We have studied the impact of PEEP on particle flow in exhaled air by gradually increasing PEEP from a baseline level of 5 cmH2O to 25 cmH2O. The particle count from the small airways stayed at a similar level during various levels of PEEP, but when PEEP was released from a high level, a significant increase in particle count from the small airways occurred.
A significant increase in MPC was seen when releasing PEEP compared to all levels of PEEP, as seen in Fig. 1. There could be several reasons why there is such an increase in MPC when releasing PEEP, such as an instant collapse of very extended airways or that the pressure induced on the airways does not have the same impact when increased as when decreased. A previous study with the Pexa technique showed that different tidal volumes and different PEEP resulted in different particle count from the airways in vivo, post mortem and during ex vivo lung perfusion [8]. In another study performed in the operating theatre showed that patients on mechanical ventilation with the use of PEEP compared to normal breathing patients display a lower particle count [14]. These findings indicate that an open airway during mechanical ventilation generates lower levels of particles compared to an airway that repeatedly collapses and reopens. We hypotheses that the particle count pattern can give information of a successfully opened small airways without repeated collapse and reopening.
Important to mention is that in all animals there was no clinically important changes in the commonly used indicators for changes of lung function such as oxygen levels or the animals’ saturation and still we saw changes in the particle count when releasing PEEP. There were changes in the ventilator’s peak and mean pressures but, those can be directly related to the gradual increase of PEEP. All animals were stable during all measurements and significant changes are displayed in Table 1 and 2 and are most likely related to the increased level of PEEP and its subsequent effect on the cardiovascular system. We could see a change in blood gases and predominantly in SvO2 which was lower at finish of the PEEP trial, and this may be the effect of PEEP on the cardiac function and not directly due to impaired lung function. PEEP’s effect on the cardiovascular system with reduced cardiac output has previously been shown in several studies [15–18]. Patients’ respiration during mechanical ventilation in intensive care are routinely monitored by evaluating their blood gases, saturations, ventilator peak pressures, volumes, and compliance. In this study we can show that these markers didn’t change without a reliable cause, but that particle count from the airway did. In the future this technique may prove to be an additional parameter used as monitoring and thereby facilitate mechanical ventilation treatment. It is of importance to further study and evaluate this technique’s usability in a clinical setting and study what physiological changes can lead to changes in particle count from the airway.
Blood pressure was stepwise decreased when PEEP was stepwise increased which also reflects the impact increased PEEP has on blood pressure, as seen in Table 2. In a human population it is a known, studied and a stated fact that PEEP has an effect on the cardiac output [15–20]. The MPC from the airway was fairly similar when using PEEP 5, 10, 15, 20 or 25 cm H2O regardless of changes in blood pressure. Blood pressure significantly decreased in higher levels of PEEP but there were no similar significant changes in MPC in higher levels of PEEP, only in relation to the release of PEEP down to 5 cmH2O. In a previous study with a lung animal model, we have shown that by stepwise increasing blood flow through the lung there is a stepwise increase in particle count from the airways. [8]. From the results in the current study, it’s hard to state why there is a similar MPC when using any PEEP level. A similar particle count from the airways was seen at both low and high levels of PEEP, one reason for this may be that the time in every PEEP level was too short to reach a steady state. We hypothesis that possibly the impact of a low PEEP such as 5 or 10 cm H2O may have a larger impact than expected and needs to be further studied.
PEEP is commonly used in intensive care when patients are on mechanical ventilation both continuously in coherence with the open lung concept but also used in various ways in RM [2, 21, 22]. In the open lung concept it has been shown that a lower PEEP level, i.e. 5–10 cmH2O, is most likely preferable to achieve an open lung and limiting the collapse of the distal airways and at the same time improved pulmonary function without causing further harm [21–24]. In patients with the risk of one of the most severe forms of lung injury, those who are at risk for or with ARDS, lung-protective ventilation strategies with the use of low tidal volume and low to moderate PEEP have been shown to improve outcomes in relation to both morbidity and mortality [25–27]. Different recruitment maneuverers are extensively used in intensive care with either changes in PEEP, I:E ratios or tidal volumes or a bit of all three [2, 28, 29]. There is no real consensus on which RM is the most optimal [1, 30, 31]. We have observed in this study an increase in particle count when releasing PEEP from a high level to a low level and this finding may contribute to further knowledge about the effect of RM, i.e., increased PEEP to different pressure levels.
Using an optical particle counter which can online none invasively analyse different particle count from the airways has a potential to be an additional and harmless way to gather further information on the impact of mechanical ventilation. Patients’ respiration during mechanical ventilation in intensive care are routinely monitored by evaluating different parameters and particle count from the airway may in the future be an additional parameter used as monitoring and thereby improve the mechanical ventilation treatment.