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Nick Mark MD

@nickmmark

Apr 30

Shortly before 3am on June 4, 1993, a mechanic at Miami airport looked in the wheel well of a DC-8 cargo jet from Bogotá. He saw the body of a teenager, curled in a ball, wearing only a t-shirt and shorts and frozen like an "ice cube."

The first paramedic pronounced him dead. The second found a weak pulse.

Somehow he had just survived 5 hours at 35,000 feet without heat or air pressure.

This should have killed him three different ways.

A🧵& blog post on how he survived.
1/

At 35,000 ft, there are three simultaneous killers:
🫁 Hypoxia: PO₂ is ~37 mmHg, well below the consciousness threshold of ~60 mmHg. Most peopple lose consciousness is 15-30 seconds. Even fully acclimatized Everest summiteers (at 29,000) survive only by driving PaCO₂ to ~8 mmHg through maximal hyperventilation.

🥶 Hypothermia: Ambient temp is –55°C. Accidental hypothermia causes fatal arrhythmia below ~28°C core temp. The coldest recorded accidental hypothermia survivor (13.7°C) lived only because of ECMO.

💥 DCS: Barometric pressure 179 mmHg (23% of sea level). The risk of decompression sickness and nitrogen gas embolism approaches 100% above 30,000 ft without a pressure suit.

No reasonable physiologist, handed these parameters, would predict survival. Yet somehow a 17 year old stow-away survived all three.

2/

The key is that hypothermia and hypoxia are mutually protective. The mechanism:
1️⃣ Hypoxia disables the thermostat
The preoptic anterior hypothalamus is exquisitely sensitive to hypoxia. As PaO₂ falls during ascent, it loses the ability to defend core temperature. The body becomes poikilothermic: temperature tracks the environment and the stow-away gets cold without shivering.

2️⃣ Hypothermia suppresses VO₂
The Q10 for brain CMRO₂ is 2.2. By the time core temp hits ~27°C (threshold for unconsciousness), brain O₂ consumption is ~45% of baseline. Demand meets the catastrophically low supply.
Cardiac surgeons exploit this in deep hypothermic circulatory arrest (DHCA), cooling the brain to 15-18°C to permit operating on a bloodless field.
The stowaway essentially did this to himself!

3/

Read 7 tweets

Nick Mark MD

@nickmmark

Sep 5, 2025

Lots of news articles reporting "Smartphone use on the toilet increases risk of hemorrhoids" citing a small single center study.

Great headlines but also a textbook example of *reverse causation* - a common methodological flaw in observational studies

A 🧵

Reverse causation occurs when we flip the arrow of cause→effect.

Protopathic bias is a subtype: An exposure (often a treatment/behavior) is started because early symptoms are already present, making it look like the exposure caused the outcome.

2/

A common example of reverse causation/protopathic bias is increased inhaler use --> increased risk of asthma hospitalization.

Did the inhaler use cause the hospitalization?

No! The person was developing symptoms which is why they were using the inhaler...

More examples:

3/

Read 12 tweets

Nick Mark MD

@nickmmark

Aug 19, 2025

This is a fascinating case:

A 60 yo man presents with paranoia, new-onset facial acne & cherry angiomas, fatigue, insomnia, ataxia, and polydipsia.

He has an extremely unusual diet.🧂

His labs are👇 (note that anion gap!):

What could be going on?
1/

Turns out he was inspired by a conversation with chatGPT to eliminate all sodium chloride from his diet.

🧪 He decided to replace sodium chloride with sodium bromide instead.

This had some unusual effects…

2/

Bromide toxicity - or bromism - is extremely rare today, but it was once quite common.

Prior to modern alternatives, lithium and potassium bromide were widely used as sedatives, with frequent side effects.

At one point 5-10% of the psychiatric admission were due to bromide!
3/

Read 6 tweets

Nick Mark MD

@nickmmark

Jun 1, 2025

Well designed RCT shows patients randomized to an exercise program had substantially improved survival after adjuvant chemotherapy for colon cancer.
- 5 yr disease-free survival 80.3% vs
73.9% (HR 0.72)
- 8 yr overall survival 90.3% vs 83.2% (HR 0.63)

This is groundbreaking!
1/

Some deets on the CHALLENGE trial

A 55 center trial done over 15 years (2009-2024) that randomized n=889 people with resected colon cancer after adjuvant chemotherapy to either:
- participate in a structured exercise program
- or to receive health-education materials alone

2/

The intervention was pretty comprehensive:
Personal activity consultant (PACs) - essentially trainers - got to know the participant 1:1, introduced them to the gym and came up with personalized activity goals
Regular every 2 week sessions helped participants reach the goals

3/

Read 16 tweets

Nick Mark MD

@nickmmark

May 18, 2025

Tragic news today about former president Biden's prostate cancer diagnosis. I wish him well.

As someone who follows presidential health reporting, I noticed something odd: unlike his predecessors, Biden's physician's never reported PSA.

How to interpret this absence? A🧵
1/

There are two possibilities:
1️⃣ Biden’s PSA was never checked
2️⃣ Biden’s PSA was checked but it wasn't reported

Strictly speaking, not checking PSA could be a medically correct option. Whether or not to test PSA is a complex question and is not the topic of this thread.

2/

Like many VIPs, presidents tend to have excessive testing that is not always strictly evidence-based.

For example, Bush 43 had an exercise treadmill test and a TB test for no apparent reason.

3/

Read 13 tweets

Nick Mark MD

@nickmmark

May 4, 2025

In honor of #MayThe4thBeWithYou let's consider the most difficult airways in the Star Wars universe:

1. Darth Vader
Species: human
Vader presents several challenges: Vent dependent at baseline, airway burns from Mustafar, limited neck mobility.
Discuss GOC before saving him

2. Fodesinbeed Annodue
Species: Trog
All airways require teamwork, but intubating Fodesinbeed Annodue's two heads really will require two operators.
Consider double simultaneous awake fiberoptic intubation
Be sure to consent both heads.

You will never find a more wretched hive of scum & challenging airways than Mos Eisley (except maybe at Jabba's)

3.Greedo
Species: Rodian
Micrognathia, posterior airway, no nasal intubation, green skin so no pulse ox
Approach: VL + bronchoscope. Intubate quickly (shoot first)

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