Experimental Models to Study End-Organ Morbidity in Sleep Apnea: Lessons Learned and Future Directions

Abstract

Sleep apnea (SA) is a very prevalent sleep breathing disorder mainly characterized by intermittent hypoxemia and sleep fragmentation, with ensuing systemic inflammation, oxidative stress, and immune deregulation. These perturbations promote the risk of end-organ morbidity, such that SA patients are at increased risk of cardiovascular, neurocognitive, metabolic and malignant disorders. Investigating the potential mechanisms underlying SA-induced end-organ dysfunction requires the use of comprehensive experimental models at the cell, animal and human levels. This review is primarily focused on the experimental models employed to date in the study of the consequences of SA and tackles 3 different approaches. First, cell culture systems whereby controlled patterns of intermittent hypoxia cycling fast enough to mimic the rates of episodic hypoxemia experienced by patients with SA. Second, animal models consisting of implementing realistic upper airway obstruction patterns, intermittent hypoxia, or sleep fragmentation such as to reproduce the noxious events characterizing SA. Finally, human SA models, which consist either in subjecting healthy volunteers to intermittent hypoxia or sleep fragmentation, or alternatively applying oxygen supplementation or temporary nasal pressure therapy withdrawal to SA patients. The advantages, limitations, and potential improvements of these models along with some of their pertinent findings are reviewed.

Keywords: CPAP withdrawal; airway obstruction; animal model; cell model; human model; intermittent hypoxia; oxygen supplementation; sleep apnea pathophysiology; sleep fragmentation.

Figure 1

Experimental in vitro system for cellular exposures to constant or variable oxygen concentrations. (A) schematic view of the cell culture setting to apply controlled intermittent hypoxia (IH) to cultured cells (see text for explanation). (B) actual oxygen concentration measured on top of the membrane (cell culture level) when applying IH with different magnitudes (1–20% O₂ (blue line) and 4–13% O₂ (black line) at a frequency of 60 cycles/h. Reprinted with permission from Ref. [11]. 2017. American Physiological Society.

Figure 2

Model to apply airway obstruction mimicking sleep apnea. (A) Diagram of the system. V′: flow generated by both air pumps (reversed directions). R1 and R2: resistors. EV: three-way electrovalve. The resistance of R1 was greater than that of R2 to generate higher positive (30 cmH₂O) than negative (−5 cmH₂O) pressure. Four airbags were connected to the source of alternant pressure. For the sake of simplicity and illustration, the figure shows only two airbags in place: one inflated (30 cmH₂O) to apply airway obstruction to the mouse (top) and the other one deflated (−5 cmH₂O) to allow the mouse to breathe spontaneously. (B) Example of esophageal pressure (Peso) recording in one mouse during application of two of apneas (6 s each at a rate of 120/h). Reprinted with permission from Ref. [55]. 2011. Elsevier.

Figure 3

Diagrams of the most common experimental animal models of sleep apnea. (A) Application of intermittent hypoxia: air with cyclic O₂ fraction (FiO₂) in the animal cage induces recurrent hypoxemia (SaO₂). (B) Induction of sleep fragmentation: smooth cyclic movement of a bar (blue) in the cage ground induces cyclic arousal, as reflected by electromyography (EMG) and electroencephalography (EEG) signals in the mice. See text for explanation. Reprinted from Ref. [6]. Creative Commons License.

Figure 4

Experimental patient models to study the mechanisms involved in obstructive sleep apnea (OSA) consequences. (A) Model of supplementary nocturnal oxygen therapy. Patients with OSA experiencing upper airway collapse (with associated intermittent hypoxemia, sleep fragmentation and increased intrathoracic pressure swings) can be subjected either to continuous positive airway pressure (CPAP) therapy (equivalent to “switching off” OSA if CPAP is effective) or to supplementary oxygen therapy to avoid only the recurrent oxygen arterial desaturations induced by OSA. (B) Model of CPAP withdrawal. Patients effectively treated with CPAP (therefore not experiencing OSA challenges) are modelling “normal” subjects as compared with the same patient after CPAP withdrawal, an intervention equivalent to “switching on” OSA. Resumption of CPAP after withdrawal recovers the baseline condition. Comparison of patient status after “switching on and off” OSA provides information on the mechanisms involved in the consequences of this sleep-breathing disorder. Reprinted with permission from Ref. [173]. 2021. European Respiratory Society.

Figure 5

Experimental models to investigate the end-organ consequences of SA.