Help! I Can’t Breathe! – Simple Asphyxiants

It is August 26th, 1986 in Nyos, Cameroon, a town next to a lake, which underwent a limnic eruption – essentially a belch of carbon dioxide from the lake – sending a wave of carbon dioxide through the town at approximately 60 mph killing people up to 15 miles away.^1,2

So what is a “simple” asphyxiant? Simple asphyxiants are generally inert gases. Common examples are noble gasses (helium, argon, neon, etc…), carbon dioxide (dose-dependent), nitrogen (dose-dependent), and many others. Simple asphyxiants are gases which reduce the partial pressure/concentration of oxygen by displacing oxygen in the air, see Figure 1. When simple asphyxiants displace oxygen in the alveolar space, there is a reduction in gas-dissolved oxygen resulting in both decreased oxyhemoglobin and aerobic cellular respiration.³ In contrast, complex asphyxiants have other physiologic effects, such as setting off signaling pathways or causing direct tissue damage.

Figure 1. A) normal mixture of Nitrogen, Oxygen and other gases; B) normal air mixed with a simple asphyxiant.

As mentioned, one of the most famous examples of mass poisoning by an asphyxiant was by carbon dioxide due to Lake Nyos. Only 6 of the 800 residents of the town of Nyos survived. In total, the gas killed 947 people and 3,500 livestock (not including wild animals). The treatment for simple asphyxiants is the removal of the patient from the gas and oxygenation with 100% O2. However, time is what’s most important and many of these people die very quickly or suffer anoxic brain injury.

Table 1 Clinical Findings at Specific Inspired Percentage of Oxygen at Sea Level³

The clinical manifestations are based on the length of exposure and partial pressure of oxygen. Table 1 lists symptoms at different percent inspired oxygen concentrations at sea level. As you might expect, these symptoms are very similar to what mountain climbers experience. For example, at the summit of Mt. Everest, the FiO2 is only 6%, and with hyperventilation (reduces pCO2), you get an alveolar partial pressure of 35 mmHg compared with 150 mm Hg at sea level.⁴ 

Simple asphyxiants other than CO2 don’t affect CO2 exchange, so pCO2 should remain normal until severe hypoxemia sets in. An interesting consequence of this is that simple asphyxiants, excluding CO2, won’t cause an early change in respiratory rate, as respiratory rate is predominantly driven by CO2 concentration.⁵

Carbon dioxide is actually not a true “simple” asphyxiant, due to its many cellular responses in the human body, including direct action on multiple signaling pathways and respiratory drive. For this reason, it is usually considered a “complex asphyxiant.” However, in high concentrations, as in the Lake Nyos incident, it may manifest as a simple asphyxiant and can cause death within minutes.^3,6 

Nitrogen gas exposure at low levels presents similarly to ethanol intoxication, causing decreased mentation, giddiness, and euphoria. Nitrogen gas, like CO2, also has direct cellular effects making it a complex asphyxiant. At higher levels, nitrogen exposure causes depressed mental status. This is often seen in divers as nitrogen narcosis requiring the substitution of a more inert gas such as helium in diving gas mixtures.^7,8 Also like CO2, in massive exposures, it functions like a simple asphyxiant.³ 

Your patient is one of the very few lucky residents of Lake Nyos. He was extricated, supplied with supplemental O2, and recovered with no deficits. Most of the other residents weren’t so lucky.

1. 1. 1. Scene safety is extremely important with simple asphyxiants; rescuers will often fall victim to the very same exposure
   2. 2. Removal from the area and supplemental oxygenation are the key aspects of management
   3. 3. Most simple asphyxiants won’t cause dyspnea, tachypnea, or breathlessness
   4. 4. Labs will reflect hypoxia and decreased cellular respiration
   5. 5. Most exposures are occupational
   6. 6. CO2 and nitrogen at lower concentrations are complex asphyxiants but in massive exposures, they behave like simple asphyxiants