Saturday, February 27, 2021
Friday, February 12, 2021
|Proning is the latest modality for augmenting ventilator
therapy. Some vintage measures were not so effective
Recognition of acute respiratory distress syndrome (ARDS) in the early 1970s and treatment with Engstrom ventilators was a game changer, with mortality plunging from 100% to about 40%. As more experience was gained mortality plunged even further. Deducing what worked and what didn't with ventilators was a rocky road.
It's human nature that clinicians faced with an unstable, critically ill patient want to do everything possible to rescue the person. We referred to situations like this as kitchen sink medicine when just about anything and everything was added to the armamentarium. Sometimes, desperation in medicine results in untoward outcomes. I'm thinking about radical mastectomies for all breast cancers and surgeries like hemipelvectomies. Some pioneering accompaniments to mechanical ventilation bore little fruit, and did little to avert a vegetative outcome, but just about anything seemed worth a try when the clinical situation seemed so bleak.
Early practitioners in the art of mechanical ventilation were not like the experienced critical care medicine experts of today, but surgeons and anesthetists who saw the benefit ventilators made with ARDS treatment. They were drafted into the new role of managing ventilators and much of what ensued was on the job training.. Science was sometimes, in short supply when empirically based notions were applied as we shall soon see.
Everyone takes in a deep breath from time to time, so why not try this with ventilated patients? It was fairly easy to adapt those ancient, chugging, Engstroms to deliver an occasional deep ventilation, all it took was some monkeying around with that gizmo on top of the Engstrom that looked like an expresso machine, and PRESTO, the "sigh" was invented. A sigh was an occasional cycle with increased tidal volume and the frequency was highly variable.
Intermittent sighs were a source of dread for sedation deprived ventilator patients, imagine having a hurricane force of air, unpredictably, blasted into your chest via a skinny little tube. A United airlines pilot recovering from pneumonia said, "Now I know what it's like to suck on an engine of a 747." Unpleasant does not begin to describe the patient experience when the sigh cycle kicked in.
Ventilator driven sighs never really caught on in the hospital where I worked. Surgeons blamed the sigh cycle for putting undue stress on suture lines and in the event of a rare evisceration, the sigh was always blamed. The elevated intra thoracic pressure was also blamed for barotrauma to vulnerable alveoli.
Positive end expiratory pressure or PEEP evolved to be the replacement for sighs. PEEP entailed maintaining a low steady pressure (5cm/H20) in the lungs just slightly above ambient atmospheric pressure. Some overzealous physicians figured that if a little bit of PEEP was good, then more is even better. Super PEEP was born with pressure of 25 cm and above which was like blowing an automotive tire up to 100psi. Talk about a rough ride!
Super PEEP worked for a time until complications surfaced. High intra thoracic pressure compromised blood flow in the great vessels which caused big problems. A hemodynamically unstable critically ill patient is not a good thing. Renal problems often developed as a result of compromised circulation. Super PEEP was not such a good idea, but did persist when technology for cardiac output monitoring was developed which enabled fine tuning to allay complications. The father of super PEEP at Montefiore Hospital in Pittsburgh was Arnold Sladen MD. He was either a hero or a scoundrel depending on who you talked to.
Endotracheal tubes exert a seal by an inflatable cuff which contacts and forms a seal with the trachea. Sustained pressure exerted by the cuff can limit the amount of time a patient can be maintained on the ventilator before an invasive tracheotomy must be done. Long term unremitting pressure from the cuff can cause problems. Intermittent endotracheal tube cuff inflation was thought to be a way a kinder gentler way of sealing an airway. The cuff was inflated only on inspiration.
Intermittent endotracheal cuff inflation required some complex additional equipment and lengthening the inflation tubing increased dead space and exacerbated the potential for leaks. There was also the ever present risk of aspiration when the cuff was deflated. This overly complicated modality was usually abandoned with much haste as it just didn't work very well.
There is that old joke about anesthetists passing gas, but in reality, they are passing gases. Fiddling around with the inspired mix of gases was second nature when novice anesthesia folks began overseeing ventilator therapy. Traces of helium mixed with the FiO2 were thought to aid in alveolar dispersion, but in the long run seemed to make little difference.
Life on a ventilator was unpleasant at best. Before propofol came along, anesthetists would sometimes "trace an agent," or install an in line vaporizer to sneak in a whiff of halothane to settle things like "bucking" down. Progress in IV sedation put a halt to anesthesia vaporizers on ventilators except, of course, in the OR.
Ventilators are an unforgiving entity and that ominous click...hiss...pause always overshadowed the cheerful cacophony of melodious alarm tones, quickly becoming the dominant noxious noise in the ICU. Ventilators really strummed a different sort of tune that frequently foreshadowed impending doom.
In the rapidly gathering storm leading up to intubation and subsequent ventilation, impending consequences were often conveniently overlooked. Rendering someone mute with an endotracheal tube and lashing them to a machine forcefully converting air into breathe has all the grace of getting clobbered by a linebacker on steroids.
Late at night, numbed by fatigue induced hopelessness, strange thoughts percolated through my mind when caring for a poor soul on a ventilator. We all want a nice linear, progressive, and predictable path from illness to health. Sometimes, though, ventilator patients are too far along on their journey to the other side. If I'm ever in this predicament, I hope my caregivers reconsider my path and don't interfere with my final journey.
When I'm on that peaceful river journey to the other side, I better not come across a ventilator masquerading as a life boat!