Acid/Base Flashcards
1
Q
5 main things measured in ABG
A
- pH
- HCO3-
- Base excess
- PaCO2
- PaO2
2
Q
Additional potential labs in an ABG
A
- Hb/Hct
- K+
- Glucose
- Ca++
- COHb
- MetHb
3
Q
What is the technical term definition for base excess?
A
The amount of acid or base needed (at 100% SaO2 and 37 C) to return
- Blood pH to 7.4
- PaCO2 to 40 mmHg
4
Q
Clinical definition of base excess
A
- ABG value that reveals if the pt has too much or not enough base in blood
- Refers to metabolic acid base status
5
Q
Refers to pt’s respiratory acid/base status
A
PaCO2
6
Q
Normal base excess
A
-2 to 2 mmol/L
7
Q
Negative base excess
A
> -2 mmol/L
- deficit of base in body
- metabolic acidosis
- treated with bicarb
8
Q
Positive base excess
A
> 2 mmol/L
- metabolic alkalosis
- treated by reversing cause of alkalosis
9
Q
High H+ concentration causes a (high/low) pH
A
Low pH (acidic)
10
Q
Low H+ concentration causes a (high/low) pH
A
High pH (alkalotic)
11
Q
Normal pH range
A
7.35-7.45
12
Q
What determines pH?
A
The HCO3- to PaCO2 ratio
13
Q
Consequences of acidosis
A
- Decreases cardiac contractility
- Decreases response to catecholamines
- Impairs coagulation and increases bleeding
- Increases PVR
- Lowers the vfib threshold (makes vfib more likely)
- Increases plasma K+ concentration (K+ exits cells)
14
Q
Consequences of alkalosis
A
- Shifts oxyhemoglobin dissociation curve to the left
- Increases SVR
- Cerebral vasoconstriction
- Decreases PVR
- Decreases plasma K+ concentration (K+ enters cells)
15
Q
Normal venous CO2
A
24-30 mEq/L
16
Q
Normal arterial HCO3-
A
22-26 mEq/L
17
Q
Normal PaCO2
A
35-45 mmHg
18
Q
Normal PvCO2
A
40-50 mmHg
19
Q
The PvCO2 to PaCO2 gradient increases if ____
A
the patient is poorly perfused
20
Q
Normal PaO2 of the atmosphere at sea level
A
160 mmHg
21
Q
Normal PaO2 in arterial blood
A
70-100 mmHg
22
Q
PaO2 in arterial blood decreases ____
A
with age
23
Q
normal PvO2
A
30-40 mmHg
24
Q
Normal CaO2
A
16-20 mL/dL
25
Normal CvO2
12-16 mL/dL
26
Normal DO2
1000 mL of oxygen delivered per minute
27
Normal DO2 assumes what?
Normal Hgb, normal SaO2, normal CO
28
Normal mvO2
- 60-80% in awake patients
| - Up to 90% on 100% FiO2
29
Normal SaO2
93-98%
30
Normal ScvO2
70-75%
31
Normal A-a gradient on room air
5-15 mmHg
32
Normal A-a gradient in elderly patients
15-25 mmHg
33
Normal A-a gradient on 100% FiO2
10-110 mmHg
34
Normal minute ventilation
7-8 L/min
35
Normal Va (alveolar ventilation)
2/3 of minute ventilation in a healthy patient
36
Normal dead space ventilation (Vd)
1/3 of minute ventilation in a healthy patient
37
Normal VCO2
200 mL/min
38
How much is VCO2 decreased under GA?
