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Acid-Base Lagniappe
(The first three should seem familiar and serve as a warm-up)
Assume all ABGs are on room air.
1.
A 53 year-old woman complaining of palpitations and flushing followed by lightheadedness and tingling
hands.
ABG: 7.56/20/65
142 | 110 | 15 / 105
4.5 | 20 | 0.9 \
Albumin: 4.0
anion gap = 142 – (110 + 20)
= 12 (normal)
What is her acid-base disturbance?
Acute respiratory alkalosis. The HCO3 has decreased by 4 which is appropriate and suggests no
further disturbances. This history fits best with a panic attack.
2.
A 23 year-old man brought in by EMS after being found down.
ABG: 7.28/55/80
Albumin: 3.8
138 | 100 | 24 / 90
3.2 | 26 | 1.3\
anion gap = 138 – (100 + 26)
= 12
delta gap = 12 – 12 (about the normal gap for albumin of 3.8)
=0
corrected HCO3 = 26 + 0
= 26
What is the acid-base disturbance?
Acute respiratory acidosis. The HCO3 is appropriately increased from 24 to 26 (25.5 rounded
up).
3.
An 18 year-old non-compliant insulin dependent diabetic brought in with mental status changes.
ABG: 7.28/28/80
124 | 80 | 36 / 586
5.1 | 14 | 1.3\
Albumin: 4.0
Serum Osms: 295
Urine Cl: 4
anion gap = 124 – (80 + 14)
= 30
delta gap = 30 – 12 (normal gap for albumin of 4.0)
= 18
corrected HCO3 = 14 + 18
= 32
Winter’s Formula = 1.5 (14) + 8
= 29 (expected pCO2)
What is the acid-base disturbance? Are there additional disturbances?
4.
Acute anion gap metabolic acidosis. There is a secondary metabolic alkalosis based on the
corrected HCO3 of 32. Winter’s formula shows adequate respiratory compensation as the
expected pCO2 is 29 and the actual pCO2 is 28, within our range of  2. This patient is in DKA.
A 75 year-old woman with five days of profound diarrhea.
ABG: 7.30/30/83
146 | 119 | 46 / 101
3.2 | 18 | 2.0\
Alb: 3.0
anion gap = 146 – (119+18)
= 9 (normal for albumin of 3.0)
Winter’s Formula = 1.5 (18) + 8
= 35 (expected pCO2)
What is the disturbance? Is there appropriate compensation?
Acute non-anion gap acidosis. There is a secondary mild respiratory alkalosis as our expected
pCO2 of 35 is greater than the actual pCO2 of 30. The diarrhea explains the non-gap acidosis.
5.
An 18 year-old man with asthma.
ABG: 7.52/25/70
136 | 102 | 12 / 126
3.9 | 22 | 0.9
Alb: 4.0
anion gap = 136 – (102 + 22)
= 12 (normal)
What is the acid-base disturbance?
There is a typo. The pCO2 should be 25, not 55. Acute respiratory alkalosis. There is not
adequate compensation as we would expect the HCO3 to decrease by 3 and it has only decreased
by 2. This suggests an additional metabolic alkalosis. As we learn acid-base it is good to use
absolute values such as a normal pH of 7.4 or a normal HCO3 of 24, but there are ranges for all
normal values. In this case, there probably is not a secondary metabolic alkalosis because this
man could have easily had a starting HCO3 of 25, which is within the normal range of 22-28.
6.
A 60 year-old woman with COPD presents complaining of cough and fever.
ABG: 7.32/60/65
140 | 100 | 14 / 101
4.3 | 30 | 0.7
Alb: 3.2
anion gap = 140 – (100 +30)
= 10 (normal for albumin of 3.2)
What is the disturbance? Compensation?
Chronic respiratory acidosis. There is adequate compensation by the kidneys. There are no
further acid-base disturbances.
7.
An 18 year-old high school student presents with nausea, vomiting, abdominal pain, mental status changes
and difficulty seeing. He says it feels like he’s in a “snowstorm.”
