base excess refers to a patient’s (metabolic/ respiratory) acid/base status
metabolic
What does base excess tell us clinically?
an ABG value that reveals if patient has too much or too little base in the blood
Normal base excess
-2 to 2 mmol/L
What is negative base excess value?
What does it mean?
How do you treat it?
below -2mmol/L
there is not enough base in the body; metabolic acidosis
treated with bicarb
What is a positive base excess value?
What does it mean?
How do you treat it?
above 2mmol/L
there is too much base in the body; metabolic alkalosis
treated by reversing the cause of alkalosis
A high concentration of H+ ions will = (high/low) pH?
low= acidotic (acid)
a low concentration of H+ ions will = (high/low) pH?
high= alkalosis (basic)
What is the normal pH range?
7.35-7.45 (7.4)
pH > 7.45 = (acidosis/alkalosis)?
alkalosis
pH < 7.35= (acidosis/alkalosis)?
acidosis
What is pH determined by?
HCO3-/ PaCO2 ratio
6 consequences of Acidosis:
- contractility?
- catecholamines?
- bleeding?
- vasculature?
- arrythmias?
- ion concentrations?
- decrease cardiac contractility
- decrease the response to catecholamines
- impair coagulation and increase bleeding
- increase PVR
- Makes Vfib more likely
- increases plasm K+
- H+ enters the cell and K+ exits
5 consequences of Alkalosis:
- hemoglobin?
- vasculature? (3)
- ion concentrations?
- Shifts the oxyhemoglobin dissociation curve to the left
- hemoglobin will bind oxygen more tightly - increase SVR
- Cerebral vasoconstriction
- because of the left shift of the oxyhemoglobin curve - Decreases PVR
- decreases plasma K+ concentration
- H+ exits the cell and K+ enters
What is a normal venous CO2 measurement?
24-30 mEq/L
*includes both the CO2 dissolved in plasma and the HCO3- dissolved in plasma
What is a normal arterial HCO3- measurement?
22-26 mEq/L
normal PaCO2
35-45 mmHg
normal PvCO2
40-50 mmHg
~5 mmHg higher than PaCO2
What does the PaCO2 and PvCO2 gradient tell you about your patient?
If the PaCO2 and PvCO2 gradient increases, the patient is poorly perfused
PaO2 of the atmosphere (at sea level)
160mmHg
-O2 makes up 21% of atmospheric pressure 760mmHG
normal PaO2
70-100 mmHg
-decreases with age
normal PvO2
30-40mmHg
normal CaO2
16-20 mL/ dL
normal CvO2
12-16 mL/dL
normal DO2 (total delivery of oxygen per minute)
1000 mL of oxygen delivered per minutes
*assumes normal hemoglobin (15g/dL), normal SaO2 of (97.5%), and normal cardiac output (5 L/min)
normal mvO2
60-80% (in awake patients)
90% with 100% FiO2
normal SaO2
93-98%
normal ScvO2
70-75%
normal A-a gradient in room air
5-15mmHg
normal A-a gradient in elderly patients
15-25mmHg
normal A-a gradient on FiO2 of 100%
10-110mmHg
normal minute ventilation
7-8 L/min
normal Va
alveolar ventilation 2/3 of minute ventilation of healthy patient
normal Vd (dead space ventilation)
1/3 of minute ventilation in a healthy patient
normal VCO2 (CO2 production)
200 mL/min
how much does VCO2 decrease in GA?
by 60%
VO2 (oxygen consumption)
250mL/min (for a normothermic 70kg adult)
VO2 (oxygen consumption) in infants
6-8mL/kg/min in infants
VO2 (oxygen consumption) in adults
3-4mL/kg/min in adults
%MetHb (methemoglobin) on an ABG sample
<2%
%COHb (carboxyhemoglobin) on an ABG sample
<3%
HCO3-/PaCO2 ratio
20: 1
- An abnormal HCO3-/PaCO2 ratio will always lead to an abnormal pH
PaO2/FiO2 ratio
480
Anion Gap
8-16mEq/L
Serum lactate concentration
<2mmol/L
Lactic acidosis value
Serum lactate > 5mmol/L
CaO2 equation
CaO2 = (SaO2)(Hb)(1.34) + (PaO2)(0.003)
*can calculate CvO2 from venous sample
What are the 2 primary determinants of CaO2?