By up to 60%
39
Normal VO2 (oxygen consumption)
- 250 mL/min for a normothermic, 70kg adult
- 6-8 mL/kg/min in infants
- 3-4 mL/kg/min in adults
40
Normal %MetHb on ABG
<2%
41
Normal %COHb on ABG
<3%
42
Normal HCO3-/PaCO2 ratio
20:1
43
Normal PaO2/FiO2 ratio
480
44
Normal anion gap
8-16 mEq/L
45
Normal serum lactate concentration
<2 mmol/L
46
Lactic acidosis value
serum lactate >5 mmol/L
47
Total arterial oxygen content equation
CaO2= (SaO2)(Hb)(1.34) + (PaO2)(0.003)
48
Purpose of CaO2 equation
1. PaO2 only makes up a small portion of total oxygen content in the arteries (1.5%)
2. SaO2 and Hb are the primary determinants of CaO2
3. SaO2 and Hb concentration have the same effect on CaO2
49
Percentage of O2 in the body bound to hemoglobin (SaO2)
98.5%
50
Percentage of O2 in the body dissolved in plasma (PaO2)
1.5%
51
What changes does CO poisoning have on SaO2 and PaO2
Lowers SaO2
Lowers CaO2
No change on PaO2
52
Equation for CvO2
CvO2= (SvO2)(Hb)(1.34) + (PvO2)(0.003)
53
Normal SvO2
Same as ScvO2, 70-75%
54
How is ScvO2 measured?
Venous sample off a central line
55
What is the Fick equation?
VO2 mL/min = (CO)(CaO2-CvO2)(10)
56
How do you calculate CO with the Fick equation?
CO L/min = VO2/ [(CaO2-CvO2)(10)]
57
What are the units for cardiac output with the Fick equation?
L/min
58
What are the units for VO2 with the Fick equation?
mL/min
59
Delivery of oxygen equation
DO2= (CaO2)(cardiac output)(10)
60
What does DO2 represent?
The amount of oxygen available for tissue perfusion
61
The tissue perfusion/delivery of oxygen is mostly dependent on
SaO2 and Hb concentration
62
What 3 things determines the pressure of inspired oxygen (PiO2)?
1. FiO2
2. Pressure of air in atmosphere
3. Water vapor pressure
63
PiO2 equation
PiO2 = (FiO2)(barometric pressure-water vapor pressure)
PiO2= (FiO2)(713)
64
Alveolar gas equation (PAO2)
PAO2 = PiO2 - (1.2)(PaCO2)
or
PAO2 = (FiO2)(713)-(1.2)(PaCO2) for FiO2 <60%
PAO2 = (FiO2)(713)-(PaCO2) for FiO2 >60%
65
For each L/min on NC, the FiO2 ___
Increases 4%
66
Estimation for PAO2
PAO2 = 102 - (Age/3)
67
Volume of air that is expired in one breath, including dead space volume
Tidal volume
68
Total volume of air that we breathe in one minute, including dead space
Minute ventilation
69
Minute ventilation equation
=RR(Vt)
70
Normal dead space
1/3 of pt's tidal volume
71
Dead space in a pt with pulmonary disease
more than 1/3 of tidal volume
72
Equation for dead space ventilation
=Vd(RR) or
| =(PaCO2-EtCO2)/PaCO2
73
The amount of dead space a patient has is proportional to
the difference in PaCO2 and EtCO2
74
Amount of air in one breath that actually reaches the alveoli and participates in gas exchange
Alveolar volume
75
Alveolar volume equation
Vt-Vd
76
The amount of air in one minute that actually reaches the alveoli and participates in gas exchange
Alveolar ventilation
77
Equation for alveolar ventilation
VA= (Vt-Vd)(RR)
78
PaCO2 equation
=(VCO2*0.863)/VA
79
With the PaCO2 equation, if alveolar ventilation is low, PaCO2 will be...
High
80
With the PaCO2 equation, if CO2 production is high, PaCO2 will be....
High
81
What are the units for VCO2?
mL/min
82
What are the units for VA?
L/min
83
What is the main determinant of pH?
The ratio of HCO3- to PaCO2
84
If you increase the amount of HCO3- or decrease the amount of PaCO2, pH will...
Increase
85
If you decrease the amount of HCO3- or increase the amount of PaCO2, the pH will...
Decrease
86
How is it possible for pH to be normal even if HCO3- and PaCO2 are abnormal?
