglobin that is bound with O2
Normal, depends on the patient, ideally should be ≥ 95%
What is PaO2 and what is the normal?
Arterial oxygen tension in the plasma
Measured by blood gas
In general less than 80mmHg abnormal
What is A-a gradient?
The difference between the oxygen tension in the alveoli (PAO2) and the arterial oxygen tension
The PaO2 from the blood gas can aid in assessment of what?
the function of the alveolar-capillary membrane
What does the different between alveolar oxygen partial pressure (PAO2) and arterial oxygen partial pressure (PaO2)
PAO2 – PaO2 measure?
Measures the integrity of the alveolar-capillary unit
- what is the Normal A-a gradient?
- How do we calculate it?
- Should it be higher at the alveolar side or capillary side?
- Normal values change with age (increase)
Usually the normal is less than 10 mmHg - (age/4) + 4 example: (40/4 + 4 = 14)
- alveolar side
So if the alveolar O2 pressure is 100 mmHg for this person the arterial O2 pressure should be at least (100-14) or 86 mmHg.
What is measured on the blood gas PaO2 or PAO2?
PaO2 is measured on the blood gas
PAO2 is calculated
What is the alveolar gas equation?
PAO2 = (FIO2 x [Patm – PH2O]) – (PaCO2 ÷ R)
Factors that go into calculation of the A-a gradient
What do the following stand for?
- FIO2
- Patm
- PH2O
- PaCO2
- R
- Fraction of inspired oxygen (.21 in room air)
- Atmospheric pressure (760 mmHg at sea level)
- Partial pressure of water at normal body temp (47 mmHg)
- Arterial carbon dioxide tension from blood gas
- respiratory quotient which is .8 at a steady state (reflection of cellular metabolism – O2 used and CO2 produced)
PAO2 = (.21 X [760-47]) – (40/.8)
150 – 50 = 100
A-a gradient in the workup of hypoxemia:
Hypoxemia with a normal A-a gradient is caused by what? 2
Hypoxemia with an increased A-a gradient is caused by what? 3
- Hypoventilation
- High altitude
- Diffusion defect
- ventilation-perfusion mismatch
- right-to-left shunt
What is the most common cause of hypoxemia?
V/Q mismatch
How is the Aa gradient changed with a V/Q ,mismatch?
increased
What are the most common causes of V/Q mismatch?
3 and 3
Ventilation:
COPD
Asthma
Pneumonia
Perfusion:
PE
Pulm HTN
Cardiac arrest
- What would cause a low V/Q?
- What would cause a high V/Q?
- How does this affect the Aa gradient?
- What is a normal value for V/Q?
- Shunt perfusion: Alveoli perfused but not ventilated. Its a problem with ventilation. Alveoli is not getting the air it needs. 0.5 for a value as an example
- Deadspace ventilation: Alveoli ventilated but not perfused. Its a problem with perfusion. Blood flow is not there to pick up O2. 1.2 for a value as an example.
- 0.8 is normal
- increases it
ABG analysis Step 1 (PaO2): Assess the PaO2 Is the PaO2 less than 80mmHg? No? Yes?
If no = adequate oxygenation and proceed to Step 2
If yes proceed to Step 1A to determine if there is an A-a gradient
Step 1A (Assess ventilation)
How do we know if the pt is hypoventilating?
Is the patient hypoventilating?
Yes?
No? and why?
If the CO2 is high = hypoventilation, proceed to Step 2
In this scenario assuming the hypoxemia is secondary to hypoventilation so no need to calculate A-a gradient
If the CO2 is normal = no hypoventilation, proceed to Step 1B to calculate A-a gradient
In this scenario need to investigate the etiology of hypoxemia to determine the degree of abnormality of the pulmonary capillary membrane (what is going on with diffusion?)
Step 1B (What is the A-a gradient?)
Yes?
NO?
If the A-a gradient is elevated then there is a problem on either side of the alveolar-capillary membrane
How do you calculate the Aa gradient?
What should be the normal Aa for a patient?
