Question

Meera is a 24 year old woman who has been studying at a local university. She recently returned from a 14 hour flight to Delhi, where she had been visiting family. She came to your emergency room worried that she is having a heart attack; she describes 2 hours of a sharp pain in her chest and back, which is worse when she takes a deep breath. She states she is having a hard time breathing.

Her vital signs are as follows: Blood pressure is normal at 120/72, pulse elevated at 112, normal body temperature of 98.1℉, and an increased respiratory rate of 40. Meera’s past medical history is unremarkable. She has never been pregnant, and is currently taking an oral contraceptive to prevent pregnancy. In addition to her oral contraceptive, she takes a multivitamin daily and sometimes uses ibuprofen for menstrual cramps. She occasionally drinks alcohol, is a ½ pack per day smoker, and denies any illicit drug use.

Treatment (part 2)

You have ordered a set of arterial blood gases (ABGs), a D-dimer, an EKG, and chest imaging for Meera. Her EKG did not show evidence of cardiac ischemia or myocardial infarction, and her Chest CT revealed a large pulmonary embolism. Her D-dimer was elevated. Her ABGs are as follows:

PaO2: 75 mmHg    Normal range: 80–100 mmHg

pH: 7.5    Normal range: 7.35–7.45

pCO2: 30 mmHg    Normal range: 35–45 mmHg

HCO3: 22 mmol/L    Normal range 22–26 mmol/L

• Thinking back to Chapter nine, does Meera’s lab work reveal an acidotic or alkalotic state?
• Which buffer system(s) will work to restore her pH to a normal value? Briefly (2-3 sentences) describe how that buffer system works to restore normal pH.
• Meera’s PaO2 and CO2 are low. What is the definition of low PaO2? What is the definition of low pCO2?
• What type of alteration in normal physiology is driving Meera’s low PaO2? Explain the alteration in her normal physiology.
• What general treatment(s) do you anticipate will be ordered for Meera? Explain briefly how those treatments will restore normal respiratory function.

Answer 1

In the given case of Pulmonary embolism,

PaO2: 75 mmHg [Normal range: 80–100 mmHg]

pH: 7.5 [Normal range: 7.35–7.45]

pCO2: 30 mmHg [Normal range: 35–45 mmHg]

HCO3: 22 mmol/L [Normal range 22–26 mmol/L]

According the the ABG, PH is greater than the normal limit [i.e Alkalosis]. Also the HCO3 is almost normal. so it is not due to a metabolic problem. Here, PH has increases due to a decrease in the pCO2.

Therefore the abnormality here is RESPIRATORY ALKALOSIS.

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The most important buffer system to restore her pH to normal is the carbonic acid - bicarbonate buffer system.

In Respiratory alkalosis, the fall in pH occurs due to decrease in the concentration of CO2 in the blood due to hyperventillation. HCO3- functions as an alkalotic substance. CO2 functions as an acidic substance. So, decrease in CO2 increases t.he ratio of bicarbonate concentration to CO2 and, thereby, increases the pH level.

The primary pH buffer system in the human body is the HCO3/CO2 chemical equilibrium system. Where:

H + HCO3 <---------> H2CO3 <---------> CO2 + H2O

Renal compensation for respiratory alkalosis involves a decrease in HCO3. Hydrogen combines with bicarbonate to form carbonic acid, which then disassociates to form water and CO2, thus replenishing the depleted pCO2 and removing bicarbonate  from the blood through kidneys, thus counteracting increase in pH.

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PO2 (partial pressure of oxygen) reflects the amount of oxygen gas dissolved in blood. The normal value is 80–100 mmHg. A PO2 les than 80 mm Hg is Low PO2

PCO2 (partial pressure of Carbon dioxide) reflects the amount of carbon dioxide dissolved in blood. The normal value is 35 - 45 mmHg. A PCO2 les than 35 mm Hg is Low PCO2

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In Pulmonary embolism, the blood flow to the lungs is blocked by a clot in the pulmonary vasculature. So even when the breathing at the level of alveoli is normal, exchange of gases across the respiratory epithelium becomes low. (because the blood flow to alveoli is reduced). This is called Ventilation - Perfusion (V/Q ) mismatch.

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The first step in treating pulmonary embolism is to provide oxygen therapy. This can correct the hypoxia to an extend. In some cases pulmonary embolectomy, i.e removal of the clot is done. Anticoagulant medications,such as heparin are usually given to help thin the blood and prevent further clotting.

Providing oxygen can corect the hypoxia or low pO2. Once you do an embolectomy, the Ventilation - Perfusion (V/Q ) mismatch, is corrected and the respiratory function returns to normal.