If they are abnormal by the same percentage
87
pH decreases ~0.1 unit per 10 mmHg (increase/decrease) in PaCO2
Increase
88
pH increases ~0.1 unit per 10 mmHg (increase/decrease) in PaCO2
Decrease
89
Normal A-a gradient
5-15 mmHg
90
A larger difference in PAO2 and PaO2 indicates ___
lung disease
91
The A-a gradient will increase with:
1. Impaired gas exchange (COPD)
2. Age
3. Supplemental oxygen (FiO2 100% A-a gradient can be 10-110mmHg)
4. R to L intracardiac shunting
92
Disadvantages to the A-a gradient
1. Anesthetist has to calculate PAO2
| 2. Can vary greatly in patients who are breathing supplemental oxygen
93
Normal PaO2/FiO2 ratio
>400
or
if the PaO2 is close to 5x the patient's FiO2
94
What does a lower value from the PaO2/FiO2 ratio suggest?
A higher degree of lung disease
95
A PaO2/FiO2 ratio of <300 indicates ___
Acute lung injury
96
A PaO2/FiO2 ratio of <200 indicates ___
ARDS
97
Advantages to the PaO2/FiO2 ratio
1. Don'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
98
What is the maximum value attainable by adding the values obtained for SaO2, %COHb and %MetHb from a single blood sample?
100%
99
If the PaCO2 and FiO2 of a patient both increase by 50% what is most likely to happen to their PAO2?
It will increase
100
If both barometric pressure and the PaCO2 of a patient fall by half, what is most likely to happen to their PAO2?
Decrease
101
If a patient's Hb decreases by half, what will happen with her PaO2, SaO2 and CaO2?
PaO2 and SaO2 are unchanged, CaO2 is reduced
102
What is least likely to change the PAO2 of a patient?
SaO2
103
Conversion of CO2 and HCO3- is made possible by what enzyme?
Carbonic anhydrase enzyme
104
What are the 3 forms of CO2 in the body?
1. PaCO2 (5-10%)
2. HCO3- dissolved in plasma (60-65%)
3. HCO3- that is attached to hemoglobin (30%)
105
Normal HCO3- to PaCO2 ratio
20:1
106
Other names for venous labs
BMP
Venous chem 7
Electrolyte panel
107
Lab values in the venous labs
1. Na+ ~135
2. Cl- ~102
3. BUN
4. K+ ~4
5. HCO3- ~24-30
6. Cr
7. Glucose ~70-100
108
Why is the venous CO2 listed as HCO3- or CO2?
It includes both forms of CO2
| -HCO3- dissolved in the veins and CO2 dissolved in the veins
109
Normal value for venous CO2/HCO3-
24-30 mEq/L
110
Normal arterial HCO3- value
22-26 mEq/L
111
Is calculated and refers to the concentration of CO2 in arterial blood
HCO3-
112
Is measured and refers to the partial pressure of CO2 in the arterial blood
CO2 (PaCO2)
113
If HCO3- and PaCO2 double from their normal baseline values, what is most likely to happen to the patient's pH?
Stays the same
114
What does the Henderson-Hasselbalch equation predict will happen if a patient's PaCO2 increases from 40-60 mmHg?
the pH will fall
115
If the initial PaCO2 is 30 mmHg and the alveolar ventilation drops by 1L/min, the CO2 only changes by
4 mmHg
116
If the initial PaCO2 is 68 mmHg and the alveolar ventilation drops by 1L/min, the CO2 changes by
24 mmHg
117
The (higher/lower) the initial PaCO2, the less it changes for a given change in minute ventilation
Lower
118
Refers to oxygen being displaced from hemoglobin as PaCO2 increases
Bohr effect
119
Refers to CO2 being displaced from hemoglobin as oxygen concentration increases
Haldane effect
120
Dexoygenated hemoglobin is (more/less) affinitive for CO2
More
121
Oxygenated hemoglobin is (more/less) affinitive for CO2
Less
122
Degree of pulmonary shunt in all humans
up to 3%
123
Refers to any decrease in blood oxygen content
Hypoxemia
124
Why does pH decrease in respiratory acidosis?
