Troubleshooting Hypercapnia and Hypocapnia
Rebecca A. Johnson
Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin, USA
- Q. How is carbon dioxide (CO2) carried in the blood?
- 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].
- Q. How are O2 and CO2 transport in the blood related to each other?
- 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].
- Q. How is CO2 related to ventilation?
- A. Hypo- and hyperventilation are defined by alterations in the partial pressure of arterial CO2 (PaCO2). Whereas hypoventilation is associated with hypercapnia, i.e., increased PaCO2 values (and respiratory acidosis), hyperventilation results in hypocapnia, i.e., decreased PaCO2 levels (and respiratory alkalosis). Normal PaCO2 is somewhat species dependent but is approximately 31–37 mmHg in small animals (see: normal blood gas values in Chapter 18) . The determinants of PCO2 are based on the alveolar ventilation equation (simplified below), which describes the relationship between ventilation and CO2 production (CO2):