Understanding Acid-Base Balance
Understanding acid-base balance is a core concept that can help you think critically about your patients and recognize how imbalances can impact their overall wellbeing. As you recall from your anatomy and physiology course, having a proper balance between acid and base in the body is critical to maintaining homeostasis and optimal cellular function. Recall that the normal range for a serum pH level is 7.35 to 7.45. If the serum is too acidic then the pH number will be low; too alkaline and the pH number will be high. In normal conditions, the body manages to keep the pH in an optimal range through compensatory mechanisms and buffering. The main ways we’ll talk about here are via the kidneys and the lungs because those are the most obvious ways it will occur in your patients.
When the body’s pH is too low, we say the patient is in a state of acidosis. This acidos can be categorized as either metabolic in nature or respiratory in nature. Some of the reasons a patient may be acidotic include:
- Respiratory Acidosis
- Excess CO2 building up in the blood, often occurs in COPD
- Metabolic acidosis
- Deficient bicarbonate reabsorption due to kidney disease
- Build up of waste products in renal failure (common when patients miss dialysis)
- Build up of acid (ketone bodies) in the blood due to metabolic disorders such as diabetic ketoacidosis
- Excessive losses of sodium bicarbonate (can happen with severe diarrhea)
- Accumulation of lactic acid, which can occur in sepsis, alcoholism, prolonged hypoperfusion, and hepatic failure
- Ethylene glycol or aspirin poisoning
And, some of the reasons a patient may be in an alkalotic state are:
- Respiratory Alkalosis
- Low levels of CO2 in the blood secondary to hyperventilation
- Being at a high altitude
- Metabolic Alkalosis
- Extreme losses of chloride or hydrogen ions secondary to excessive vomiting or GI suctioning.
How the body regulates pH
The first thing to know about the regulation of pH is that there are two kinds of acids: volatile acids and fixed acids. Volatile acids are those that form gas in a solution. For example, carbonic acid forms the gas CO2. Volatile acids leave the body via the lungs (it’s called ‘gas exchange’ for a reason, folks!).
Fixed acids, on the other hand, must leave the body through the kidneys as they cannot be converted into a gas. An example of a fixed acid is sulfuric acid which is a byproduct of protein metabolism. Another example is lactic acid which accumulates in cases of severe sepsis. So, looking at these two types of acids it’s easy to see why it’s the renal system and the respiratory system that play such a key role. But these aren’t the only ways the body regulates pH…there are chemical buffers as well.
The main chemical buffers in the body are:
- Protein buffer systems: these involve amino acids binding and releasing hydrogen ions as needed. Protein buffers account for about 66% of the buffering action going on in the plasma and most of the buffering action occurring inside the cell.
- The carbonic acid-bicarbonate buffer system (remember this equation from your physiology class?). This is a key buffer in the extracellular fluid.
- The phosphate buffer system, which is important in the intracellular fluid and in the urine.
We won’t go into detail on these, but just know that they exist and certain conditions can lead to alterations in these buffer systems. But for the most part, the obvious way you’ll witness acid/base regulation occurring in your patients is by monitoring their renal and respiratory response.
How the lungs affect pH
The lungs affect pH balance essentially by altering the rate of gas exchange. Slower breathing (bradypnea) or conditions like COPD cause CO2 to increase. When this occurs, we say the patient is “retaining” as in, they are “retaining their CO2.”
Hyperventilation or tachypnea, on the other hand, causes CO2 to decrease. In these cases we say the patient is “blowing off their CO2.” A key thing to know about the lungs and their ability to affect pH is that changes can occur rather quickly.
How the kidneys affect pH
The kidneys affect pH much more slowly through the excretion of hydrogen ions in the distal tubules and through the reabsorption of bicarbonate through the proximal tubules. Recall that hydrogen ions are acidic, and bicarbonate is a base. The kidneys can also generate “new” bicarbonate in the distal nephron. This is achieved through two mechanisms…a complicated process involving hydrogen phosphate and through the metabolism of anions. If you really want to dive into the chemistry of this, here’s a very in-depth article. Have at it!
A key concept to understand when you’re learning about acid-base disorders is compensation. For example, in a metabolic acidosis, the respiratory system will kick in to help compensate. It does this by changing the rate of ventilation to “blow off” CO2 more quickly. In cases of respiratory acidosis or alkalosis, the kidneys can compensate by altering how much bicarbonate is reabsorbed and how much hydrogen ions are removed. When the lungs are compensating for an imbalance, the changes can occur pretty quickly…often within hours, with full compensation often present within 24 hours. Kidneys, on the other hand, take much longer to make a difference…usually several days at least. Patients with chronic COPD will have an elevated CO2 at baseline, but the kidneys compensate by increasing bicarbonate, so their pH stays more-or-less within the normal ranges.
A quick word about compensation…it can either be complete or partial. It’s important to note that there are some limiting factors. For example, in cases of metabolic alkalosis, the respiratory drive will decrease in an effort to retain more acid and maintain balance. However, driving ventilation down too low leads to hypoxia, which wouldn’t be good either! On the other hand, renal compensation is limited by a lot of different factors such as renal blood flow, tubular flow rates and overall health of the kidneys. So, even though compensation can occur, it’s not always a perfect process.
