Arterial Blood Gas

A Patient With Respiratory Acidosis: Multidisciplinary Dialogue: Clinical Rounds and Case Reviews, Ep. 5

This podcast series aims to highlight the prevention, diagnosis, and treatment of patients with diseases commonly seen in internal medicine. Host, Anil Harrison, MD, discusses patient cases with residents and with prominent experts to help educate clinicians in treating patients using a multidisciplinary approach.


In this episode, Dr Harrison speaks with Paul Shiu, DO, about arterial blood gases, including a patient with respiratory acidosis and another patient with respiratory alkalosis. They also speak about the A-a gradient.

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Anil Harrison, MD

Anil Harrison, MD, is the Associate Program Director of the Internal Medicine Residency Program and the Ambulatory Care Director at Touro University and St Joseph’s Medical Center-Dignity Health (Stockton, CA). Dr Harrison is board certified in India and the United States.

Paul Shiu, MD
Paul Shiu, DO, is a second-year internal medicine resident at St Joseph's Medical Center (Stockton, CA).

Dharminder Singh, MD

Dharminder Singh, MD, is an internal medicine chief resident at St Joseph’s Medical Center (Stockton, CA).


 

TRANSCRIPTION:

Moderator: Hello everyone. And welcome to Multidisciplinary Dialogue: Clinical Rounds and Case Reviews with your host, Dr Anil Harrison, who is the Associate Program Director for the Internal Medicine Residency Program and the Ambulatory Care Director at Touro University in St. Joseph's Medical Center, Dignity Health in Stockton, California.

Today we have a case review that Dr Harrison and Dr Paul Shiu will analyze and provide treatment insights. Dr Shiu is a second-year internal medicine resident at St. Joseph's Medical Center in Stockton, California.

In this episode we'll discuss a patient with respiratory acidosis. The views of the speakers are their own and do not reflect the views of their respective institutions or the views of Consultant360.

Dr Paul Shiu: Good morning, sunny, California.

Dr Anil Harrison: Good morning, Paul.

Dr Paul Shiu: Today we have a very, very important topic to discuss. It's actually part of our asset base series. This is episode five.

Dr Anil Harrison: Correct.

Dr Paul Shiu: And Dr Harrison, if I recall correctly, today's presentation is going to be on respiratory acidosis and alkalosis and the A-a Gradient. Big A, little a Gradient.

Dr Anil Harrison: Correct. Yes. Paul, if you remember-

Dr Paul Shiu: Yes.

Dr Anil Harrison: The last one was I think, on a normal anion gap metabolic acidosis and a high anion gap metabolic acidosis.

Dr Paul Shiu: Shameless self-promotion, folks. If you missed that episode, please, I invite you all to check us out. That podcast is particularly foundational, just like this one, and the next one. So you don't want to miss any of them, really. I saw a really interesting patient to start us off.

Coincidentally, serendipitously, we have a patient to talk about. A 22-year-old is evaluated in the ER for altered mental status, a heart rate of 50 per minute, blood pressure of 100/60, and respiratory rate of six per minute. The patient is afebrile, with pinpoint pupils. Blood gasses reveal the pH of 7.3, PCO2 of 50, PO2 of 80, and a Bicarb of 26.

Dr Anil Harrison: Yeah, that's a good one, Paul. So to understand acidosis and alkalosis, as we know, carbon dioxide, which is produced by cells, combines with water. This is the respiratory component to form carbonic acid, which is H2CO3, the H2CO3 then breaks down into hydrogen ions and bicarbonate ions, the latter being the metabolic component. Therefore, it makes sense if there is too much of carbon dioxide, either because of an excess production or because it cannot be blown out, this results in an increase in hydrogen ions, which constitutes acidosis. The hydrogen ions and bicarbonate ions are then managed by the kidneys. With respiratory acidosis, the pH is less than 7.35, and the PCO2 is more than 45. As the kidneys tend to retain Bicarb or dump excessive hydrogen ions as a compensatory mechanism.

On the other hand with respiratory alkalosis, the pH is greater than 7.45, and the PCO2 is less than 35, while the kidneys again, compensate by getting rid of Bicarbonate or retaining hydrogen ions. So the question might be what happens with acute versus chronic respiratory acidosis?

So Paul, with acute respiratory acidosis, the PCO2 acutely goes above 45. Hence, the pH goes below 7.35, which is in contrast with chronic respiratory acidosis or an acute on chronic respiratory acidosis, where the PCO2 is much greater than a baseline of 45. The pH, however, might be less than 7.35, or it might be normal because of the compensatory mechanisms of the kidney in trying to retain Bicarbonate over a period of time.

Dr Paul Shiu: Ah, so what happens with acute versus chronic respiratory alkalosis then?

Dr Anil Harrison: Sure. In acute respiratory alkalosis, the PCO2 acutely goes below 35 and the pH goes above 7.45. In chronic respiratory alkalosis or with acute on chronic respiratory alkalosis, the PCO2 is much less than 35 and the pH can be normal or slightly alkaline. This is because, once again, the kidneys have had adequate time in dumping bicarbonate over a period.

Dr Paul Shiu: So then what causes respiratory acidosis where the pH is less than 7.35 and the PCO2 is greater than 45?

