Normal Levels of Substances in the Arterial Blood:
pH
pCO2 (partial pressure of carbon
dioxide)
pO2 (partial pressure of oxygen)
Hemoglobin - O2 saturation
[HCO3-]
7.40 + 0.05
40 mm Hg
90 - 100 mm Hg
94 - 100 %
24 meq / liter
Case Study #1
A 14-year-old girl with cystic fibrosis has complained of an increased cough
productive of green sputum over the last week. She also complained of being
increasingly short of breath, and she is noticeably wheezing on physical
examination. Arterial blood was drawn and sampled, revealing the following
values:
pH
pCO2
pO2
Hemoglobin - O2 saturation
[HCO3-]
7.30
50 mm Hg
55 mm Hg
45 %
24 meq / liter
1. What causes cystic fibrosis? Describe the pathophysiologic mechanisms
of the disease.
Cystic fibrosis is a disease characterized by abnormally thick mucus
secretions from the epithelial surfaces of various organ systems. CF is
caused by an abnormal mutation of the cystic fibrosis transmembrane
conductance regulator (CFTR). In individuals with CF, CFTR’s failure to
function properly results in thick viscous secretions that eventually lead to
obstruction of the glands and ducts in the affected organs. CF patients
tend to experience an increase of repeated lower respiratory tract
infections.
2. How would you classify this girl's acid-base status?
The girl’s acid-base status can be classified at respiratory acidosis. This
shows in her pH level, which is a bit lower than normal. Along with her
other labs, listed above, which are not in normal ranges.
3. How does cystic fibrosis cause this acid-base imbalance?
CF causes this acid-base imbalance due to the excessive amounts of
thick mucus, which obstructs the airways. This leads to the abnormal
levels that are listed above, causing an imbalance in her overall system.
4. How would the kidneys try to compensate for the girl's acid-base
imbalance?
The kidneys would try to compensate for the girl’s acid-base imbalance by
increasing the tubular secretion rate of hydrogen ions into the renal
tubules. However, since her levels are still normal, the kidney’s most likely
have not started to compensate for her respiratory acidosis.
5. This girl has also had a long history of diarrhea and poor weight gain.
Explain why.
She is most likely dehydrated and experiencing an electrolyte imbalance.
Also, the thick mucus secretions can accumulate in the pancreatic ducts
as well. With this occurring, normal functions of the digestive enzymes are
not taking place. Along with blocking the bile ducts, which bile is then
restricted in its normal movement from the liver and gallbladder to the
duodenum.
6. List some other causes of this type of acid-base disturbance.
Other causes of this type of acid-base disturbance include asthma,
chronic bronchitis, emphysema, muscular dystrophy, choking, and an
overdose of morphine.
Normal Levels of Substances in the Arterial Blood:
pH
pCO2 (partial pressure of carbon
dioxide)
pO2 (partial pressure of oxygen)
Hemoglobin - O2 saturation
[HCO3-]
7.40 + 0.05
40 mm Hg
90 - 100 mm Hg
94 - 100 %
24 meq / liter
Case Study #2
A 76-year-old man complained to his wife of severe sub-sternal chest pains that
radiated down the inside of his left arm. Shortly afterward, he collapsed on the
living room floor. Paramedics arriving at his house just minutes later found him
unresponsive, not breathing, and without a pulse. CPR and electroconvulsive
shock were required to start his heart beating again. Upon arrival at the
Emergency Room, the man started to regain consciousness, complaining of
severe shortness of breath (dyspnea) and continued chest pain. On physical
examination, his vital signs were as follows:
Systemic blood pressure
Heart rate
Respiratory rate
Temperature
85 mm Hg / 50 mm Hg
175 beats / minute
32 breaths / minute
99.2oF
His breathing was labored, his pulses were rapid and weak everywhere, and his
skin was cold and clammy. An ECG was done, revealing significant "Q" waves in
most of the leads. Blood testing revealed markedly elevated creatine
phosphokinase (CPK) levels of cardiac muscle origin. Arterial blood was sampled
and revealed the following:
pH
pCO2
pO2
Hemoglobin - O2 saturation
[HCO3-]
7.22
30 mm Hg
70 mm Hg
88 %
2 meq / liter
1. What is the diagnosis? What evidence supports your diagnosis?
The diagnosis for this man is that he suffered a heart attack, or myocardial
infraction. He experienced severe sub-sternal chest pains that radiated
down his left arm, collapsed, sever shortness of breath (dyspnea), cold
and clammy skin, significant “Q” waves on the ECG, and elevated creatine
phosphokinase (CPK) levels of the cardiac muscle origin.
2. How would you classify his acid-base status? What specifically caused
this acid-base disturbance?