ABG: 7.20/21/78
141 | 103 | 22 / 103
5.0 | 9 | 1.2\
Alb: 4.0
Serum Osms: 328
anion gap = 141 – (103 +9)
= 29
osmolal gap = 328 – (2 x 141 + 22/2.8 + 103/18)
= 328 – (282 + 7.9 + 5.7)
= 32.4
delta gap = 29 – 12
= 17
Winter’s Formula = 1.5 (9) + 8
= 22 (expected pCO2)
corrected HCO3 = 9 + 17
= 26
What is the disturbance? Are there additional Acid-Base disturbances? What did he ingest? What is the
treatment?
Acute anion gap metabolic acidosis. There is a significant osmolal gap. The most likely cause by
history is methanol. The treatment is IV ethanol. The corrected HCO3 is within the normal range
and Winter’s Formula tells us that there are no respiratory disturbances.
8.
A 78 year-old man with coronary artery disease presents with mental status changes.
ABG: 7.34/26/71
135 | 102 | 19 / 154
4.3 | 19 | 1.1
Alb: 3.0
Serum Osms: 290
anion gap = 135 – (102 + 19)
= 14
osmolal gap = 290 – (2 x 135 + 19/2.8 + 154/18)
= 290 – (270 + 6.8 + 8.6)
=5
delta gap = 14 – 9 (normal anion gap for albumin of 3)
=5
corrected HCO3 = 19 + 5
= 24
Winter’s Formula = 1.5 (19) + 8
= 36.5 (expected pCO2)
What is the acid-base disturbance? What bottle of over-the-counter pills did he ingest?
Acute anion gap acidosis. There is a secondary respiratory alkalosis. This is classic for salicylate
overdose.
9.
A 42 year-old man with refractory hypertension presents with fatigue.
ABG: 7.56/45/70/32
146 | 102 | 15 / 252
2.8 | 32 | 0.9\
Alb: 3.9
Urine Cl: 30
anion gap = 146 – (102 + 32)
= 12
Evaluate respiratory compensation in metabolic alkalosis
= 0.7 (HCO3) + 21 (+ 2)
= 43
What is the disturbance? What hormone(s) cause(s) this presentation?
Acute metabolic alkalosis. The alkalosis is likely due inappropriately elevated aldosterone as the
urine Cl is greater than 10 and this patient is known to have refractory hypertension. The
definitive treatment is to remove whatever is causing the elevated aldosterone… small cell lung
tumor, pancreatic tumor, adrenal tumor etc. There are equations similar to the Winter’s Formula,
but for metabolic alkalosis. These equations are not as reliable as the Winter’s Formula and
should be interpreted with caution. I prefer 0.7 (HCO3) + 21 (+ 2).
10. A 55 year-old woman presents with weight loss, bone pain, and fatigue.
ABG: 7.41/41/82/24
138 | 114 | 25 / 95
3.8 | 24 | 2.3\
Alb: 2.0
anion gap = 138 – (114 + 24)
=0
What is going on in this scenario? What might be the underlying disease?
There is no real acid-base disturbance, but the anion gap is zero. There are a few things that can
do this, but the one you don’t want to miss is multiple myeloma. The IgG produced from the
malignant plasma cell line obtains a positive charge and behaves as a cation. Remember the anion
gap = unmeasured anions – unmeasured cations. By increasing the unmeasured cations with this
IgG, the anion gap decreases and can even become negative.
11. A 32 year-old man with acute renal failure.
ABG: 7.16/20/68
138 | 110 | 130 / 90
5.1 | 8 | 6.1
anion gap = 138 – (110 + 8)
= 20
delta gap = 20 -12
=8
Alb: 3.9
Urine Na: 55
Urine K: 18
Urine Cl: 25
osmolal gap = assume it is normal for this patient.
urine anion gap = (55 + 18) – 25
= 48
corrected HCO3 = 8 + 8
= 16
What is the primary acid-base disturbance? What is the secondary disturbance?
Acute anion gap metabolic acidosis. There is a secondary non-anion gap acidosis due to renal
tubular acidosis (RTA). In this situation, always make the anion gap acidosis the primary
disturbance as it is the more acute and usually most serious problem. Based upon the urine anion
gap being positive we can deduce that the cause of the secondary non-gap acidosis is due to RTA.
