In several small mammals, including rodents, the chest wall is significantly more compliant than larger animals (6
); a mechanical feature reminiscent of newborn mammals (8
). In the setting of a compliant chest wall, the static balance between opposing elastic recoil of the lung and chest wall results in low resting or relaxation volumes of the lung. In the mouse, the low relaxation volume is only slightly above the volume at which all airways are closed (19
). This small expiratory reserve volume is inconsistent with efficient gas exchange.
Using continuous arterial oximetry monitoring in mice, we confirmed the inefficiency of sedation-associated gas exchange. Sedation resulted in a dose-dependent decrease in oxygen saturation. The effect of the sedation appeared to be related to breathing frequency and tidal volume. Mechanical ventilation with either increased frequency or tidal volume restored oxygen saturation to awake baseline levels. These observations are consistent with the hypothesis that lung volumes, and indirectly gas exchange, are dynamically determined.
In awake mice, an additional mechanism has been implicated in maintaining an increased functional residual capacity; namely, expiratory braking (19
). Braking is persistent inspiratory muscle activity or increased glottal resistance that results in the retardation of expiratory flow (14
). Prior to these studies, we suspected that PEEP would have a similar functional effect in maintaining expiratory airway pressure and functional residual capacity. An unexpected observation was the adverse effect of PEEP on oxygen saturation; as little as 2cmH20 PEEP resulted in significant arterial desaturation.
There are several plausible mechanisms for the adverse effect of PEEP on murine arterial oxygen saturation. The commonest explanation in humans for adverse effects of PEEP is an increase in mean intrathoracic pressure. The increased airway pressure can result in systemic circulatory insufficiency and/or the inefficient redistribution of blood flow in the lungs. The potential effect of PEEP-associated crowding within the chest likely contributes to inefficient gas exchange because pneumonectomy reduced the adverse effects of PEEP-induced desaturation. A second possibility is that the increased airway pressure associated with PEEP may have a direct effect on juxta-alveolar vascular resistance. Increased vascular resistance may redistribute blood flow away from well-aerated alveoli and toward nonaerated regions of the lung. A third possibility is that the dominant effect of PEEP was to maintain airway patency and blood flow to otherwise poorly ventilated alveoli. The finding that supplemental oxygen normalized oxygen saturation is consistent with this possibility. Because of the complex interactions between breathing frequency, lung volume, tidal volume and volume history, the ability to distinguish between these mechanisms will be limited until quantitative and instantaneous measurements of interstitial pressure and blood flow are available.
A contribution of this study was the concomitant use of continuous oxygen saturation monitoring and the FlexiVent mechanical ventilator. Measuring the rapid changes in arterial oxygen saturation was enabled by the recent development of continuous arterial oxygen saturation monitoring in mice. An extension of photoplethysmography (PPG) technology, mouse oximetry uses the optical measurement technique that has been clinically adapted in humans (5
). PPG is commonly employed as the detected reflectance of a red or a near infrared wavelength light; the most recognizable waveform feature of the reflected light is the peripheral pulse. The pulsatile component of the PPG waveform (often called the 'AC' component) has a fundamental frequency linked to the heart rate (1
). The challenge of adapting pulse oximetry to the mouse is not only the small scale, but a heart rate in mice that is 8-fold higher than in humans (10
). The recent development of analytic algorithms that permit noninvasive pulse oximetry is particularly useful given the limitations of invasive monitoring in mice.
The other technology used to assess murine physiology was the FlexiVent. The FlexiVent is a rodent ventilator, requiring endotracheal intubation, designed to provide an instantaneous assessment of the pressure-flow relationship (impedance). Based on impedance measurements, the flow resistance and reactance of the respiratory system can be calculated. Two major limitations of the FlexiVent are 1) the requirement for tracheal intubation, and 2) the requirement for complete relaxation to avoid extrapulmonary artifact. In survival experiments, tracheal intubation must be performed skillfully to avoid glottic edema and airway obstruction. Because of the need for complete relaxation, and the limitations of paralytic agents in mice, the use of the FlexiVent requires careful data analysis.
Mice have well-established differences in lung mechanics over time and between strains (18
). A practical limitation of our study is that we examined only one strain of mice (C57/B6) within a narrow age range. Although specific ventilatory parameters may vary with age and strain, we suspect the basic management principles will be equally applicable to not only other mouse strains, but rats and many small mammals as well.
The results of our study highlight the both the similarities and differences between mice and humans. In humans, sedation results in desaturation only after prolonged hypoventilation and hypercarbia. In contrast, sedation in mice results in rapid and significant oxygen desaturation. Our studies suggest that these differences are not the result of intrinsic differences in lung function, but a consequence of the hypercompliant chest wall and the high rate of oxygen consumption characteristic of the mouse. The awake mouse uses breathing frequency and tidal volume to maintain functional residual capacity and efficient gas exchange. Any anesthetic or surgical procedure that compromises ventilatory rate and tidal volumes risks substantial hypoxemia. The practical consequence of these observations for murine procedures is the meticulous attention to mechanical ventilation parameters and the optional use of supplemental oxygen.