SaO2 and Hb
-have the same effect on CaO2
What is the primary determinant of arterial oxygen content?
SaO2
Fick equation (for cardiac output)
Cardiac Output = “VO2” /”(CaO2−CvO2)(10)”
*measures cardiac output by oxygen consumption
DO2 equation
DO2 = (CaO2)(Cardiac Output)(10)
10 converts mL/d to mL/L
What does DO2 represent?
the amount of oxygen available for tissue perfusion
delivery of oxygen to tissues per minute
According to the DO2 equation, what is tissue perfusion most dependent on?
tissue perfusion (delivery of oxygen) is mostly dependent on the SaO2 and hemoglobin concentration (minimally determined by PaO2)
what is PiO2
pressure of inspired oxygen
3 things that determine PiO2
- FiO2
- if this increases, PiO2 increases - The pressure of air in the atmosphere (barometric/atmospheric pressure
- If atmospheric pressure INCREASES (elevation decreases), the pressure of oxygen we inspire INCREASES (and vice versa) - Water vapor pressure (WVP)
- As the water vapor pressure in the air INCREASES, the pressure of oxygen we inspire DECREASES (and vice versa)
PiO2 equation
PiO2 = (FiO2)(Barometric Pressure - Water Vapor Pressure)
at sea level:
barometric pressure= _____
Water vapor pressure =________
so, PiO2 equation is?
760 mmHg
47 mmHg
760-47=713
PiO2 = (FiO2)(713)
Alveolar Gas equation (PAO2)
PAO2 = PiO2 – (1.2)(PaCO2)
*if the FiO2 is greater than 60%, don’t need to use 1.2
At sea level, a patient has a nasal cannula placed at 4L/min. A recent arterial blood gas shows a PaCO2 of 40mmHg. What is the partial pressure of oxygen in this patient’s alveoli?
PAO2 = (0.35)(713) – (1.2)(40) = 201.55mmHg
What is PAO2 primarily determined by?
FiO2
What is the estimation for PAO2?
PAO2 = 102 - “(Age)” /”3”
Minute ventilation equation
TV X RR
equation for dead space ventilation (Vd/Vt)
Vd/Vt = (Vd)(RR)
The amount of dead space a patient has is proportional to what 2 values?
PaCO2 and EtCO2
alternate equation for dead space ventilation
Vd/Vt = “(PaCO2−EtCO2)” /”(PaCO2)”
equation for alveolar volume
Alveolar Volume = Vt– Vd
equation for alveolar ventilation
VA = (Vt– Vd)(RR)
PaCO2 equation
PaCO2 = “(VCO2)(0.863)” /”Va”
A patient has an alveolar ventilation of 4.5L/min and a CO2 production of 200mL/min. What is their estimated PaCO2?
PaCO2 = “(200)(0.863)” /”4.5” = 38.35L/min
What is the estimated alveolar ventilation of a patient if they have a CO2 production of 200mL/min and a PaCO2 of 50mmHg?
Va = “(200)(0.863)” /”50” = 3L/min
Carbonic anhydrase equation
CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-
henderson hasselbalch equation in how it relates to pH
“(HCO3−)” /”(PaCO2)”
What are 4 things that increase the A-a gradient?
- impaired gas exchange (COPD)
- age
- supplemental oxygen
- 100% O2, A-a gradient can vary from 10-110mmHg - Right to left intracardiac shunt
2 disadvantages of the A-a gradient
- anesthetist has to calculate PAO2
2. it can very greatly in patients who are breathing supplemental oxygen
What is an alternative to the A-a gradient in assessing the patient’s lung function
PaO2/ FiO2
What is a normal PaO2/ FiO2 ratio?
> 400
a normal PaO2 on room air in a healthy patient is roughly ______ times the amount of FiO2 on room air
5X
a shortened way of doing the PaO2 to FiO2 ratio is to just see if the PaO2 is close to 5x the patient’s FiO2
a PaO2/ FiO2 ratio of less than ____ indicates acute lung injury
300
a PaO2/ FiO2 ratio of less than _____ is diagnostic of ARDS
200
What are 2 advantages of using PaO2/ FiO2 over the A-a gradient?
