Tuesday, November 18, 2014

Metabolic Acidosis and Alkalosis

A common problem faced by trainees in Internal Medicine is acid-base disorders. Here, we will discuss the causes of acid-base disorders rather than the diagnosis on the basis of arterial blood gases. Specifically, we will discuss metabolic disturbances.

1) How does potassium affect acid-base status?
Changes in potassium often affect the metabolic acid/base status and vice versa. This is explained by the movement of excess potassium into cells which is balanced in part by intracellular hydrogen ions moving out to the extracellular fluid, and vice versa. This is termed "Internal Potassium Balance:" 

Metabolic acidosis tends to be associated with hyperkalemia (exceptions below).
Metabolic alkalosis tends to be associated with hypokalemia (exceptions below).

2) How does volume status affect acid-base status?
Hypovolemia can predispose patients to contraction alkalosis in some cases. There are a few reasons for this:
1. Loss of large volumes of low-bicarbonate, sodium-containing fluid leads to bicarbonate concentration rises because of contraction of the ECF volume with a constant quantity of  bicarbonate. This often occurs with IV loop diuretics.
2. High aldosterone - In the cortical collecting duct, high aldosterone released due to volume contractions leads to potassium and hydrogen ion excretion to balance sodium reabsorption. Hydrogen ion loss causes alkalosis, as does hypokalemia itself for the reasons stated above.
3. Angiontensin II - Released as part of RAAS activation in hypovolemia, angiontensin II acts at the Na+/H+ exchanger in the proximal tubules to enhance sodium absorption and H+loss, leading to alkalosis.

Therefore, contraction alkalosis can be seen in some patients with volume depletion.

Metabolic Acidosis

A) Increased Anion Gap
In these cases, extra anions (paired with H+) are introduced to the blood. The His consumed by HCO3- in the buffer system and the only component that remains is the anion, which creates the anion gap. One mnemonic that can be used to remember these causes is MUDPILES:

Uremia (chronic kidney disease)
Diabetic Ketoacidosis
Propylene glycol/Paraldehyde
Iron overload/Isoniazid use
Lactic acidosis
Ethylene glycol

B) Non-Anion Gap
In these cases, no new anions are being introduced to the blood. Instead, either HCO3-  is excessively lost or renal acid is not being excreted. 

Note that "hyperchloremic metabolic acidosis" is interchangeable with "non-anion gap metabolic acidosis." This is because chloride is proportionately increased when HCO3-  is lost or H+ accumulates (no extra anions are present).

1. Diarrhea - Loss of bicarbonate rich fluid results in acidosis. It is  important to note that although diarrhea typically causes a metabolic acidosis, it can also cause a metabolic alkalosis in the following cases:
- Laxative abuse
- Villous adenoma
- Congenital chloroidorrhea (rare) 

2. Bowel conduits for urine - These conduits are created during ureteric or bladder operations. When urine is exposed to gut mucosa, it reabsorbs chloride in exchange for HCO3-, leading to acidosis.

3. Renal Tubular Acidosis
These diseases result from the kidney's inability to acidify the urine due to several reasons. Note that potassium and acid/base disturbance associations in these cases do not follow the usual pattern listed above.

Metabolic Alkalosis

Metabolic alkalosis results from additional creation of bicarbonate combined with a process which prevents renal excretion of the excess bicarbonate. Patients with metabolic alkalosis compensate by hypoventilating and should raise the PCO2 by about 0.7 mm Hg for every 1 meq/L elevation in the bicarbonate. This initially lowers the arterial pH toward normal and is most effective acutely, but then becomes less effective over time.

The differential diagnosis for metabolic alkalosis can be broken down into two groups, according to the urinary chloride levels:

Of these, the two most common causes are vomiting and diuretic therapy. One additional differential diagnosis for metabolic alkalosis of relevance is contraction alkalosis, mentioned earlier.

Here's a more physiological approach to acid-base disturbances from NEJM.

Thursday, November 6, 2014


During morning report this week, we discussed a patient who returned form Ghana with Falciparum malaria. Let's discuss the approach to treatment of malaria - which literally means 'bad air.'

