Carbon dioxide is a natural by product of our cell metabolism. It is not merely a waste product, as it is usually described, but an important cerebral vasodilator for the brain.

1] CO2 helps improve blood flow to the brain.

Why is blood flow to the brain important? Cerebral circulation is the blood flow in your brain. It’s important for healthy brainfunction. Circulating blood supplies your brain with the oxygen and nutrients it needs to function properly. … In other words, it needs a lot of blood circulating through it to stay healthy.
Cerebral Circulation: Definition and Patient Education

2] Carbon Dioxide also helps the lungs;

Carbon dioxide enhances delivery of oxygen to the tissues of the body – an increase in CO2 will decrease the pH and induce oxygen unloading. 2, 3-DPG is a glycolytic intermediate produced in higher amounts in the low ATP and high acid state. It binds directly to deoxyhemoglobin and favors unloading of the remaining O2 atoms, enhancing delivery.and induces bronchodilation in the small airways of the lungs {i.e. CO2 will relax the muscles in the lungs and widen the airways (bronchi)}. Thus Carbon Dioxide in the blood helps us to breathe.

As in all substances in the word, and especially in the field of chemistry, too little is not good, too much is not good, just enough is best.

“The Dose Makes the Poison”

Nearly 500 years ago, Swiss physician and chemist Paracelsus expressed the basic principle of toxicology: “All things are poison and nothing is without poison; only the dose makes a thing not a poison.” This is often condensed to: “The dose makes the poison.” It means that a substance that contains toxic properties can cause harm only if it occurs in a high enough concentration.

In other words, any chemical—even water and oxygen—can be toxic if too much is ingested or absorbed into the body. The toxicity of a specific substance depends on a variety of factors, including how much of the substance a person is exposed to, how they are exposed, and for how long. chemicalsafetyfacts.org.

All doctors and biological scientists  should be able to “describe the physiological consequences of a progressive rise in blood carbon dioxide levels.” In effect, this describes the dose-response relationship of CO₂, and consequently, this chapter treats CO₂ as one might treat a drug. The discussion revolves around its physicochemical properties, relevant features of its transport in the body, and most importantly the physiological effects of hypercapnia and hypocapnia. For the CICM answer, the college also expected “a mechanistic description the neuro-cellular events”, which apparently meant some discussion of the ventilatory responses to CO₂, something which is discussed in more detail in the chapter on the relationship of arterial carbon dioxide and alveolar ventilation. 

In summary:

The physiological consequences of hypercapnia are:

• Depressed airway reflexes with severe hypercapnia
• Changes in respiratory drive:
  • Increased respiratory drive with mild hypercapnia
  • Depressed respiratory drive with severe hypercapnia
  • These changes are controlled by central chemoreceptors
  • The chemorecetors are sensitive to changes in the pH of CSF
  • Their output is maximal with a PaCO2 of 60-65 mmHg
  • With extremely high PaCO2, the neurodepressant effect of hypercapnia actually depresses the respiratory drive
• Changes in respiratory function:
  • Bronchodilation
  • Right shift of the oxygen-haemoglobin dissociation curve
• Cardiovascular stimulation:
  • Sympathetic overactivity, thus:
  • Hypertension
  • Tachycardia
  • Serum catecholamine excess
  • Increase in cardiac output
  • Prolonged QT interval
• Vasoactive effects:
  • Systemic arterial vasodilation
  • Pulmonary arterial vasoconstriction
• CNS effects
  • Progressively increasing sedation
  • Increased intracranial pressure
• Acid-base effects
  • Acidosis
  • Increased serum bicarbonate
• Other organ system effects
  • Increased renal vascular resistance
  • Decreased GFR
  • Decreased urine output
  • Increasd portal venous pressure and vascular resistance

We think that this is what happened to Paula when she could not raise her “very slow” breathing rate” to deal with her respiratory challenges, when she became quietly delirious 20 years ago. She never went to hospital and never got her HCO3 tested , but due to her usual intake of only 1.5 L in health and presumably in illness as well, we cannot be sure of what to expect with respect to HCO3 or even pulse oximetry O2. All bets are off, as they say. This needs further study in other people with “different” minute ventilation than expected; in health and in illness.

What seems clear is the dose-response relationship of carbon dioxide in the blood to the changes in vital signs in the body in illness [which seem to be the same as the ones described above] and the changes to mental status and overall function, perhaps due to the slowly rising hypothesized increase in intercranial pressure.