1. The number of H+ ions increases
| 2. PaCO2 increases by a greater percentage
125
Diagnosis of respiratory acidosis
1. Low pH
| 2. High PaCO2
126
Body's compensation for respiratory acidosis
Kidneys reabsorb HCO3-
| but then PaCO2 increases even more
127
Treatment for respiratory acidosis
Increase minute ventilation and lower PaCO2
128
Diagnosis of respiratory alkalosis
1. High pH
| 2. Low PaCO2
129
Body's compensation for respiratory alkalosis
Kidneys excrete HCO3-
| but then PaCO2 decreases more
130
Treatment for respiratory alkalosis
Lower minute ventilation and raise PaCO2
131
Causes of metabolic acidosis
1. Direct, actual, physical loss of HCO3- (diarrhea)
| 2. Increase in acid and lowers HCO3- indirectly (lactic acidosis, diabetic ketoacidosis)
132
Diagnosis of metabolic acidosis
1. Low pH
| 2. Low HCO3-
133
Body's compensation for metabolic acidosis
Increasing ventilation and lowering PaCO2
| but lowers HCO3- even more
134
Treatment for metabolic acidosis
Give bicarb
135
Sodium bicarb dose
(0.3)(kg)(base excess)
136
Causes of metabolic alkalosis
1. Direct loss of acid (H+) from the body (vomiting, diuretics, gastric drainage, bowel obstruction)
2. Build up of HCO3- in the body (massive blood transfusion)
137
Diagnosis of metabolic alkalosis
1. High pH
| 2. High HCO3-
138
Body's compensation for metabolic alkalosis
Decreasing ventilation and increasing PaCO2
| but increases HCO3- more
139
Treatment for metabolic alkalosis
Reverse what is causing it
140
For each 10 mmHg increase in PaCO2, HCO3- will increase by ____ if acute
1 mEq/L
141
For each 10 mmHg increase in PaCO2, HCO3- will ______ if chronic
Increase by 4 mEq/L
142
For each 10 mmHg decrease in PaCO2, HCO3- should ____ if acute
decrease 2mEq/L
143
For each 10 mmHg decrease in PaCO2, HCO3- should ____ if chronic
Decrease 4 mEq/L
144
During metabolic acidosis, PaCO2 should decrease ___ the HCO3- decrease
1.2x
145
During metabolic alkalosis, PaCO2 should increase ____ the HCO3- increase
0.7x
146
Occurs when cells receive too little oxygen (hypoxia)
Lactic acidosis
147
Inadequate oxygen delivery to the tissues
Type A
148
Adequate oxygen delivery, but the tissues cannot use the oxygen normally
Type B
149
Causes of lactic acidosis
1. Sepsis
2. Shock/inadequate cardiac output/perfusion
3. Hepatic failure
4. Exercise
150
Treatment of lactic acidosis
1. Restore normal pH
| 2. Improve tissue oxygenation (perfusion) with fluids and/or vasopressors
151
Cations in the body
1. Na+
2. K+
3. Ca++
4. Mg++
5. H+
152
Anions in the body
1. HCO3-
2. Cl-
3. Lactate
4. Proteins
5. Phosphates
153
Measured cations
Na+, K+
154
Measured anions
Cl-, HCO3-
155
Unmeasured cations
Mg++, Ca++, H+
156
Unmeasured anions
Lactic acid, phosphates, sulfates, protein
157
The difference in the number of measured cations and measured anions
Anion gap
158
Anion gap equations
Measured cations - measured anions
or
unmeasured anions - unmeasured cations
159
Normal anion gap (without K+)
12 +/- 4 mEq/L
160
Causes anion gap to increase
1. Inc in measured cations
2. Dec in measured anions
3. Unmeasured anions inc
4. Unmeasured cations dec
161
Causes anion gap to decrease
1. Measured cations dec
2. Measured anions inc
3. Unmeasured anions dec
4. Unmeasured cations inc
162
When is an elevated anion gap most commonly observed?
When there is an increase in unmeasured anions (lactate)
163
Why does the anion gap increase if both unmeasured anion and unmeasured cation increase?
H+ doesn't technically increase because it binds HCO3-
1. An increase in the number of unmeasured anions
2. A decrease in the number of measured anions
164
What is a normal anion gap caused by?