Dr Anil Harrison: Sure. So simplistically, we divide this into the patient won't breathe, can't breathe, or can't breathe enough. What do I mean by that? So if you think about it, if the problem is in the central nervous system, where there is not enough stimulus to the respiratory centers in the brain, the patient won't breathe.

Or the patient can't breathe adequately due to issues either with the peripheral nerves, the neuromuscular junction or chest muscle weakness or chest wall and plural issues or with issues in the upper area airways.

The last one is where a person can have respiratory acidosis, which can occur because the patient cannot breathe enough, which is due to an abnormal gas exchange between the alveoli and the capillaries. The latter can be discerned with an A-a Gradient. A capital A, which stands for alveoli, and a small a, which stands for arterial.

Dr Paul Shiu: Wait, wait, wait, wait, Dr Harrison. You're talking about this A-a Gradient, the big A, little a. First of all, what does the big A stand for and what does the little a stand for? And how does one measure it?

Dr Anil Harrison: Sure. So once again, Paul, the big A is alveola and the small a is arterial. The Alveolar-Arterial Gradient is important in discerning issues with gas exchange between the alveolis and the arterial end of the pulmonary capillaries. The big A represents alveola oxygen and, the small a represents arterial oxygen. An increased gradient, called increased capital A, small a gradient represents an issue either in the alveolis, the interstitial space or on the arterial end of the pulmonary capillaries. Hence, gas exchange is not occurring efficiently.

A-a gradient = 2.5 + 0.21 x age in years. A simpler way to calculate the A-a gradient is to add 4 to the age and then divided the number by four. The normal A-a gradient ranges between, I would say, five to 20. And when the A-a Gradient is above 20, one starts worrying about an abnormality, which we've discussed before. A simple way to calculate pAO2 is150-pCo2/0.8, while the paO2 or pO2 is derived from blood gases. There is another one that is the Pao2/ FIO2 ratio, which is another common measure of oxygenation and is most often employed in ventilated patients or patients who are on ventilators. A normal Pao2/ FIO2 ratio is about 300 to 500 mm of mercury. With values less than 300 millimeters of mercury indicating abnormal gas exchange, and values less than 200 indicating severe hypoxemia.

Dr Paul Shiu: There's just so much high-yield information in what Dr. Harrison just said in the preceding three paragraphs. We talked about the A-a Gradient. What is expected for the normal range based on age, a simple formula age plus four, divide by four, as well as the P/F ratio. Which many of you probably have heard of the Horowitz, in which the P/F ratio tells us the severity of a patient's... I guess, what is it? Pulmonary edema, anything that might affect the oxygenation?

Dr Anil Harrison: Absolutely.

Dr Paul Shiu: We have to cover this in another podcast, Dr. Harrison, talking about ventilation and ICU patients.

Dr Anil Harrison: Would be happy to.

Dr Paul Shiu: Excellent. Well, not to derail this entire podcast. So could you give us some examples of respiratory alkalosis, Dr. Harrison?

Dr Anil Harrison: Sure. So respiratory as alkalosis is when the pH is greater than 7.45, and the PCO2 is less than 35. So if you think about it, disorders like asthma or pulmonary embolism, pulmonary edema, and pneumothorax. And if it's in the nervous system disorders such as central neurogenic disorders, pain and panic disorders, endocrine disorders such as with pregnancy and thyrotoxicosis. And of course liver issues, such as cirrhosis, systemic disorders, such as sepsis, or if the person's on medications. And, of course, Salicylates to Topiramate, Progesterone, Theophylline, nicotine, and caffeine, they can all cause respiratory alkalosis.

Dr Paul Shiu: Now that we have covered the basics, what are your thoughts on our 22-year-old who's evaluated for altered mental status with pinpoint pupils, bradycardia, and bradypnea? Blood gas is revealed pH of 7.3, a PCO2 of 50, a PO2 of 80 and a Bicarb of 26.

Dr Anil Harrison: I think the patient with a pH of 7.3 and a PCO2 of 50, with altered mental status and pinpoint pupils, with a slow heart rate and is breathing slow, who has acute respiratory acidosis. It could be a secondary to an opioid overdose. And you might agree in considering giving Naloxone though, looking out for opioid withdrawal, as well.

Dr Paul Shiu: Oh, all right. I have another one to throw at you then. What about a 24-year-old pregnant female with blood gasses revealing a normal pH, but a low PCO2 of 34 and a slight low Bicarb of 20. What are your thoughts?

Dr Anil Harrison: Okay, so with a normal pH and a low PCO2, this patient has probably chronic respiratory alkalosis. Secondary to, I think, Progesterone and her pregnancy. The Bicarb, Paul, if you notice this low, as the kidneys have had time to compensate. And, from a previous podcast, if you remember, if you add 15 to the Bicarb of 20, it equals 35, which is close to the patient's PCO2 of 34. So I feel this is chronic respiratory alkalosis secondary to pregnancy.

Dr Paul Shiu: Well, certainly adding 15, it's a very quick way than running Winter's Formula.

Dr Anil Harrison: I believe so. Yes.

Dr Paul Shiu: Well, folks, that's all we have for you for this episode, covering arterial blood gases. Next episode, we'll continue the series and we'll continue to provide practice cases so that you guys can follow along.

Feel free to reach out if you guys have any questions about anything that we've talked about. Thank you so much for joining us, guys.

Dr Anil Harrison: Thank you, Paul.

Dr Paul Shiu: All right. Take care guys