Mechanical Ventilation Basics

ventilator

Volume control (VC) and pressure control (PC) are two common modes of positive pressure mechanical ventilation. In VC, the clinician sets the tidal volume that is given for every breath; pressure is allowed to vary over the course of the breath. In PC, the ventilator is programmed to deliver the same pressure throughout inspiration, so tidal volume is allowed to vary based on the pressure and timing settings, as well as the patient’s own lung compliance.

The timing of ventilation can be set according to a trigger. Continuous mandatory ventilation (CMV) involves setting the respiratory rate and having the ventilator deliver breaths at exactly that rate. This is generally used in paralyzed patients (e.g., general anesthesia), where the patient is not expected to trigger any breaths. In Synchronized Intermittent Mandatory Ventilation (SIMV), mandatory breaths are still given but they are synchronized to the patients’ own respiratory efforts (if present). Also, the patient is allowed to take additional breaths on their own. SIMV is often used to wean patients from the ventilator, by decreasing the rate of mandatory breaths and having patients take more of their breaths spontaneously.

Pressure support (PS) is another mode that is used for weaning. No mandatory breaths are programmed. The patient actively takes their own breaths, and the ventilator simply gives an additional boast of inspiratory pressure to help them out.

Positive End Expiratory Pressure (PEEP) is a setting that is used to prevent alveolar collapse, increase functional residual capacity, and generally improve gas exchange. PEEP involves programming a small amount of additional airway pressure (often ~5-10 cmH2O) to be present at the end of expiration.

  • Nugent K, Nourbaksh E (Eds.). 2011. A bedside guide to mechanical ventilation. Createspace.
  • Owens W. 2012. The ventilator book. First Draught.
  • Kacmarek RM, Hess DR. 2008. Mechanical ventilation for the surgical patient. In: Anesthesiology (Longnecker DE, Brown DL, Newman MF, Zapol WM, Eds.). McGraw Hill, New York.

 

Assessing the severity of pneumonia – CURB-65

When you see someone with suspected pneumonia you think “well, they have pneumonia, but how sick are they reaaalllly?” (you would ideally think this while cocking an eyebrow and tilting your head slightly)

To help you determine whether you send the person home on their merry way or admit them to hospital, you can use the CURB-65 pneumonia severity score.

  1. Confusion
  2. Urea >7
  3. Respiratory rate >30 breaths per minute
  4. Blood pressure <90 systolic or <60 diastolic
  5. 65 years or older

Each of the five criteria is worth 1 point, add them together:

  • 0 – 1 points: mild pneumonia (there’s only a 3% chance that your patient will go home and die, leaving you racked with guilt)
  • 2 points: moderate (9% risk of dying) – you should probably hospitalize this person
  • 3 – 5 points: severe (13-53% chance of dying) – you better admit this person, maybe even to the ICU

Lim WS, van der Eerden MM, Laing R, et al. Defining community acquired pneumonia severity on presentation to hospital: an international derivation and validation study. Thorax. 2003 May;58(5):377-82.
Lim WS, Baudouin SV, George RC, et al. BTS guidelines for the management of community acquired pneumonia in adults: update 2009. Thorax. 2009 Oct;64 Suppl 3:iii1-55.

Lung surface anatomy and chest tubes vs needle decompression

Surface anatomy of the lungs

  • Lungs extend from about 2 cm above the clavicle down to the 6th rib in the midclavicular line and 8th rib in the midaxillary line
  • The oblique fissure goes from the 6th rib midclavicular line to T3 in the back
  • The horizontal fissure (only on the right) starts at the 4th rib at the sternum and then meets the oblique fissure at the 5th rib in the midaxillary line.
  • The pleura generally is 2 ribs below

Chest Tube

Insert a chest tube in the 4th or 5th intercostal space in the anterior axillary line. When making the incision, make it one rib below the intercostal space you want to insert the tube into. Also, remember to go above the rib, as the neurovascular bundles travel along the underside of the ribs.

These can be done to relieve a pneumothorax, drain a malignant pleural effusion, drain a empyema, or drain a hemopneumothorax. They can also be placed post-operatively following a thoracotomy, esophagectomy or cardiac surgery.

