Carbon dioxide in arterial blood

Cardiac arrest IV Initially, 1 mEq/kg (as 7.5%-8.4% solution). May repeat with 0.5 mEq/kg qlOmin during continued cardiopulmonary arrest. Use in the postresuscitation phase is based on arterial blood pH, partial pressure of carbon dioxide in arterial blood (PaCOj) and base deficit calculation. 

With the exception of nitrous oxide, all inhaled anesthetics in current use cause a dose-dependent decrease in tidal volume and an increase in respiratory rate. However, the increase in respiratory rate is insufficient to compensate for the decrease in volume, resulting in a decrease in minute ventilation. All volatile anesthetics are respiratory depressants, as indicated by a reduced response to increased levels of carbon dioxide. The degree of ventilatory depression varies among the volatile agents, with isoflurane and enflurane being the most depressant. All volatile anesthetics in current use increase the resting level of Paco2 (the partial pressure of carbon dioxide in arterial blood). 

Problem 7.23. What is the solubility of carbon dioxide in arterial blood ... 

Figure 6.7. Partial pressures of oxygen and carbon dioxide in arterial blood (y-axis) as a function of time (x-axis). At time zero, ventilation ceases, e.g. as a result of a plastic bag put over the head. Inset the oxygen dissociation curve shown for reference see text for description.

Acute exacerbations superimposed on advanced chronic disease require the effective administration of noninvasive ventilation (NIV) to improve morbidity and mortality. NIV should also enable a steady improvement in spontaneous ventilatory capacity. A decrease in partial pressure of carbon dioxide in arterial blood (Paco2) during ventilation and subsequently during unassisted respiration is the best indicator of effective NIV (1,2). 

Figure 3 Ventilatory polygraphy of a patient with severe OHS IAH = 26 Pao = 9.6 kPa Paco = S 5 kPa (O2 3L/imn) lh/Jviarw OHS, obesity hypoventilation syndrome lAH, intra-abdominal hypertension Paco2, partial pressure of carbon dioxide in arterial blood.

Abbreviations. NIPPV, noninvasive positive pressure ventilation Cl, cardiac index FEVi, forced respiratory value in 1 second FVC, forced vital capacity Pa02, partial pressure of oxygen in arterial blood Pac02, partial pressure of carbon dioxide in arterial blood. 

As every medical student learns, it can be shown, from estimates of the total amount of carbon dioxide dissolved in the blood and from estimates of the difference in the amount of dissolved carbon dioxide in arterial and venous blood, that most of the 200 milliliters of carbon dioxide produced per minute in an average adult human cannot be transported in simple physical solution to the lungs. (1998, p. 132)... 

With large doses, thiopental causes dose-dependent decreases in arterial blood pressure, stroke volume, and cardiac output. This is due primarily to its myocardial depressant effect and increased venous capacitance there is little change in total peripheral resistance. Thiopental is also a potent respiratory depressant, lowering the sensitivity of the medullary respiratory center to carbon dioxide. 

Two umbilical arteries from the foetus carry blood to the placenta and a single umbilical vein returns blood from the placenta back to the foetus. The functions of the placenta in pregnancy are to supply oxygen and nutrients from the maternal circulation to the foetus and to remove waste materials, such as urea and carbon dioxide, from foetal blood. 

A pulmonary artery (Swan-Ganz) catheter can be used to determine central venous pressure (CVP) pulmouary artery pressure CO aud pulmonary artery occlusive pressure (PAOP), an approximate measure of the left ventricular eud-diastolic volume aud a major determinaut of left veutricular preload. CO (2.5 to 3 L/min) and mixed venous oxygeu saturatiou (70% to 75%) may be very low in a patient with extensive myocardial damage. Respiratory alkalosis is associated with low partial pressure of O2 (25 to 35 mm Hg) and alkaline pH, but uormal bicarbouate. The first two values are measured by arterial blood gas, which also yields partial pressure of carbon dioxide and arterial oxygen saturation. Circulating arterial oxygen saturation can also be measured by an oximeter, which is a noninvasive method that is fairly accurate and useful at the patient s bedside. 

Pac02 Partial pressure of carbon dioxide present in arterial blood normal is 35 5 mm Hg, but higher levels are acceptable to minimize ventilator support. 

Clinical assessment of abnormalities in the regulation of respiration is usually limited to measurements of the concentration of carbon dioxide and oxygen in arterial blood (P COg and These measurements are made... 

If the patient anesthetized with halothane is allowed to breathe spontaneously, an increased partial pressure of carbon dioxide in the arterial blood is common and is indicative of ventilatory depression. There also is an increased difference between the partial pressure of oxygen in the alveolar gas and in the arterial blood, indicating less efficient exchange of gas. Halothane thus influences both ventilatory control and the efficiency of oxygen transfer. To compensate for these effects, ventilation frequently is assisted or controlled by manual or mechanical means, and the concentration of inspired oxygen is increased. 

Carbon dioxide (CO ) is produced by the body s metabohsm at approximately the same rate as consumption, about 3 mL/kg/minute at rest, increasing dramaticahy with heavy exercise. CO diffuses readily from the ceUs into the bloodstream, where it is carried partly as bicarbonate ion (HCO3 ), partly in chemical combination with hemoglobin and plasma proteins, and partly in soln-tion at a partial pressnre of 6 kPa (46 mm Hg) in mixed venons blood. CO is transported to the lung, where it is normaUy exhaled at the same rate at which it is produced, leaving a partial pressure of 5.2 kPa (40 mm Hg) in the alveoh and in arterial blood. An increase in Pco results in a respiratory acidosis and may be due to decreased ventilation or the inhalation of CO, whereas an increase in ventilation results in decreased Pco and a respiratory alkalosis. Since CO is freely diffusible, changes in blood Pco and pH soon are reflected by intraceUular changes in Pco and pH. 

As the subject of acid-base physiology is developed, it will become apparent that carbon dioxide and bicarbonate play a crucial role. Carbon dioxide is not itself an acid but in aqueous solution it reacts with water to yield hydrogen ions (Table 1.4 reaction 1) and such a chemical is an acid. It also yields bicarbonate and this is the corresponding (conjugate) base. The C02-bicarbonate system is thus a buffer pair in which we will refer, rather loosely, to CO2 as the buffer acid and bicarbonate as the buffer base. Typical values for the concentrations in arterial blood are [CO2] = 1.2 m M and [HCO3 ] = 24 m M. 

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