Partial pressure of carbon dioxide in arterial

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). 

F ure 6.8. Partial pressure of carbon dioxide in arterial blood (y-axis) as a function of time (x-axis). At time zero, hypoventilation suddenly supervenes, e.g. as a result of a severe attack of asthma. As indicated, immediately following the onset of hypoventilation, there is a period during which carbon dioxide accumulates in the body. The PCO2 rises exponentially towards a steady state value. When a steady state is established, the rate of carbon dioxide excretion in the lungs equals the rate of production in the tissues. 

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. 

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. 

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. 

Pregnancy is also associated with a partially compensated respiratory alkalosis that may affect the protein binding of some drugs. Respiratory changes in pregnancy include a decrease in arterial partial pressure of carbon dioxide to 30.9 mm Hg, most likely due to the effect of progesterone (23,24). In compensation, serum bicarbonate decreases, and maternal serum pH increases slightly to 7.44 (23). 

Opioids are potent respiratory depressants, causing a dose-dependent decrease in respiratory frequency, tidal volume and minute ventilation and increased arterial partial pressure of carbon dioxide (PaC02) (Carvey 1998). Opioids depress chemosensors in the brainstem, decreasing the ventilatory response to carbon dioxide. Opioids also depress rhythmicity in the dorsal respiratory group in the nucleus tractus solitarius, attenuating the respiratory cycle. Opioids, however, do not diminish hypoxic ventilatory drive. Significant elevations in Paco2 can result in increased ICP after opioid administration. 

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. 

In a patient in whom there is a sudden rise in the alveolar partial pressure of carbon dioxide, the change in composition of the patient s arterial blood is represented in Figure 3.1 as a move from the normal, represented by point N, along the normal blood line to point A. The increase in the x coordinate represents the rise in the PCO2 of the patient s arterial blood. The increase in the y coordinate represents the rise in bicarbonate concentration the move to a new isohydric contour reflects the increase in hydrogen ion concentration. These changes in composition of the arterial blood are summarized in Table 3.1 Column A. 

In hypoventilation, where aeration of the lungs is inadequate but not zero, the arterial partial pressure of carbon dioxide rises until the rate of excretion of carbon dioxide equals the rate of production (Figure 6.8). The ventilation is reduced but the concentration of carbon dioxide in the expired air rises. The... 

Arterial blood gases for evaluating partial arterial pressure of carbon dioxide (Pco2) should be considered for patients in severe distress, suspected hypoventilation, or when PEF or FEYT, is less than or equal to 30% after initial treatment. 

FIGURE 11-3. Emergency department and hospital-based management of asthma exacerbation. FEV-, forced expiratory volume in 1 second Pco2, partial arterial pressure of carbon dioxide PEF, peak expiratory flow 02, oxygen. (From Kelly HW, Sorkness CA. Asthma. In DiPiro JT, Talbert RL, Yee GC, et al, (eds.) Pharmacotherapy A Pathophysiologic Approach. 6th ed. New York McGraw-Hill 2005 517, with permission.)... 

---