- the anesthetist does’t have to calculate PAO2
2. The PaO2/FiO2 ratio doesn’t vary as much as the A-a gradient does with supplemental oxygen administration
Which of the following is least likely to lower a patient’s alveolar PO2?
Increase in altitude
Elevated PaCO2
A left to right intracardiac shunt
None of the above
left to right intracardiac shunt
What is the maximum value attainable by adding the values obtained for SaO2, %COHb, and %MetHb from a single blood sample?
100%
150%
200%
300%
100%
If the PaCO2 and FiO2 of a patient both increase by 50%, what is most likely to happen to their PAO2?
Increase
Decrease
Remain the same
Impossible to determine from the information provided
increase
If both barometric pressure and the PaCO2 of a patient fall by half, what is most likely to happen to their PAO2?
Increase
Decrease
Remain the same
Impossible to determine from the information
Decrease
At 10:00am, a patient has a PaO2 of 85mmHg, and SaO2 of 98%, and a Hb of 14g/dL. At 10:05am, she suffers a severe hemolytic reaction that suddenly leaves her with a Hb of 7g/dL. Assuming no lung disease occurs from the hemolytic reaction, what is most likely to occur with her PaO2, SaO2, and CaO2?
PaO2 unchanged, SaO2 unchanged, CaO2 unchanged
PaO2 unchanged, SaO2 unchanged, CaO2 reduced
PaO2 reduced, SaO2 unchanged, CaO2 reduced
PaO2 reduced, SaO2 reduced, CaO2 reduced
PaO2 unchanged, SaO2 unchanged, CaO2 reduced
Which of the following is least likely to change the PAO2 of a patient?
SaO2
PaCO2
Barometric pressure
FiO2
SaO2
A patient has the following ABG values: Hb = 8mg/dL PaO2 = 100mmHg SaO2 = 60% PaCO2 = 65mmHg Which of the following would have the greatest impact on increasing oxygen delivery to the tissues?
Increasing the PaO2 from 100mmHg to 500mmHg
Increasing the SaO2 from 60% to 90%
Hyperventilating a patient down from a PaCO2 of 65 to a PaCO2 of 30
Increasing the patient’s hemoglobin from 8mg/dL to 10mg/dL
Increasing the SaO2 from 60% to 90%
Which of the following situations would be most likely to lower the PaO2 of a patient?
Carbon monoxide poisoning
Abnormal hemoglobin that holds oxygen with twice the affinity of normal hemoglobin
Anemia
Lung disease with intrapulmonary shunting
Lung disease with intrapulmonary shunting
CaO2 is reduced in all of the following situations EXCEPT:
Severe anemia Carbon monoxide poisoning Severe V-Q imbalance High altitude None of the above
None of the above
What does the PaCO2 equation tell us?
That the higher the initial PaCO2, the greater it is affected by minute ventilation
-with each minute of apnea, CO2 will increase exponentially
If HCO3- and PaCO2 double from their normal baseline values, what is most likely to happen to the patient’s pH?
Stay the same
Increase
Decrease
Cannot determine from the information provided
stay the same
What does the Henderson-Hasselbalch equation predict will happen if a patient’s PaCO2 increases from 40-60mmHg?
That pH will fall
That bicarbonate will fall
That bicarbonate will rise
Cannot determine from the information provided
the pH will fall
A patient’s Va is 8.63L/min and their VCO2 is 300mL/min. How much will their PaCO2 rise if their Va drops 1L/min?
What is the initial PaCO2?
What is the PaCO2 after a decline a 1L/min drop in alveolar ventilation?
What is the total change in CO2 after a 1L/min drop in alveolar ventilation?
30 mmHg
33.9 mmHg
4 mmHg
A patient’s Va is 3.8L/min and their VCO2 is 300mL/min. How much will their PaCO2 rise if their Va decreases 1L/min?
What is the initial PaCO2?
What is the PaCO2 after a decline a 1L/min drop in alveolar ventilation?
