Measurement of airway pressure in a ventilator is fundamental to the management of ventilation (using both pressure and volume modes). Display of the pressure trace, as opposed to a digital display or an analog display, enables much clearer analysis of the pattern of ventilation that is actually being administered.

There is some data that airway pressures measured at the endotracheal tube are similar to those measured in the ventilator circuit (Cannon et al. 2000). However, Nasiroglu et al. (2006) compared the pressure measured in the ventilator circuit proximal to the endotracheal tube with the pressure in the trachea on a group of 30 children (aged 29 days to 5 years) and showed that there were significant differences. Peak inspiratory pressures measured in the ventilator circuit overestimated tracheal pressure during positive-pressure ventilation and underestimated tracheal pressures during spontaneous ventilation. Similar findings have been reported by other authors (Dela Cruz et al. 2005). Nikischin et al. (2007; Nikischin and Lange 2003) have examined the impact of leaks on the relationship between airway and tracheal pressures and have suggested methods for correction of data.

Airflow during ventilation can be measured using a range of pneumotachographs placed at a variety of places within the ventilation circuit. Care must be taken to ensure that the pneumotachographs are appropriately calibrated and that the geometry of the ventilatory circuits does not interfere with effective function (Kreit and Sciurba 1996). Respiratory impedance plethysmography can also be used to measure gas flows (Brooks et al. 1997) with the advantage that there is no insertion to the respiratory circuit with additional resistance and dead space; however, respiratory impedance plethysmography does require calibration and may require more sophisticated resources.

There is substantial evidence that flows and tidal volumes measured at the endotracheal tube may be substantially different to those measured within ventilator circuits (Cannon et al. 2000; Al-Majed et al. 2004; Castle et al. 2002; Chow et al. 2002; Heulitt et al. 2005, 2009), and these differences may be affected by issues such as lung compliance and leaks around the endotracheal tube (Main et al. 2001; Herber-Jonat et al. 2008) or the use of heat moisture exchangers for humidification (Fujita et al. 2006).

This is of particular concern, as there is substantial evidence that tidal volumes are clearly related to inflammatory responses both systemically and within the lungs (Wolthuis et al. 2008) and many studies have highlighted the potential benefits of ventilation with lower tidal volumes in adults and children with ARDS and other lung conditions (Wolthuis et al. 2008; Amato et al. 1995, 1998; The Acute Respiratory Distress Syndrome Network 2000; Meade et al. 2008; Schultz 2008; Choi et al. 2006; Ferguson et al. 2005; Rimensberger et al. 1999; Brochard et al. 1998). And ventilation at lower tidal volumes has been associated with improved outcomes in pediatric patients with lung disease (Khemani et al. 2009; Randolph 2009; Hanson and Flori 2006; Mehta and Arnold 2004; Albuali et al. 2007).

Ideally flow and volume data should be graphically displayed (in conjunction with pressure data), as this enables optimization of inspiratory and expiratory times. Utilization of this data can prevent overdistension of the lung from “stacking” (Pohlman et al. 2008).

Particular attention needs to be paid to leaks around endotracheal tubes, which are almost the norm in smaller children where uncuffed endotracheal tubes have been used routinely for many years. With modern cuffed endotracheal tubes, it may be possible (and appropriate) to use cuffed tubes on younger infants, particularly those who are unstable at the time of intubation (Newth et al. 2004). Elimination of the leak makes it substantially easier to evaluate respiratory function in ventilated children.

A wide variety of curves and graphics can be generated using the interactions of pressure, flow, volume, and time. However, although they may provide promising and interesting data regarding lung and respiratory physiology, there is not data showing that their use changes patient outcomes (Terragni et al. 2003).

Much physiological data has been derived from static compliance curves, and much of the theory on optimal ventilation has derived from study of static compliance curves. However, as shown by Adams et al. (2001), the dynamic compliance curves that are displayed on many ventilators are very different to static curves and should not be used to define ventilatory strategies. In particular inflection points on compliance curves are often difficult to identify and may not be present on tidal pressure-volume curves (Albaiceta et al. 2008).