Pleural Disease
Reference: https://www.nejm.org/doi/full/10.1056/NEJMra1403503
##Physiology
- generally, pleural pressure is below atmosphere.
- water seal = 1 way valve where fluid (gas or liquid) will flow if a small pressure is overcome (e.g. 2 cmh2o)
Why do elephants not have pleural spaces?
When snorkeling under water using their trunk, the pressure in the abdomen, vessels (including inside the pleura), and outside of thorax are at increase pressure due to the column of water (150 mmHg ish). Pressure in the air space is 0 (column of air is minuscule). Would lead to very large pressure gradient inside pleura = pneumothorax, vessel rupture (hemothorax), or huge transudate from starling forces. Thus, they have dense connective tissue their instead.
Question: will a primary spontaneous pneumothorax (without ongoing air leak) re-expand if just put to water seal? How could you accelerate resorption
- Answer: yes - because the valve is 1-way, if the pressure in the pleural space is ever 2 cm-H2O or more above atmospheric pressure, it will burp some air out and the lung will re-expand a bit. This will continue until the lung is expanded (or, if the lung is entrapped or trapped) the intra-pleural pressure decreases such that it never reaches more than 2 above atmospheric. This occurs even though the intrapleural pressure is usually below atmospheric. This would be accelerated by having the patient cough or forced exhalation (increases intrathoracic pressure).
How does an effusion / pneumothorax cause dyspnea?
Answer: by uncoupling the transfer of force (pressure) from the chest wall into the lung. Thus, breathing work is done by the muscles but not transmitted into expansion of the lung.
###Re-expansion pulmonary edema Question: How much fluid can you take off before causing this?
Not clear, but volume is probably not the right metric.
- probably a result of injury to the lung from transplueral pressure: think of this as analagous to VILI - instead of a high driving pressure via the positive pressure ventilator, it is a higher (more local) driving pressure as a result from lower pressure in the pleural space driving lung expansion.
- thus, it's reasonable to keep taking fluid as long as the intrapleural pressure is not dropping significantly below atmospheric.
###Non-expanding lung Reference; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1440-1843.2004.00630.x Respirology 2014
Lung entrappment = active physiologic process (e.g. inflammation) is limiting the ability of the lung to expand. However, may not be permanent. Manifests as an intrapleural pressure drop during drainage that is not quite so steep (because there is some give in the pleura). Effusion will be exudative.
Trapped lung = physical process (e.g. scarring) has occurred such that there is no chance that the lung will expand. Manifests in a very steep pressure drop as fluid is removed. Pleural fluid will be transudate. Should re-expand with pleural peel debridement (decortication)
What happens if you lower intrapleural pressure progressively more in these cases? (e.g. - 20 cmH2O)
- answer: as you continue to pull off fluid and lower the pressure in the intrapleural cavity (especially once the lung is no longer expanding) - you will then begin to transudate more and more fluid (due to starling forces) which will lead to re-accumulation of the fluid. Thus, applying the additional pressure is of limited benefit.
Pleural Plaques
If asbestos related, will generally be asymptomatic - located on the parietal pleura, posteriorly, rib space 5-8 (because a plaque on the parietal pleural will not cause symptoms - only on the visceral pleura = impairs expansion). Thus, if they have symptoms, should evaluate for pulmonary asbestosis.
Tension Pneumothorax
What circumstances is it possible for a tension pneumothorax to NOT be complete (e.g. collapsed into a fist sized ball at the hilum):
- Adhesions
- Positive pressure ventilation
- Bullous lungs disease with partially communicating airways
Otherwise, if pleural pressure is above atmospheric pressure, would continue to collapse.
Causes
Exudates (increased permeability / inflammatory) vs Transudates (increased starling forces / non-inflammatory).
Light's Criteria: calibrated to prioritize sensitivity to exudates (bigger consequence if missed) over specificity due to greater harm in missing.
Typical pleural fluid production = 0.26 ml/kg per day. Absorption by lymphatics can increase to 20x that.
- remember 5 6 7: 0.5 t prot plerual serum, 0.6 ldh pleural serum, 0.7 (actually 2/3) ldh ULN
- particularly less accurate in effusions due to HF after diuresis. In this case, use the gradient between albumin (1.2) or t prot (3.1) as a threshold for classifying.
- cholesterol level 0.3x serum (or cutoff of 45 mg/dl)can also increase accuracy of Lights.
"Pseudo-exudate" = transudate followed by diuresis - classically, a serum protein gap of 3.1+ or albumin gap of 1.2+ discriminates (though not particularly well)
JAMA 2014 Meta-analysis of Transudate vs Exudate determination
Empyema:
Data supports that chest tubes 12 french and smaller have higher failure rate draining empyema.
TPA/Dornase - MIST trial
Recurrent Malignant Effusions:
Options: tunneled pelural drain vs pleurodesis. Evidence suggests talc pleurodesis has higher incidence of adverse events and repeat procedures.
Pneumothorax
Large: 2-3 cm or larger.
Rec: small bore (14-french) chest bue rather than large bore.
3-5 days of air leak = persistent
Note: mechanism of post-procedure pneumothorax is usually not traumatic injury to the visceral pleura - more often related to non-expandable lung.