Capnographic identification of end-expiratory flow limitation.

Patients with end-expiratory flow limitation (eEFL) demonstrate a terminal rise in capnography slope. The high slope could represent phase 5, a phenomenon described for single breath N2 tests but previously unreported during capnography. This study evaluated 6 healthy subjects exhaling from total lung capacity to residual volume at several set constant rates. We measured the volumes of flow limitation (VFL) and phase 5 (VP5) for CO2 and N2. A distinct phase 5 occurred shortly after eEFL for both gases. Increased expiratory flow rate resulted in parallel increases in VFL and VP5. The two quantities differed on average by the volume of dead space. These data suggest that phase 5 on capnography identifies eEFL with a small delay resulting from transit of expired gas through dead space. Following phase 5 by volumetric capnography could be useful for monitoring anesthetized patients, who in some circumstances may have lung volumes close to residual volume. eEFL could be treated with lung volume-increasing maneuvers, such as positive end-expiratory pressure.

Measurement of blood pressure.

Blood pressure is overwhelmingly the most commonly measured parameter for the assessment of haemodynamic stability. In clinical routine in the operating theatre and in the intensive care unit, blood pressure measurements are usually obtained intermittently and non-invasively using oscillometry (upper-arm cuff method) or continuously and invasively with an arterial catheter. However, both the oscillometric method and arterial catheter-derived blood pressure measurements have potential limitations. A basic technical understanding of these methods is crucial in order to avoid unreliable blood pressure measurements and consequential treatment errors. In the recent years, technologies for continuous non-invasive blood pressure recording such as the volume clamp method or radial artery applanation tonometry have been developed and validated. The question in which patient groups and clinical settings these technologies should be applied to improve patient safety or outcome has not been definitively answered. In critically ill patients and high-risk surgery patients, further improvement of these technologies is needed before they can be recommended for routine clinical use.

Noninvasive continuous beat-to-beat radial artery pressure via TL-200 applanation tonometry.

The Tensys TL-200(®) noninvasive beat-to-beat blood pressure (BP) monitor displays continuous radial artery waveform as well as systolic, mean and diastolic BP from a pressure sensor directly over the radial artery at the wrist. It locates the site of maximal radial pulse signal, determines mean BP from maximal pulse waveform amplitude at optimal artery compression and then derives systolic and diastolic BP. We performed a cross-sectional study of TL-200 BP comparisons with contralateral invasive radial artery (A-Line) BP values in 19 subjects during an average 2.5 h of general anesthesia for a wide range of surgical procedures. Two hundred and fifty random sample pairs/patient resulted in 4,747 systolic, mean and diastolic BP pairs for analysis. A-Line BP ranged from 29 mm Hg diastolic to 211 mm Hg systolic, and heart rate varied between 38 and 210 beats/min. Bland-Altman analysis showed an average 2.3 mm Hg TL-200 versus A-Line systolic BP bias and limits of agreement (1.96 SD) were ± 15.3 mm Hg. Mean BP showed a 2.3 mm Hg TL-200 bias and ± 11.7 mm Hg limits of agreement, while diastolic BP showed a 1.7 mm Hg bias and ± 12.3 mm Hg limits of agreement. Coefficients of determination for TL-200 and A-Line BP regression were r² = 0.86 for systolic, r² = 0.86 for mean, and r² = 80 for diastolic BP, respectively, with no apparent change in correlation at low or high BP. Bland-Altman analysis suggested satisfactory agreement between TL-200 noninvasive beat-to-beat BP and invasive A-Line BP. Paired TL-200/A-Line BP comparisons showed a high coefficient of determination.

Alveolar recruitment versus hyperinflation: A balancing act.

PURPOSE OF REVIEW: To address lung recruitment according to pressure/volume curves, along with regional recruitment versus hyperinflation evidence from computed tomography and electrical impedance tomography. RECENT FINDINGS: Cyclical tidal volume recruitment of atelectatic lung regions causes acute lung injury, as do large breaths during pneumonectomy. Using the lower inflection point on the static pressure/volume inflation curve plus 2 cmH2O as a positive end-expiratory pressure setting limits hyperinflation in acute lung injury, but may not provide enough positive end-expiratory pressure to avoid cyclical recruitment/derecruitment injury in more severe acute lung injury regions. Both computed tomography and electrical impedance tomography can help titrate positive end-expiratory pressure in these regions, thereby assuring an 'open lung' ventilatory pattern. Regional pressure/volume curves show that adequate positive end-expiratory pressure for severe acute lung injury regions may not be reliably determined from whole lung pressure/volume curves. Balancing positive end-expiratory pressure requires both arterial PO2 and PCO2 values to determine at what level hyperinflated regions become seriously underperfused (develop very high ventilation-perfusion ratios), adding to the hypercarbia from increased deadspace. SUMMARY: Positive end-expiratory pressure levels must be high enough to minimize recruitment/derecruitment cycling. Balancing recruitment versus overdistension may require thoracic tomography, to assure sufficient improvement in oxygenation while limiting hypercarbia.