Last summer I had the privilege of climbing Denali (formerly known as Mt. McKinley), one of the world's seven summits and it was one of the hardest things I have ever done. 20,322'. My measured oxygen saturation on the summit was in the 80's. The air temperature was well below zero with wind speeds approaching 30-50 miles per hour. The only thing I could focus on was eating, drinking, staying warm, and keep on moving.
As a pediatric cardiac anesthesiologist, I started thinking about my single ventricle patients living with hypoxia every day, often with sats in the 70's and even high 60's on rare occasions. At high altitude, we are compromised by low barometric pressure rather than a low concentration of oxygen. Denali is unique in that because of its extremely north latitude (63 deg north) the cold temperature drops the barometric pressure even further. A mountain of higher altitude closer to the equator could have actually have higher barometric pressures than on Denali. But it is specifically the low pressure that makes climbers hypoxic and on Denali, it is more than most climbers have ever experienced. I recall many arguments over the years with young college students who would tell me that there is less oxygen on Mount Everest. Hah! It's all so beautifully illustrated in this simple yet painful equation:
PA02 = Fi02 * (Pb - PH2O) - (PaCO2/RQ)
On the top of Denali, the barometric pressure is around 360 mm Hg which translates into more than a 50% reduction in the oxygen available to your lungs.
But what does that feel like? After I got home and indulged in all the pleasures of modern life, I begin wondering what does it feel like for the single ventricle baby who goes home after their Norwood where their acceptable sats are between 75-85 %? Well from my experience, lower sats do not feel good, period. Although you can function, everything is hindered from your appetite to your cognitive skills. At altitude, everything harder. Sleeping above 18,000' can be a real nuisance. Imagine getting winded just rolling over in bed. And getting up to go to the bathroom outside? Forget about it! The energy it takes to get out of your sleeping bag and put on your warm layers followed by mountaineering boots is such a colossal use of your energy that it simply isn't worth it. That's why high altitude climbers have a bedside bottle. The gastrointestinal tract is less perfused so that even if you eat well, you may absorb less than half of the nutrients you consume. We had to hunker down at high camp for a few days waiting for the weather to clear and we just laid in our tents. Food was not especially great, we did not sleep that well, it was cold, very cold. And life was hard. I've always been fascinated by how remarkably well the body will tolerate low sats with good cardiac output and that high sats with poor cardiac output is a deal breaker. We see evidence of this when our single ventricles go through their glenn and fontan stages. If the pulmonary pressures are too high, a fontan an be fenestrated to allow a pop-off which lowers the saturation but maintains perfusion. I see it happen in the mountains all the time. Climbers function with mild hypoxia but they do not tolerate dehydration.
Summit day is such an unusual day. Its beautiful yet dangerous. It takes a tremendous amount of determination to not only train for a big mountain, but the hard work on the mountain is harder than any training you could simulate. I made attempts at cold winter camping alone, pulling a sled while wearing a heavy backpack. But you just cannot replicate these workouts with the same time of cold and altitude. When we reached the summit, we could only stay for 10 minutes to take some photos, eat and drink. You can't even take your glove off for 1 second as it was instantly numb. And you could not miss an opportunity to drink and eat. Your system had to be perfectly efficient. It became so abundantly clear to me more so than ever that we were really right on the edge of danger. This was an extreme environment and the margin of safety was always very slim. We gave a few celebratory hugs and started back knowing that reaching the summit meant were only halfway home. The most dangerous part of an expedition was the descent and it was very long. Every step counted as we were on steep ice. A total of 13 hours was our round trip summit day. And every second was crucial for any misstep meant a fall and risk of major injury to ourselves and our entire team. It's hard to think about being able to concentrate in the operating room for that long and thats under the best of conditions. I am amazed at how we achieved such a feat of extraordinary concentration under such harsh conditions.
But one of things I had to remind myself was that single ventricle babies acclimatize starting at birth. Although they have a chance to get used to their hypoxic state, they have to live with it constantly. I guess we shouldn't be surprised then that most of our Norwood babies need feeding tubes. They just don't eat well nor gain weight without some help. It's the same high up on the mountain. I knew I had to eat, but my appetite was terrible. I forced the food down anyway but every day at high altitude, we all lost weight.