What is the total change in CO2 after a 1L/min drop in alveolar ventilation?
68 mmHg
92 mmHg
24 mmHg
A patient has the following respiratory parameters: Tidal Volume = 500mL Respiratory Rate = 12 Alveolar Ventilation = 3L/min What is this patient’s dead space volume (the amount of dead space in one breath)? 150mL 200mL 250mL 300mL
250 mL
A patient has a respiratory rate of 24, a tidal volume of 300mL, a dead space volume of 150mL, and a CO2 production of 300mL/min. Based on this information, what is the patient’s PaCO2?
24.9
39.5
55.7
71.9
Impossible to determine from the information provided
71.9
The Bohr effect refers to _____ being displaced from hemoglobin as PaCO2 increases
oxygen
*right shift
The Haldane effect refers to ____ being displaced from hemoglobin as oxygen concentration increases
CO2
refers to impaired oxygen perfusion
hypoxia
refers to any decrease in blood oxygen content (CaO2)
hypoxemia
A patient is diagnosed with respiratory acidosis if they have?
- A low pH, and
2. A high PaCO2
Whenever a patient is in respiratory acidosis (high PaCO2, high H+ concentration, low pH), what does the body do to compensate? why is this a problem?
Kidney will try to reabsorb more bicarb
shift reaction to left
will decrease H+ (good)
but will INCREASE CO2 (already a problem)
What is the best way the anesthetist can treat respiratory acidosis?
increase the minute ventilation
-hyperventilate the patient
A patient is diagnosed with respiratory alkalosis if they have:
. A high pH, and
2. A low PaCO2
Whenever a patient is in respiratory alkalosis, what does the body do to compensate? why is this a problem?
kidney will excrete more HCO3-
shift the reaction to the right
will increase H+( good)
the excretion of HCO3- DECREASES CO2 (already a problem)
What is the best way for an anesthetist to treat respiratory alkalosis?
decrease the minute ventilation
-hypoventilation
What is the cause of metabolic acidosis?
decrease in HCO3-
What are 2 ways HCO3- can decrease( Metabolic acidosis)?
- actual loss of HCO3- (diarrhea)
2. Indirect loss of HCO3- from the increase in H+
H+ bind HCO3-
-lactic acidocis
-ketoacidosis
a patient is diagnosed with metabolic acidosis if they have:
- low pH
2. low HCO3-
Whenever a patient is in metabolic acidosis, they will “compensate” by?
Why is this a problem?
increasing their ventilation and lowering their PaCO2
drive reaction to the left and decrease H+ (good)
but hyperventilation causes HCO3- to decrease even more
the correct treatment for metabolic acidosis is?
to give HCO3-
What is the Sodium Bicarb dose?
Bicarb dose = (0.3)(kg)(base excess)
A 50kg patient has a base excess of -3. What is the dose of bicarb recommended to correct the base excess?
40mEq
45mEq
50mEq
55mEq
45 mEq
2 ways in which you can get Metabolic Alkalosis?
- direct loss of H+ from the body
vomiting, diuretics, gastric drainage, bowel obstruction
2.buildup of HCO3- in the body
massive blood transfusion, where the citrate preservative is converted to HCO3-
Therefore, a patient is diagnosed with metabolic alkalosis if they have
- A high pH and
2. A high HCO3-
Whenever a patient is in metabolic alkalosis they will “compensate” by? Why is this bad?
decreasing their ventilation and increasing PaCO2
shifts the reaction to the right and increases H+ (good)
but hypoventilation will increase HCO3- even more
What is the best treatment for metabolic alkalosis?