Needle Decompression

These are used in a pinch when a patient is suspected to have a tension pneumothorax and needs immediate decompression. A 14 or 16 gauge needle is inserted above the 2nd or 3rd rib in the midclavicular line.

A tension pneumothorax is recognized by:

  • Dyspnea
  • Hypotension
  • Decreased breath sounds on the affected side
  • Distended neck veins
  • Trachea deviating away from affected side

 

Cephalosporins (generations and spectrum of activity)

Cephalosporins work much like penicillins, inhibiting peptidoglycan cell wall synthesis in bacteria (remember those sites of action and mechanisms?)

Of course the issue is that they just keep making new cephalosporins and each generation is a little bit different in terms of its spectrum and whether it’s better at fighting Gram positive or Gram negative bacteria. Generally the newer the generation, the more broad spectrum and less Gram positive coverage. To add another layer to the confusion, there are separate oral and IV cephalosporins for each generation and all of the cephalosporins are usually recognizable by starting with “CEF-” or “KEF-” (except for Suprax and Ancef, who ever came up with those brand names didn’t get the memo)

 

Asthma Triad

Asthma is a reactive airway disease where there is inflammation of the bronchioles.

Patients usually have the characteristic symptoms of:

  • Wheeze (high-pitched, usually upon exhalation)
  • Cough (worse at night)
  • Shortness of breath or difficulty breathing

These symptoms usually have characteristic triggers and are episodic.

The triad of atopy, nasal polyps and ASA sensitivity (also known as Samter’s triad) is the hallmark for Aspirin Exacerbated Respiratory Disease, and affects around 5-20% of asthmatics. Patients with this condition experience a worsening of symptoms 30 min to 3h after ingestion of aspirin or other NSAIDs.

Acid/Base (alkalosis vs acidosis, metabolic vs respiratory)

This is the general way to approach an acid-base disturbance. They’re not really as bad as they seem at first. You just need to remember that CO2 is acidic and HCO3- is basic. So an increase in CO2 makes the body acidotic and an increase in HCO3- makes the body alkalotic.

It’s also good to remember to calculate the anion gap when doing these calculations.

AG = Na – (Cl- + HCO3-) it’s just the cations minus the anions. If this gap between the cations and anions is large, it means that the anions are stacking their team and have an extra anion helping out.

The classic mnemonic is MUDPILES

  • Methanol
  • Uremia
  • Diabetic ketoacidosis
  • Paraldehyde
  • Isopropyl alcohol
  • Lactic acidosis
  • Ethylene glycol
  • Salicylates

If the anion gap is big, it’s good to look at the ratio between the change in the gap and the change in the HCO3-.

  • Increase in AG < decrease in HCO3- = coexisting non-anion gap metabolic acidosis
  • Increase in AG > decrease in HCO3- = coexisting metabolic alkalosis

Antibiotic classes and mechanism

These are the different classes of medications and their mechanism of action. I’ve sorted them by which ones are exclusively Gram + or Gram – and which ones do both.

Here’s a handy mnemonic for remembering the antibiotics that act on the ribosomes (I find they’re the most confusing to remember):

Buy AT 30
Aminoglycosides
Tetracyclines

CELL for 50
Clindamycin
Erythromycin
Linezolid

Sites of antibiotic actions

Antibiotics are the current bane of my existence. Most medical conditions there are a couple of different classes of drugs, but overall things are straight forward. Not so with antibiotics, not only are there a bunch of different classes, they all have different indications!

This is part 1 of my antibiotics doodles. This one outlines where the different commonly used antibiotics act on the cell, the class they belong to, and whether they are used mostly for Gram + (Staph, Strep), Gram -, or a little bit of both. Keep in mind that the bacterial cell in the drawing is a Gram + coccus.

Part 2 will go into a little more detail about the different classes.

Pulmonary function test patterns

Same old pulmonary function tests (PFTs) that we’ve all probably memorized countless times. The only thing to remember that is a little odd is that you can get a restrictive type pattern of the FEV1/FVC ratio (but not quite the perfect flow loop) if you have air trapping in COPD.

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