Two to four types of necrotizing fasciitis

The first two are the classics, defined by a 16-patient case series:

  1. Type I: polymicrobial infections caused by Enterobacteraciae, non-Group A streptococci, and anaerobes
  2. Type II: monomicrobial infections, usually Gram-positive, and usually Group A streptococci

The next two were added later on:

  1. Type III: monomicrobial Gram-negative infections, generally caused by water-associated Vibrio vulnificans or Aeromonas hydrophila
  2. Type IV: fungal, caused by Candida species, and exceedingly rare

I’ll cite this review for those four, although there appears to be some disagreement on what constitutes Type III; many other articles refer to Type III as gas gangrene caused by Clostridium species. In this matter, I’ll defer to Mandell: it says Vibrio, so I’ll say Vibrio.

Three types of peritonitis

Peritonitis refers to inflammation (usually infectious) of the peritoneum. Infectious peritonitis is broken into three classes, although the terms don’t necessarily make sense:

  1. Primary peritonitis: also called spontaneous bacterial peritonitis (SBP), this is a monomicrobial infection of ascites usually caused by enteric bacteria and Strep. pneumoniae. It can present with minimal or no abdominal symptoms, so it should be considered in almost everyone who has ascites and is unwell. Ascitic fluid classically has more than 250 neutrophils and a positive culture, but either of these alone also constitutes primary peritonitis. It’s called primary because the source of the infection is unclear*.
  2. Secondary peritonitis: this is an intraabdominal infection secondary to a clear process, usually trauma or surgical misadventure. The usual bacteria is whatever is in poop (so your enteric Gram-negatives and some Gram-positives).
  3. Tertiary peritonitis: this is what we call secondary peritonitis when it’s gone on for too long. The organisms that grow in culture are often low-virulence bacteria that may or may not be contaminant or colonization, though yeast and resistant organisms can also be found. Basically, it’s secondary peritonitis after you’ve killed off all of the usual suspects with course of broad-spectrum antibiotics.

* It’s poop. It’s just not by direct spread, like you might think, but rather translocation of bacteria into the lymphatics, which drains into the bloodstream via the thoracic duct, then hematogenously seeds the ascitic fluid.

Read more: Chapter 76: Peritonitis and Intraperitoneal Abscesses, in Mandell Principles and Practice of Infectious Diseases 8e.

The five causes of eosinophilia

A short mnemonic for eosinophilia is CHINA:

  • Connective tissue diseases, like eosinophilic granulomatosis with polyangiitis (aka Churg-Strauss) and rheumatoid arthritis
  • Helminths, especially Strongyloides
  • Idiopathic, which is simply called hypereosinophilic syndrome
  • Neoplasms, like Hodgkin lymphoma, CML, and some solid-organ cancers
  • Allergies etc, which includes asthma and drug-induced eosinophilia (don’t forget about DRESS)
    • Also, Addison’s disease, for some reason

Of interest, helminth infections can cause a Loffler’s syndrome, also known as eosinophilic pneumonia, where you see a peripheral eosinophilia on the blood count and pulmonary infiltrates on the chest x-ray.

Read more: Eosinophils in Chapter 80: Disorders of Granulocytes and Monocytes, Harrison’s Principles of Internal Medicine 19e.

Why you might avoid beta blockade in severe aortic insufficiency

It comes down to coronary perfusion.

Heart muscle needs blood and oxygen like any other muscle. In the case of the myocardium, coronary perfusion occurs during diastole, when the muscle relaxes. The pressure gradient between the aortic and left ventricular pressures drives blood forward through the coronaries. The coronary perfusion pressure, in the context of chest compressions, is that difference between at end-diastole, for example.

In a normal heart, the gradient is present throughout diastole, so the myocardium gets lots of good, oxygenated blood throughout that part of the cycle (Figure 1). In severe aortic insufficiency, though, the blood from the aorta quickly rushes back into the ventricle during diastole, dropping the gradient between aortic and ventricular pressures to zero (Figure 2). The myocardium, therefore, only has a short period of time, in early diastole, to get that oxygenated blood.

