Assessment of lung function in the ventilated patient

Abstract

IntroductionThe ability to accurately assess lung function is essential in the management of patients requiring ventilation for acute respiratory failure. Such measurements aid in diagnosis, in optimising mechanical ventilatory support and in predicting the likelihood of success at weaning. Undertaking and interpreting these investigations in the critically ill poses significant challenges. Patients are often unable to undertake complex respiratory maneuvers. Further, normal values for many of the standard respiratory function tests are lacking, when applied in the postures and with the inspired oxygen concentrations (F I O 2 ) required by the critically ill. However, the presence of an endotracheal tube and the monitoring system incorporated into modern microprocessor controlled ventilators can significantly aid the clinician in undertaking certain investigations. Gas exchangeGas exchange is assessed by the intermittent analysis of arterial blood gas tensions. Continuous arterial gas tension monitoring is now available and utilizes miniaturized fibre-optic probes placed within the arterial cannula [1], although its clinical utility has not been assessed to date. Hypoxaemia may be caused by intra-pulmonary shunting or ventilation perfusion inhomogeneity [2]. A number of indices which are functions of FIO 2 have been described to quantify the severity of hypoxaemia including alveolar-arterial PO 2 difference (AaDO 2 ), arterial/alveolar ratio (PaO 2 /PAO 2 ) and PaO 2 : FIO 2 ratio. However, due to the non linear relationship between oxygen tension and content as demonstrated by the oxygen haemoglobin dissociation curve, all these indices are affected by changes in FIO 2 . The PaO 2 : FIO 2 ratio exhibits the most stability, particularly when used at FIO 2 above 50 % in patients with a shunt fraction in excess of 30 % [3]. Thus the PaO 2 : FIO 2 ratio is the most acceptable non-invasive index of pulmonary oxygenation in patients with lung injury and is used to differentiate acute lung injury ( > 200 mm Hg) from the acute respiratory distress syndrome ( < 200 mm Hg) [4]. Although it is the preferred method of assessing gas exchange in clinical trials carried out in patients with ARDS [5], the PaO 2 : FIO 2 ratio does not necessarily reflect the overall severity of lung injury and is not of prognostic value. Shunt fraction can be estimated from the relevant equation if samples of arterial and mixed venous blood (from a pulmonary artery catheter) are obtained [6]. However, measurements of intrapulmonary shunt may be less stable with changing FIO 2 than the PaO 2 : FIO 2 ratio and vary in proportion to the degree of V : Q inhomogeneity [3]. Shunt should be measured with an FIO 2 of 1.0, although this may increase shunt further due to diffusion atelectasis. Calculations of shunt performed at lower FIO 2 quantify the venous admixture and also reflect the shunt-like effect of V : Q inhomogeneity and diffusion limitation. Due to these limitations and the requirement for a pulmonary artery catheter, there appears to be no ...

Effects of different PEEP levels on respiratory mechanics and oxygenation after coronary artery bypass grafting

“…Oxygenation index or PaO 2 /FiO 2 ratio routinely quantifies alveolar collapse in mechanically ventilated patients in postoperative period [20], being the preferred method for pulmonary function assessment [21].…”

Subsystem organ function during coronary surgery

“…Even though there was perioperative deterioration in gas exchange in patients who underwent CPB in our study, this was to a lesser degree than in previous similar studies. 60,61 This suggests that abnormalities of oxygenation are becoming a less important problem of CPB, at least in relatively healthy patients with no underlying pulmonary dysfunction.…”

The Predictive Value of Integrated Pulmonary Index after Off-Pump Coronary Artery Bypass Grafting: A Prospective Observational Study

“…To maintain respiratory function, the cardiosurgical patients are monitored using pulse oximetry, capnography, respiratory rate, and discrete blood gas analysis ( 7 – 9 ). Although blood gas analyses are the gold standard for early detection of different types of respiratory failure, they are invasive, cannot be measured continuously, and frequently impose a delay between sampling and availability of results ( 10 ).…”