A direct loss of HCO3-
| -Can happen with GI loss or renal dysfunction
165
Why does a normal anion gap not increase with the loss of HCO3-?
It is typically replaced by a chloride ion
166
What anion gap is seen with hyperchloremic metabolic acidosis
A normal anion gap
167
Causes of a low anion gap
- Hypoalbuminemia (w/liver failure)
| - Results in retention of chloride and bicarb
168
What is the acid/base status of a patient with
pH = 7.25
PaCO2 = 36
HCO3- = 14
Uncompensated metabolic acidosis
169
What is the acid/base status of a patient with
pH= 7.29
PaCO2=60
HCO3-=29
Compensated respiratory acidosis
170
What is the acid/base status of a patient with
pH= 7.36
PaCO2=28
HCO3- = 16
Compensated Metabolic Acidosis
171
What is the acid/base status of a patient with
pH=7.53
PaCO2= 42
HCO3- = 34
Uncompensated Metabolic Alkalosis
172
What is the acid/base status of a patient with
pH= 7.37
PaCO2 = 37
HCO3- = 24
Normal
173
What is acid/base status of a patient with
pH= 7.25
PaCO2 = 24
HCO3- = 14
Compensated Metabolic Acidosis
174
How does DKA happen?
1. Type 1 diabetics have a lack of insulin
2. Cells of the body become starved
3. Body breaks down lipids in an effort to create sugar
4. Fat breakdown produces acidic ketone bodies
5. Sugar in the blood spills into renal tubules causing profound osmotic diuresis
6. Diuresis can lead to hypovolemia and electrolyte abnormalities in addition to acidosis
175
DKA symptoms
1. Hyperglycemia
2. Acidosis
3. Hypovolemia
4. Potassium disturbances
5. Sodium disturbances
6. Hyperosmolarity
176
DKA treatment
1. Administer insulin to feed the cells and reverse ketone production if K+ >3.3 mEq/L
2. Fix the acidosis by administering bicarb
3. Treat hypovolemia
4. Prevent insulin induced hypokalemia
177
When should glucose be added to the insulin infusion during DKA treatment, and why?
Once glucose gets to 250-300 mg/dL, add glucose to keep the level above that until the acidosis is corrected
178
Dosing for the glucose insulin infusion
5 units of insulin per amp of D50 (2.5-5g glucose per unit of insulin)
179
Average fluid loss during DKA
6-9 L
180
Fluid resuscitation goal during DKA
Within 24-36 hours with 50% of resuscitation fluid being administered during the first 8-12 hours with NS
181
How do you prevent insulin induced hypokalemia?
- Administer a potassium drip as necessary
| - Correct K+ prior to starting insulin
182
Significance of DKA with children
- Children may be dehydrated but have high serum osmolality caused by hyperglycemia
- Rapid administration of crystalloid solution and reduction in serum osmolality may contribute to risk of cerebral edema
183
Fluid administration of children w/DKA
- Initial bolus of isotonic crystalloid 10-20 mL/kg over 1-2 hours
- If pt is in hypotensive shock, default to more aggressive bolus fluids for shock
- Isotonic saline IV then 0.45% NaCl based on corrected serum sodium concentration to help move water into intracellular compartment
184
What would happen with CO2 if air got into your ABG?
CO2 would diffuse out of the blood and into the air, causing a falsely lowered PaCO2 reading
185
What would happen with O2 if air got into your ABG for a patient on room air?
Oxygen would diffuse out of the air and into the blood, causing a falsely elevated PaO2 reading
186
What would happen with PaO2 if air got into your ABG for a patient on 100% oxygen?
Oxygen would diffuse out of the blood and into the air, causing a falsely lowered PaO2 reading
187
How does the ABG get corrected if air gets into the blood sample?
Tell the lab the pt's FiO2
188
What is the effect of warming blood on ABG readings?
PaO2 and PaCO2 will increase, causing falsely elevated readings
189
What is the effect of cooling blood on ABG readings?
PaO2 and PaCO2 will decrease, causing falsely reduced readings
190
How do you correct for warming/cooling ABGs?
Tell the lab the pts temeprature when sending the sample