From looking at his lab values above, they indicate that he is experiencing
primary metabolic acidosis. It is partially from hyperventilation. When his
heart stopped beating, blood flow to all organs stopped. Tissues were then
forced to continue to work in the absence of oxygen, which caused an
imbalance in his acid-base status.
3. How has his body started to compensate for this acid-base disturbance?
With hyperventilation, the arterial pCO2 moved downward and caused free
H+ ion to bind with HCO3-, which forms carbonic acid. This occurs with the
purpose of removing excess H+ ions from the bloodstream, correcting the
metabolic acidosis.
4. What would his blood pH be if his body had not started compensating
for the acid-base disturbance? Show your work.
If this man had not started compensating for the metabolic acidosis by
hyperventilating, his pCO2 level would be normal (i.e. about 40 mm Hg).
The pH, in this case, can be calculated using the Henderson-Hasselbalch
equation and knowing that the arterial HCO3- concentration is 12 mEq / L
and the arterial pCO2 would be 40 mm Hg. Doing so reveals that the
arterial pH would have been 7.1 (rather than 7.22) if this man had not
started hyperventilating. While this might not seem like a big difference,
recall that the pH scale is a logarithmic scale, and a shift in arterial pH
from 7.22 to 7.1 represents a 33% increase in hydrogen ion concentration.
5. List some other causes of this type of acid-base disturbance.
Other causes of this type of acid-base disturbance include DKA, increased
amounts of methanol and ethanol, severe diarrhea, renal tubular acidosis,
and chronic renal failure.
Normal Levels of Substances in the Arterial Blood:
pH
pCO2 (partial pressure of carbon
dioxide)
pO2 (partial pressure of oxygen)
Hemoglobin - O2 saturation
[HCO3-]
7.40 + 0.05
40 mm Hg
90 - 100 mm Hg
94 - 100 %
24 meq / liter
Case Study #3
An elderly gentleman is in a coma after suffering a severe stroke. He is in the
intensive care unit and has been placed on a ventilator. Arterial blood gas
measurements from the patient reveal the following:
pH
pCO2
pO2
Hemoglobin - O2 saturation
[HCO3-]
7.50
30 mm Hg
100 mm Hg
98%
24 meq / liter
1. How would you classify this patient's acid-base status?
I would classify this patient’s acid-base status as primary respiratory
alkalosis, as proven by his increased arterial blood pH.
2. How does this patient's hyperventilation pattern raise the pH of the
blood?
The patient’s hyperventilation pattern raises the pH of the blood by
increasing the rate of CO2 excretion from the body. This drives the arterial
pCO2 downward, causing free H+ ion to bind with HCO3-, forming carbonic
acid.
3. How might the kidneys respond to this acid-base disturbance?
The kidneys would try to compensate for the girl’s acid-base imbalance by
increasing the tubular secretion rate of hydrogen ions. The patient's
arterial HCO3-concentration is within normal range, indicating that the
kidneys have not started in compensating for the respiratory alkalosis.
4. List some other causes of this type of acid-base disturbance.
Other causes of this type of acid-base disturbance include expose to
higher altitudes than the patient is used to. Stress and anxiety can also
cause respiratory alkalosis.
Normal Levels of Substances in the Arterial Blood:
pH
pCO2 (partial pressure of carbon
dioxide)
pO2 (partial pressure of oxygen)
Hemoglobin - O2 saturation
[HCO3-]
7.40 + 0.05
40 mm Hg
90 - 100 mm Hg
94 - 100 %
24 meq / liter
Case Study #4
A 28-year-old woman has been sick with the flu for the past week, vomiting
several times every day. She is having a difficult time keeping solids and liquids
down, and has become severely dehydrated. After fainting at work, she was
taken to a walk-in clinic, where an IV was placed to help rehydrate her. Arterial
blood was drawn first, revealing the following:
pH
pCO2
pO2
Hemoglobin - O2 saturation
[HCO3-]
7.50
40 mm Hg
95 mm Hg
97%
32 meq / liter
1. How would you classify her acid-base disturbance?
According to what the woman’s lab results indicate, she is experiencing
primary metabolic alkalosis.
2. Why might excessive vomiting cause her particular acid-base
disturbance?
Excessive vomiting of gastric secretions tend to be very acidic. As
indicated, she is vomiting several times every day, which can lead to a
loss in H+ ions in the stomach and bloodstream. Resulting in an increase
in pH of her arterial blood.
3. How would the kidneys compensate for this acid-base disturbance?
The kidneys would try to compensate for the girl’s acid-base imbalance by
increasing the tubular secretion rate of hydrogen ions. The patient's
arterial HCO3-concentration is above normal range, indicating that the
kidneys have not started in compensating for the respiratory alkalosis.
4. List some other causes of this type of acid-base disturbance.
Other causes of this type of acid-base disturbance include antacid
overdose, ingestion of loop diuretics, or elevated levels of the hormone
aldosterone.