Reverse what is causing it
-Zofran or fluids if they have had lots of diarrhea or vomiting
Respiratory acidosis (↑ PaCO2) For each 10mmHg increase in PaCO2:
HCO3- should increase _____ (if acute)
HCO3- should increase ___ (if chronic)
1 mEq/L
4 mEq/L
Respiratory alkalosis (↓ PaCO2) For each 10mmHg decrease in PaCO2:
HCO3- should decrease _____ (if acute)
HCO3- should decrease ____(if chronic)
2 mEq/L
4 mEq/L
Metabolic acidosis (↓ HCO3-) PaCO2 should decrease \_\_\_ the HCO3- decrease
1.2X
Metabolic alkalosis (↑ HCO3-) PaCO2 should increase\_\_\_\_the HCO3- increase
0.7X
assume that a normal starting PaCO2 is 40mmHg and a normal starting HCO3- is 24mEq/L
A75-year-old woman is scheduled to undergo ORIF of the wrist. Medical history includes COPD and type II diabetes. She takes Flovent and albuterol twice per day and wears 2L/min oxygen at night. Preoperative blood gas analysis shows pH is 7.37 and PaCO2is 60 mmHg.Which of the following isthe expected bicarbonate level? 31mEq/L 30mEq/L 27mEq/L 24meq/L 21mEq/L
31 mEq/L
assume that a normal starting PaCO2 is 40mmHg and a normal starting HCO3- is 24mEq/L
A60-year-old man is scheduled to cystoscopy for stent placement. He is currently septic. Preoperative blood gas analysis reveals the following: pH = 7.35, HCO3- = 16mEq/L, and lactate = 10mmol/L. Assuming maximum respiratory compensation, which of the following isthe expected PaCO2 level? 50mmHg 45mmHg 35mEq/L 30mEq/L 24meq/L 21mEq/L
30 mEq/L
What is elevated lactate indicative of?
tissue hypoxia/ hypoperfusion
What is type A lactic acidosis?
inadequate oxygen delivery to tissues
What is type B lactic acidosis?
adequate oxygen delivery, but the tissues cannot use the oxygen normally
4 causes of lactic acidosis
- sepsis
- shock/ inadequate cardiac output/perfusion
- hepatic failure
- exercise
2 treatments for lactic acidosis/sepsis?
- restore normal pH
2. . Improve tissue oxygenation (perfusion) with fluids and/or vasopressors
Anion gap equation
AG = unmeasured anions – unmeasured cations
Anion Gap = measured cations – measured anions
Anion Gap = (Na+) – (HCO3- + Cl-)
Anion Gap = (≈140mEq/L) – (≈102mEq/L + ≈28mEq/L) = ≈10mEq/L
What is a normal anion gap (w/o K+)
12 ± 4mEq/L
What is an high (elevated, widened) anion gap?
when there has been an increase in the number of UNMEASURED anions
These “unmeasured” anions dissociate from H+ and are considered acids because they increase the H+ ion concentration
(lactic acidic/lactic acidosis, ketoacids/ketoacidosis, etc)
2 reasons the anion gap will increase
- when there is an increase in the number of unmeasured anions
- when there has been an decrease in the number of measured anions (HCO3-)
A normal anion gap is caused by a direct loss of ____
HCO3-
-This can happen with a GI loss of HCO3− (diarrhea) or with renal dysfunction (and subsequent excretion of HCO3-)
Why doesn’t the anion gap increase when there is a direct loss of HCO3- (measured anion)?
whenever there is a direct loss of HCO3-, it is typically replaced by a chloride ion
Acidosis with an increase in the chloride concentration is referred to as ____ and is typically seen with a (normal/elevated) anion gap
“hyperchloremic metabolic acidosis”
normal anion gap
What is the most common cause of hyperchloremic, normal anion gap metabolic acidosis?
excess 0.9% N/S administration
This is apparently due to the fact that sodium normally reabsorbs with HCO3-, but with excess saline administration, sodium reabsorbs more with Cl-, which causes HCO3- excretion (hence the acidosis)
Since we’re gaining a measured anion (Cl-) and losing a measured anion (HCO3-), the anion gap remains unchanged in this type of acidosis
What is a low anion gap frequently caused by?
hypoalbuminemia ( liver failure)
-negatively charged protein
Loss of albumin from the plasma results in the retention of other negatively charged ions such as chloride and bicarbonate
Since the patient is losing unmeasured anions (Albumin) and increasing measured anions (HCO3-, Cl-), the anion gap decreases
HCO3- = 36 mEq/L
PaCO2 = 40mmHg
Compensated or uncompensated?
PaCO2 = 50mmHg
Compensated or uncompensated?
