Carbon dioxide is involved in the bicarbonate buffer system. The method that PaCO2 is involved in the regulation of minute ventilation is by bodily pH. It is with alveolar ventilation that the gases, including PaCO2, are exchanged. The minute ventilation is used routinely as a surrogate for alveolar ventilation. Under normal physiologic conditions, the minute ventilation, or the liters per minute of air exchanged in the lungs, is primarily controlled by the partial pressure of arterial carbon dioxide (PaCO2). The balance within the respiratory system depends primarily on the supply of oxygen and removal of carbon dioxide, thus regulating the body’s pH. Further study is necessary to determine the utility of peripheral venous blood gases in critically ill patients. Therefore venous PCO2 has been shown to have a weak correlation to arterial PCO2 in shock or extreme acid-base abnormalities. The peripheral venous PCO2 difference has demonstrated an increase of up to a factor of three due to ischemic changes. The difference between venous PCO2 measurement to arterial PCO2 measurement does increase in the presence of hypotension and shock. Studies in hemodynamically stable patients demonstrate that, in comparison, the central venous PCO2 is approximately 4 to 5 mmHg higher than an arterial sample, and the peripheral PCO2 is approximately 3 to 8 mmHg higher than arterial sampling. This process will ensure the circulating PCO2 is more accurate and will also give the most reliable pH. One mode of peripheral venous blood collection is to release the tourniquet after venipuncture and to allow a full minute to pass before collection. The collection of a peripheral venous blood gas during venipuncture can be the most misleading alternative to an arterial sample, as the collection must avoid ischemic changes from a tourniquet. The central venous blood gas is the most well established correlative blood gas alternative to the arterial blood gas in terms of PCO2 measurement. Traditionally, the arterial blood gas is the more reliable sample to monitor PCO2 this is facilitated with the placement of an arterial catheter for hemodynamic monitoring, as the collection of arterial blood gases becomes readily available. However, if the patient does have central venous access, then the collection of venous blood gas is acceptable. The collection of a blood sample to determine PCO2 is a significant area of clinical concern due to the need for accuracy of the measurement and its importance in clinical decision-making.
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