Chapter 19
Troubleshooting Hypercapnia and Hypocapnia

Just breathe!

Rebecca A. Johnson

Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin, USA

1. Q. How is carbon dioxide (CO₂) carried in the blood?
   
2. A. CO₂ is produced in the tissues and diffuses down a concentration gradient until it enters the blood where it is carried in three ways: (i) dissolved in solution (∼5% in arterial blood), (ii) as bicarbonate (∼75–80% in arterial blood), and (iii) combined with proteins such as carbamino compounds (∼15–20% in arterial blood) [1,2].
   

   Bicarbonate is the main type of CO₂ found in the blood and is formed by the carbonic anhydrase equation:

   
   
   
   
   

   The first reaction is slow in plasma but fast in the red blood cell due to the presence of carbonic anhydrase. The second reaction occurs quickly. The HCO₃– diffuses out of the red blood cell but the H⁺ ions become trapped since the cell membrane is relatively impermeable to cations. Chloride ions move into the cell from the plasma (the chloride shift) to maintain electrical neutrality. Carbamino compounds are formed by the chemical combination of CO₂ with terminal amine groups of proteins such as hemoglobin, resulting in carbaminohemoglobin [1,2].

   
   
3. Q. How are O₂ and CO₂ transport in the blood related to each other?
   
4. A. O₂ and CO₂ transport in the blood have mutual interactions via the Bohr and Haldane effects. For example, some of the H⁺ ions produced from the carbonic anhydrase equation above bind to reduced hemoglobin (Hb) and displace O₂:
   
   
   
   

   Thus, in the O₂-enriched lungs, Hb is a poor buffer, which aids in the unloading of CO₂, termed the Haldane effect. In the tissues, the high level of CO₂ reduces the affinity of Hb for O₂ and consequently aids in the unloading of O₂, termed the Bohr effect [1,2].

   
   
5. Q. How is CO₂ related to ventilation?
   
6. A. Hypo- and hyperventilation are defined by alterations in the partial pressure of arterial CO₂ (P_aCO₂). Whereas hypoventilation is associated with hypercapnia, i.e., increased P_aCO₂ values (and respiratory acidosis), hyperventilation results in hypocapnia, i.e., decreased P_aCO₂ levels (and respiratory alkalosis). Normal P_aCO₂ is somewhat species dependent but is approximately 31–37 mmHg in small animals (see: normal blood gas values in Chapter 18) [3]. The determinants of PCO₂ are based on the alveolar ventilation equation (simplified below), which describes the relationship between ventilation and CO₂ production (CO₂):
   
   
   
   

   Where

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