PaCO2 = 30mmHg
Compensated or uncompensated?
uncompensated
compensated
uncompensated
PaCO2 = 25mmHg
HCO3- = 20mmHg
Compensated or uncompensated?
HCO3- = 24mmHg
Compensated or uncompensated?
HCO3- = 32mmHg
Compensated or uncompensated?
compensated
uncompensated
uncompensated
Patient
pH = 7.25
PaCO2 = 36
HCO3- = 14
Acid Base status
Uncompensated Metabolic Acidosis
Patient
pH = 7.29
PaCO2 = 60
HCO3- = 29
Acid Base Status
Compensated Respiratory Acidosis
Patient
pH = 7.26
PaCO2 = 28
HCO3- = 16
Acid Base Status
compensated metabolic acidosis
Patient
pH = 7.53
PaCO2 = 42
HCO3- = 34
acid base status
uncompensated metabolic alkalosis
Patient
pH = 7.37
PaCO2 = 37
HCO3- = 24
acid base status
normal acid base status
Patient
pH = 7.25
PaCO2 = 24mmHg
HCO3- = 14
compensated metabolic acidosis
Which type of diabetics have diabetic ketoacidosis?
What is diabetic ketoacidosis?
Type 1
lack of insulin
- The lack of insulin causes the cells of the body to become starved (despite high blood sugar)
- The body responds to this starvation by breaking down lipids in an effort to create sugar
- fat breakdown = acidic ketone bodies (metabolic acidosis)
6 symptoms of diabetic ketoacidosis
- Hyperglycemia
- lack of insulin - acidosis
- protein/fat breakdown and production of ketones - Hypovolemia
- Polyuria from increased oncotic pressure (from sugar) in renal tubules - Potassium disturbances
- hyperkalemic (lack of insulin and acidosis)
(some may be hypokalemic from polyuria =rare)
- Sodium disturbances
- Hyponatremia (polyuria induced Na+ loss)
(hypernatremia from diuresis & water loss =rare)
- hyperosmolarity (dehydration and hyperglycemia)
- Symptoms include cell shrinkage, cerebral edema, altered consciousness, increased blood viscosity, and possible thrombosis
What is the treatment for diabetic ketoacidosis?
What should their K+ level be to do this?
administer insulin
-feed cells and reverse ketone production
only if K+ is >3.3 mEq/L
Once the glucose gets down to______, add glucose to the insulin infusion to keep the glucose above that level until the acidosis is corrected
250-300 mg/dL
-prevents too rapid of correction of hyperglycemia which can lead to cerebral edema
What amount should be given for a glucose/insulin infusion?
give ≈5 units of insulin per amp of D50 (2.5-5g glucose per unit of insulin)
In diabetic ketoacidocis, how should you treat the acidosis? (normalize the pH)
administer bicarb as needed
In diabetic ketoacidosis, how should you treat the hypovolemia?
What is the average fluid loss in DKA?
What type of resuscitation fluids are preferable?
goal is to replace the total volume loss within 24–36 hours with 50% of resuscitation fluid being administered during the first 8–12 hours
Fluid loss averages approximately 6–9 L in DKA
Normal saline
-higher incidence of hyponatremia
In DKA treatment, how do you prevent insulin induced hypokalemia?
Administer a potassium drip as necessary
If the patient is initially hypokalemic due to excessive diuresis, correct the patient’s potassium level prior to starting insulin therapy
What is the effect of air on a PaCO2 sample?
Air will cause the blood sample to have a falsely lowered PaCO2, which will cause a falsely elevated pH
What is the effect on air in a PaO2 sample on room air?
Air will cause the blood sample to have a falsely elevated PaO2
What is the effect on air in a PaO2 sample on 100% FiO2?
Air will cause the blood sample to have a falsely lowered PaO2
What is the effect on cold temperature of an ABG?
Cold blood samples will be warmed and thus have a falsely elevated PaO2/PaCO2
What is the effect on warm temperature of an ABG?
Warm blood samples will be cooled and thus have a falsely reduced PaO2/PaCO2
What are 2 things you should tell the lab before sending them an ABG sample?
The temperature and the patients FiO2