The cause of the anion gap acidosis is renal failure and the body’s inability to excrete organic
acids that are the normal byproducts of endogenous chemical reactions.
12. A 27 year-old man with a history of paranoid schizophrenia is admitted for mental status changes and
lethargy, but no seizures. He has good skin turgor and no signs of peripheral edema. Below are his labs.
115 | 85 | 6 / 80
3.2 | 18 | 0.5
anion gap = 115 – (85 + 18)
= 12
Alb: 4.0
Urine Osms: 75
Weight: 70 kg
What is causing this man’s hyponatremia? What is the treatment?
This man’s hyponatremia is caused by psychogenic polydipsia. The first thing to assess when
dealing with hyponatremia is the patient’s volume status. This man seems to be euvolemic on
physical exam. Next we must check the urine osmolality to figure out if SIADH is causing this.
A concentrated urine suggests ADH being the cause of the hyponatremia. A dilute urine suggests
psychogenic polydipsia or beer potomania (too little salt intake, large liquid intake). In
psychiatric patients, especially those with schizophrenia, this is not uncommon. It is thought that
the brain disturbance causing the schizophrenia is very proximal to the anterior hypothalamus
where the thirst center resides. This may result in an inappropriate thirst stimulus. This is all
theoretical, but makes sense. The treatment is fluid restriction.
13. A 65 year-old man with a history of hypertension is admitted for mental status changes and lethargy, but no
seizures. He has decreased skin turgor, dry mucosal membranes, dry skin, and is modestly hypotensive
(100/60).
115 | 85 | 6 / 80
3.2 | 18 | 0.5
anion gap = 115 – (85 +18)
= 12
Alb: 4.0
Urine Osms: 700
Weight: 70 kg
What might be causing this man’s hyponatremia?
He appears to by hypovolemic based upon his physical exam. His urine is very concentrated.
This man has volume depletion due to diuretics or dehydration. His BUN/Cr has remained normal
and this is unusual.
What is his sodium deficit?
Weight (kg) x 0.6 = TBW
70 x (0.6) = 42 kg
42 kg of water = 42 liters of water
How much sodium does this patient have? 115 mmol/L x 42 L = 4830 mmol
How much sodium do you want this patient to have? 140 mmol/L x 42 L = 5880
How much sodium do you need to give to this patient? 5880 – 4830 = 1050 mmol
Divide the deficit by 154, the sodium concentration in normal saline. 1050/154 = 6.81 L of NS
How fast do you want to run your fluids to average 1 meq/hr increase in sodium?
We want to go from 115 to 140, which is an increase of 25 mmol. We do not want to increase the
serum sodium by more than 1mmol/L/hr to prevent central pontine demyelinosis. This should
therefore take 25 hours. 6.81 liters of normal saline/25 hours = 272.4 ml/hr of normal saline.
14. An 85 year-old nursing home resident is found to be difficult to arouse. She is brought to the hospital for
evaluation. She has decreased skin turgor, dry mucosal membranes, dry skin, and is hypotensive (95/65).
165 | 123 | 20 / 85
4.1 | 30 | 1.0
Alb: 4.0
Urine Osms: 750
Weight: 50 kg
What is this woman’s free water deficit?
Weight (kg) x 0.6 = TBW
50 x 0.6 = 30 kg
30kg of water = 30 L of water
How much sodium should this patient have? 140 mmol/L x 30 L = 4200 mmol
How much sodium does this patient have? 165 mmol/L x 30 L = 4950 mmol
What is the sodium excess? 4950-4200 = 750 mmol
Divide excess sodium by goal sodium concentration. 750/140 = 5.36 L of D5W
How fast do you want to run fluids to correct her Na by 1 meq/hr?
Need to correct sodium from 165 to 140 = 25 mmol. This will take 25 hours of running free water
IV. 5.36/25 = 215 ml/hr of D5W. We do not often run free water initially because patients with
hypernatremia are severely intravascularly depleted. Normal saline is given until the patient
approaches euvolemia and then we switch to a hypotonic solution. More than anything, the free
water deficit characterizes the severity of disease.