When do we get concerned?
Seriously bad things happen when pH is below 7.2 or above 7.55. And how do we know what the pH is? It will be on your ABG or VBG, arterial blood gas or venous blood gas. Just know that the reference ranges vary slightly between these two tests. When we’re looking at interpreting the cause of the imbalance, we will use an ABG (learn how to analyze it here).
What are you going to do about it?
Correcting acid/base disorders is complex and tailored to the underlying cause. Let’s look at a few classic examples and how it could possibly be treated.
Patient Case 1: Respiratory Acidosis
Bob is a 68 year old male with COPD, a chronic lung condition he’s had for six years. His family brought him in to the emergency room after finding him unresponsive at home. His ABG reveals that he is in a respiratory acidosis. Patients with COPD “retain” their CO2…they don’t blow it off as effectively as those with normal lung function. When this CO2 level gets too high, they become severely acidotic and lose consciousness. These patients are treated by assisting their ventilation. We often start with BIPAP, which is a non-invasive way to provide positive pressure ventilation and get more gas exchange to occur, bringing down the CO2.
What typically happens with these patients is that they will come in to the ER or ICU very somnolent or completely unresponsive. We’ll place them on BIPAP and after a few hours or so, the CO2 level will decrease and the patient’s level of consciousness will improve. They wake up and very often want that BIPAP mask off! It is pretty uncomfortable for patients as it blows forced air into the mouth, nose and lungs pretty assertively. If the patient is alert enough, we may try them off the BIPAP for a bit and see how they do. Some patients do just fine whereas others continue to retain too much CO2. These patients will become somnolent again, requiring continued BIPAP therapy. Oftentimes, these COPD exacerbations have an underlying pathology such as an infection, so treating the pneumonia (or whatever it is) goes a long way toward returning their respiratory status back to baseline.
Patient Case 2: Metabolic Acidosis
Jackie is a 58 year old woman with end-stage renal disease who gets dialysis Mondays, Wednesdays and Fridays. Jackie caught a head cold from her grandson and didn’t feel up to going to dialysis on Friday. It is now Sunday morning and her family cannot get her to wake up. They bring her in to the emergency room where the ABG reveals she has a pH of 7.1 due to a metabolic acidosis secondary to missing her dialysis appointment. Patients in renal failure will not have that important physiological system that secretes waste products or balances hydrogen and bicarbonate, so their pH is going to be very imbalanced. Jackie is given dialysis in the emergency room and admitted to the ICU where her level of consciousness improves as her pH corrects.
Patient Case 3: Respiratory Alkalosis
Penny is a 42 year old woman with a history of severe uterine fibroids. Over the past week she has noticed an increase in bleeding, but didn’t make an appointment with her OB/GYN because she’s trying to complete her graduate thesis in nursing. She becomes progressively weaker and eventually reaches a state of acute confusion with tremors and chest discomfort. Her husband brings her into the emergency room where it is found she is severely anemic with an ABG that shows she is in respiratory alkalosis. In response to the deceased oxygen-carrying capacity of her blood, Penny began hyperventilating in response. This hyperventilation eventually “blew off” her CO2, resulting in an alkaline state. The treatment would be to correct the underlying cause…in this case we’ll treat the anemia with an emergent blood transfusion. Penny should start to feel much better pretty quickly. In severe cases, patients may be placed on a ventilator and their respiratory rate forcibly decreased…a maneuver that requires adequate sedation to overcome the body’s drive to hyperventilate. And, perhaps Penny will be getting a hysterectomy as well and kiss her fibroid troubles goodbye once and for all!
Patient Case 4: Metabolic Alkalosis
Chuck is a 52 year old male with a history of congestive heart failure. He recently moved to your area and has changed pharmacies. His new pharmacy labels his diuretic by the brand name, Lasix, and his prior pharmacy labeled it with the brand name Furosemide. Despite extensive teaching on the subject, Chuck does not realize these medications are the same. He takes them both, leading to massive volume and potassium losses. He comes in to the emergency room with severe muscle weakness. A chemistry panel reveals a K level of 2.4, but thankfully he doesn’t have severe abnormalities on his EKG. Whew!
This loss of potassium is going to cause a shift, where K moves from the cells into the extracellular fluid. In order to maintain electroneutrality, hydrogen ions will enter the cell…and in the renal tubules, the hydrogen ions are secreted into the lumen. These shifts both cause an increase in plasma bicarbonate, which raises the body’s pH.
So, to correct Chuck’s metabolic alkalosis, we’re going to correct the underlying cause, which is hypokalemia (and may need to replenish some fluids while we’re at it). So, Chuck will get potassium replacement and we’ll be sure to educate him on proper medication administration as well.
So there you have it…a quick overview of Acid-Base balance! Let me know in the comments below if this helps you on an exam or in clinical…I love hearing your success stories!
Get this on audio in Episode 65.
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