PAO2 = (FIO2 x (Patm –PH2O))-(PaCO2/R) PAO2 = [0.21 x (760-47)]-(PaCO2 from ABG/.8)
Calculated PAO2 = 150mmHg – PaCO2/.8
Then take calculated PAO2 – PaO2 (from ABG) = A-a gradient
Normal A-a less than (age/4) +4
ABG analysis step 2 (pH)
What is the pH?
Acidosis at what level?
Alkalosis at what level?
Acidosis pH less than 7.35
Alkalosis pH > 7.45
ABG analysis step 3 (CO2)
What is the PaCO2?
In primary respiratory disorders
pH and CO2 change in what direction?
In primary metabolic disorders
pH and CO2 change in what direction?
opposite
same
ABG analysis step 4 (compensation)
If it is a respiratory problem what is happening with the HCO3-?
If it is a metabolic problem what is happening with the CO2?
The HCO3- will increase to try and raise the pH (make it more alkaline)
The CO2 will go down as a result of trying to get the pH back to normal (less acidic)
ABG analysis Step 5 (Anion Gap)
Equation?
Range?
= Na+ – (Cl- + HCO3-)
6-12
Describe the thumbs up thembs down method.
pic on comp
A 60 year old male with a history of COPD presents to the ED with increasing SOB, fever, and cough productive of yellow-green sputum. Unable to speak in complete sentences. Symptoms times 2 days. Wheezing with crackles noted in the lower lobes. ABG results: pH 7.2, PaCO2 70 mmHg, HCO3- 27, PaO2 59.
How would you interpret this?
Respiratory acidosis
hypoxemia
(no metabolic compensation)
pH 7.2 PaO2 100 PaCO2 25 HCO3 10 6 year old boy with vomiting, decreased LOC. Breathing slow and deep (Kussmal). He is lethargic and irritable. Eyes sunken, mucous membranes dry, 2 week hx of polydipsia, polyuria and wt loss. Na 126, K 5, Cl 95
Interpretation of blood gas results?
What is the anion gap?
metabolic acidosis with respiratory compensation
elevated
pH 7.5 PaO2 98 PCO2 29 HCO3 24 12 year old girl with arm pain after a fall. VS HR 110, RR 32, BP 102/70. She was given pain meds and is waiting to see the PA. The nurse runs to get you and says she is hysterical and now complaining of numbness, tingling, and muscle cramps. Interpretation?
Interpretation? acute respiratory alkalosis
hypoventilating
PH 7.5 PaO2 85 PaCO2 45 HCO3 37 80 year old female with 2 day hx of vomiting. Lethargic, weak with myalgias. Mucous membranes dry.
Interpretation?
metabolic alkalosis
dehydration vomiting out the hydrochloric acid
pH 7.36 PO2 80 CO2 75 HCO3 40 interpretation?
How did it normalize?
Acidosis where pH is trying to normalize so this sounds chronic. Chronic respiratory acidosis
bicarbonate/metabolic compensation (bicarbonate high)
pH 7.36 PO2 90 CO2 30 HCO3 15 Interpretation?
chronic Metabolic acidosis
with respiratory compensation(cuz the CO2 is low)
75 year old female on 2 L of supplemental O2 pH 7.33 PO2 70 PCO2 70 Na+ 140 K+ 4.0 HCO3- 36 Cl- 94 Anion Gap = A-a gradient =
primary ventilation problem but for someone on supplemental oxygen their O2 should be higher. So we dont skip the Aa gradiant this time.
FIO2 at 2 L so its 25 to 28%. Use 28(760mmHg-47)- (70/0.8)
199.64- 87= 112 That is our PA.
PaO2= 70
112-70= 42 is our Aa gradient
75/4 + 4 = 23. Her Aa is high
This means she has a problem ay the alveolar capillary membrane
Acute on Chronic Respiratory Acidosis (this is chronic because compensation will not happen in acute cases)
Bicarb is high so she has metabolic compensation. O2 is low so we have hypoxemia
Anion Gap: Na-(HCO3 +Cl)
140-(36+94)= 10. normal