Beta blockade prolongs diastole and therefore prolongs the time that the coronaries aren’t being perfused. Based on that, some cardiologists (including where I trained) avoid beta blockers with the thought that they decrease coronary perfusion in severe AI and therefore promote ischemia. However, beta blockers also decrease myocardial oxygen demand.

So, as with most things in medicine, it’s a balance, and there are few good clinical studies.

Figure 1: Coronary perfusion in a normal heart

Figure 2: Coronary perfusion in severe aortic insufficiency

Read more: Aortic Regurgitation in Chapter 283: Aortic Valve Disease, Harrison’s Principles of Internal Medicine 19e.

The above Wiggers diagrams are modifications of: adh30 revised work by DanielChangMD who revised original work of DestinyQx; Redrawn as SVG by xavax – Wikimedia Commons: Wiggers Diagram.svg, CC BY-SA 4.0

What causes alcohol withdrawal seizures?

In my simplified view, it comes down to GABA and NMDA receptors.

Alcohol promotes GABA receptors (which are neuroinhibitory receptors) and inhibits NMDA receptors (neuroexcitatory receptors). Chronic alcohol use upregulates both, but maintains a semblance of balance between the two.

Following alcohol withdrawal, the upregulated NMDA receptors are no longer inhibited by alcohol, so their overall activity increases. The GABA receptors are no longer being stimulated by alcohol. This imbalance of GABA and NMDA signals (with NMDA activity much greater than GABA activity) lowers the seizure threshold.

The acute management and prophylaxis of withdrawal-related seizures, therefore, is typically to promote GABA activity with benzodiazepines, thus restoring the balance between GABA and NMDA signals and increasing the seizure threshold.

Read more: Pharmacology and Nutritional Impact of Ethanol in Chapter 392: Alcohol and Alcoholism, Harrison’s Principles of Internal Medicine 18e.

The six broad-spectrum antiepileptics to know

Broad-spectrum antiepileptic drugs (AEDs) can treat both focal and generalized seizures, and are often used when patients have an undifferentiated seizure disorder. Alphabetically, they are:

  1. Clonazepam
  2. Lamotrigine
  3. Levetiracetam
  4. Topiramate
  5. Valproic acid
  6. Zonisamide

I even made up a mnemonic: Cloned Lambs Love To Visit Zoos.

Also, rufinamide. Personally, never heard of it.

 

What use is the alveolar gas equation?

None! Just kidding; it’s useful for evaluating hypoxia because it can easily rule out hypoventilation as the cause.

For background, the alveolar gas equation is a way of calculating what the level of oxygen is in the alveoli, given the atmospheric pressure (pATM, usually 760mmHg), the fraction of inspired oxygen (FIO2, which is 21% for room air and increases if they’re on supplemental oxygen), the pressure of water vapour in the lungs (pH2O, usually 47mmHg), the arterial CO2 (paCO2, taken from your ABG), and the respiratory exchange ratio (RER, the amount of oxygen exchanged for carbon dioxide in one breath, usually 0.8). The equation is:

pAO2 = FIO2(PATM – pH2O) – paCO2(1 – FIO2[1 – RER]) / RER

The use of the alveolar gas equation is in the A-a gradient, the difference between what the alveolar gas equation says your alveolar oxygen is and what your ABG says your arterial oxygen is. If there’s lots of oxygen getting to the alveoli, then you should have lots of oxygen in the blood. A normal A-a gradient is approximately (age / 4) + 4, so it should be about 9 for a healthy young 20-year old and 24 for an 80-year old.

How is the A-a gradient useful? Well, there are only two things that cause hypoxia with a normal A-a gradient: hypoventilation (not moving enough air), and decreased PiO2 (that is, high altitude). Since I’m rarely doing my ABGs on a mountaintop, the A-a gradient is basically a quick and easy way to rule out hypoventilation as the cause of their hypoxia.

Read more: Adequacy of Ventilation in Chapter 306e: Disturbances in Respiratory Function, Harrison’s Principles of Internal Medicine 19e.