Malaria - What's up with that? 
Malaria is caused by the protozoan PlasmodiumThe mosquito that carries Plasmodium species called Anopheles (literally translated to 'ueseless'). Due to their transmission of malaria, mosquitos are thought to have caused half of all human deaths - more than all the wars combined.

Malaria should be suspected in anyone with a fever that has had mosquito exposure in a region where malaria is endemic. Don't expect to get a history of 'mosquito bites' from patients, just assume that they got mosquito bites when they traveled in the area. The incubation period for malaria: can range from 2 weeks (often falciparum) to a year, due to potential latency of the organisms. The diagnosis of malaria is based on light microscopy of thick and thin smears of patient blood - which are highly sensitive for detection of parasites when the load is >1%. When the parasite load is low, antigen testing is able to detect infection, though antigen testing cannot reveal the quantity of organisms infecting a patient. Also, it may or not be able to differentiate falciparum species from non-falciparum species.

The top map shows falciparum distribution and the bottom shows vivax distribution.

4 Malaria Questions to consider for treatment in a patient diagnosed with Malaria:
1) Is this uncomplicated or complicated malaria?
2) Where it was acquired, and what are the resistance patterns there?
3) What species of malaria is it?
4) What is the degree of parasitemia?

1) Presentation of Malaria: Signs vs Symptoms

Uncomplicated vs Complicated Presentation of Malaria

In general, patients who are not obtunded and can take PO medications are considered "uncomplicated".

Uncomplicated Malaria - Usually presents with non-specific symptoms, such as chills, fatigue, diaphoresis, headache, nausea/vomiting, abdominal pain, diarrhea, arthralgias, and myalgias

On physical examination, the most prominent and common findings are splenomegaly and jaundice. This may be found even in patients without complicated malaria. Common lab abnormalities in patients with uncomplicated malaria include thrombocytopenia (very common) and anemia. Elevated liver enzymes, elevated creatinine, and raised WBC are also seen.
Complicated Malaria

  • Altered consciousness +/- seizures
  • Respiratory distress or acute respiratory distress syndrome (ARDS)
  • Circulatory collapse (shock)
  • Metabolic acidosis 
  • Renal failure, hemoglobinuria ("blackwater fever")
  • Liver failure
  • Coagulopathy with or without disseminated intravascular coagulation
  • Severe anemia or massive intravascular hemolysis
  • Hypoglycemia

2) Location of Acquisition 
Location of acquisition is relevant to understanding whether the organism is expected to be resistant or not. When in doubt, treat all falciparum organisms as if they are resistant to chloroquine. Chloroquine sensitive areas include Central America, Haiti, the Dominican Republic, former Soviet Union nations, and most of the Middle East. Anywhere outside of these regions is considered to be chloroquine resistant. Potential areas of resistance to artesunate include southeast Asia (especially Cambodia).

3) Species of Malaria

  • Non-falciparum (P. vivax, P. ovale, P. malariae, P. knowlesi) cause febrile illnesses but are rarely fatal.
  • However, P. falciparum infections are more virulent and can rapidly progress to fatal illness.
  • It is often impossible to determine the species of malaria infection on clinical grounds alone. This can be established through antigen testing and can sometimes be established based on the location of acquisition

    4) Level of Parasitemia 
    This refers to the percentage of RBCs carrying the organism and is established by examining the blood smear. In uncomplicated malaria, loads tend to be >5% in low transmission areas and 10% in high transmission areas.

    With chloroquine sensitive strains (determined by location of acquisition), the first line of therapy is either hydroxycloroquine or chloroquine. Side effects of these medications include GI upset, retinopathy, and pruritus.

    With chloroquine resistant strains, first line therapy usually includes Artesunate based therapy or Atovaquone-based therapy (+proguanil). Artesunate therapy is generally very well tolerated.

    What's that picture of the top? That's the building of the Panama Canal, 1904-1914. Malaria and yellow fever were rampant during the construction. Overall, 5600 people died building the canal.

    An excellent resource for malaria, as indicated by Dr. Paul Bunce: