When the application of positive end-expiratory pressure (PEEP) results in global lungrecruitment, physiologic and alveolar dead space decrease [17]; the reverse is true when PEEP application results in lung overdistension [18]. Therefore, volumetric capnography selleckbio may also be helpful to identifyoverdistension or better alveolar gas diffusion [19].In summary, volumetric capnography has important potential for monitoring thedifficult-to-ventilate patient. Volumetric capnography needs sophisticated equipment andthis has limited its widespread use.Blood gasesThe PaO2/inspired fraction of oxygen (PaO2/FiO2)ratio is still the most frequently used variable for evaluating the severity of lungfailure and is included in the current definition of acute lung injury/ARDS [20].
The PaO2/FiO2 ratio is often a curvilinear(U-shaped) relationship, being at its lowest for moderate ranges of FiO2,depending on the shunt level, the hemoglobin value, and the arteriovenous differencein O2 content [21-23]. For a given PaO2/FiO2 ratio, the higher theFiO2, the poorer the prognosis [24]. In patients with ARDS, the PaO2/FiO2 ratio isdependent on the PEEP level and can be a surrogate, though imperfect, marker ofrecruitment [25]. Hemodynamic status (via the mixed venous oxygen tension, orPvO2) and intracardiac shunt (patent foramen ovale) also influence thePaO2/FiO2 ratio [26]. Despite its limitations, this ratio remains the most commonly used meansof assessing severity of lung disease. The oxygen index ([mean airway pressure ��FiO2 �� 100]/PaO2) accounts better for the influence ofventilator pressures on oxygenation value [27].
PaCO2-related variables are tightly correlated to outcome [28] and to lung structural changes [29] sometimes better than oxygen-related variables (such as shunt fraction) [30,31].Extravascular lung waterExtravascular lung water (EVLW) is a quantitative measure of pulmonary edema and iscorrelated, in multiple patient populations, to mortality [32]. Normal values are 5 to 7 mL/kg (indexed to predicted body weight), andquantities above 10 mL/kg are associated with adverse clinical outcomes [33].Indicator dilution techniques for measuring EVLW are available for bedside use incritically ill patients. The single-indicator technique is now well validated andoffers the additional value of simultaneously measuring cardiovascular performance(cardiac output, fluid res-ponsiveness, and filling volumes).
Current technology usesan injection of cold saline into the right atrium and assesses Batimastat transpulmonarythermodilution in the arterial system by using a femoral or brachial catheter.Limitations of the technique include requirements for good indicator mixing withoutloss and for constant blood flow and temperature. EVLW can be assessed only inperfused areas of the lung [34].EVLW measurements may be used in combination with other cardiovascular and pulmonaryparameters to diagnose pulmonary edema.