Three ways that supplemental oxygen causes hypercapneia

Supplemental oxygen can sometimes cause carbon dioxide to increase to dangerous levels, usually in patients with chronic lung diseases like COPD. I was originally taught that this was due to the extra oxygen blunting their respiratory drive, but it turns out that’s not the whole story. The mechanisms, in order of importance:

  1. V/Q mismatch: Lungs autoregulate their circulation to match ventilation, so that low-oxygen blood only goes to the parts of the lung that have oxygen, which are usually the well-ventilated parts of the lung with lots of air moving in and out. If there’s extra oxygen diffusing to places that are poorly ventilated, it can cause vasodilation within that poorly-ventilated lung. As a result, blood is going to parts of the lung that aren’t well ventilated and can’t blow off CO2. Basically, it increases perfusion to physiologic dead space. Not good for getting rid of carbon dioxide.
  2. Haldane effect: hemoglobin binds both oxygen and carbon dioxide in order to deliver oxygen from the lungs to the tissue and take CO2 from the tissue to the lungs. Unfortunately, when there’s high O2, the Haldane effect means that hemoglobin isn’t as good at carrying CO2. When there is also poor ventilation, this causes CO2 to build up in the blood.
  3. Blunting of respiratory drive: respiratory drive is controlled by oxygen-sensing parts in the periphery and pH-sensing parts in the brain. It was once thought that chronic CO2 retainers lose their pH-based respiratory drive, and require their hypoxic drive to be working well in order for them to blow off any CO2. It turns out that this isn’t the case.

Read more: Abdo WF, Heunks LM. Oxygen-induced hypercapnia in COPD: myths and facts. Critical Care. 2012;16(5):323. doi:10.1186/cc11475.

The five causes of hypoxemia

There are five basic processes that result in hypoxemia:

  1. Ventilation-perfusion (V/Q) mismatch: air isn’t getting to the parts of the lung that the blood is passing through. Causes includes pneumonia, asthma, COPD, ARDS, pulmonary embolism, heart failure, and interstitial lung diseases. V/Q mismatches usually respond well to supplemental oxygen.
  2. Right-to-left shunt: blood bypasses the lung altogether. This can happen due to an anatomic shunt in the heart itself as in an ASD, VSD, or PFO or in the lung vasculature through an AVM, or as a physiologic shunt due to severe pneumonia, ARDS, heart failure, or atelectasis. Because blood isn’t getting to the alveoli, supplemental oxygen doesn’t help–all it does it bring O2 to places without blood flow.
  3. Hypoventilation: the patient just isn’t moving enough air. It’s associated with an increase in CO2, and causes include CNS causes (sedation, stroke, tumours), neuromuscular disorders, airway obstruction (COPD, asthma, laryngospasm), and dead space ventilation.
  4. Diffusion defect: oxygen isn’t getting from the air to the blood. Causes include emphysema, PJP, atypical pneumonias, and pulmonary fibrosis.
  5. Low inspired oxygen content: high altitude! And not much else.

Read more in Chapter 49: Hypoxia and Cyanosis in Harrison’s 19e.

Speaking of hypoxemia, an anaesthesia fellow turned me on to an article from a few years back, Arterial Blood Gases and Oxygen Content in Climbers on Mount Everest by Grocott et al. (NEJM 2009), that includes the following table:

Everest ABGs Table 2

Those are some wild ABGs! If I saw those in a patient, I would be calling the ICU.

Measuring BNP

If atrial natriuretic peptide (ANP) is released by distension of the atria, then brain natriuretic peptide (BNP) must be released by distension of the… ventricles? Don’t think about it too hard.

BNP and the N-terminal fragment of proBNP (NT-proBNP) are useful for diagnosing heart failure, especially when there’s uncertainty. In fact, at least one trial1 has suggested that it should be used for screening of high-risk patients to identify those who have preclinical heart failure (if such a thing can be said to exist).

Measurements of BNP and NT-proBNP can be used interchangeably, but have different cutoffs. The cutoffs stratify people into three categories: low likelihood of having heart failure (good LR-), high likelihood (good LR+), and intermediate (not terribly helpful).

Besides diagnosis, it can also be used for prognosis as well as for tracking response to treatment. That last one hasn’t made it into the guidelines yet.

Read more:

Works cited:

  1. Ledwidge M, Gallagher J, Conlon C, et al. Natriuretic Peptide–Based Screening and Collaborative Care for Heart Failure: The STOP-HF Randomized Trial. JAMA. 2013;310(1):66-74. doi:10.1001/jama.2013.7588.

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