Hypercapnia augments resistive exercise‐induced elevations in intraocular pressure in older individuals

Abstract

The present study assessed the effect of 6 • head-down (establishing the cephalad fluid displacement noted in astronauts in microgravity) prone (simulating the effect on the eye) tilt during rest and exercise (simulating exercise performed by astronauts to mitigate the sarcopenia induced by unloading of weight-bearing limbs), in normocapnic and hypercapnic conditions (the latter simulating conditions on the International Space Station) on intraocular pressure (IOP).Volunteers (mean age = 57.8 ± 6 years, n = 10) participated in two experimental sessions, each comprising: (i) 10 min rest, (ii) 3 min static handgrip exercise (30% max), and (iii) 2 min recovery, inspiring either room air (NCAP) or a hypercapnic mixture (1% CO 2 , HCAP). We measured IOP in the right eye, cardiac output (CO), stroke volume (SV), heart rate (HR) and mean arterial pressure (MAP) at regular intervals. Baseline IOP in the upright seated position while breathing room air was 14.1 ± 2.9 mmHg. Prone 6 • head-down tilt significantly (P < 0.01) elevated IOP in all three phases of the NCAP (rest: 27.0 ± 3.7 mmHg; exercise: 32.2 ± 4.8 mmHg; recovery:27.4 ± 4.0 mmHg) and HCAP (rest: 27.3 ± 4.3 mmHg; exercise: 34.2 ± 6.0 mmHg; recovery:29.1 ± 5.8 mmHg) trials, with hypercapnia augmenting the exercise-induced elevation in IOP (P < 0.01). CO, SV, HR and MAP were significantly increased during handgrip dynamometry, but there was no effect of hypercapnia. The observed IOP measured during prone 6 • HDT in all phases of the NCAP and HCAP trials exceeded the threshold pressure defining ocular hypertension. The exercise-induced increase in IOP is exacerbated by hypercapnia.

Intraocular pressure responses to walking with surgical and FFP2/N95 face masks in primary open-angle glaucoma patients

“…Based on the widespread use of face masks due to the COVID-19 pandemic and the lack of research in this regard, we designed a randomized clinical trial to determine the IOP responses to low-intensity endurance exercise (400-m walking) while wearing surgical and FFP2/N95 masks in primary open-angle glaucoma (POAG) patients. Considering the increments in IOP values reported in previous studies when the breathing pattern was restricted during the execution of resistance training exercises [11,12] or under hypoxic and hypercapnic conditions [17,18], we hypothesized that wearing face masks during walking would counteract the IOP-lowering effect frequently observed while performing low-intensity endurance exercises [9,10].…”

The eye in extreme environments

“…With the development of high‐performance aircraft, humans can be exposed to transient changes in gravitational loading, which affect the cardiovascular system and, indirectly, vision (Lee, Mader, Gibson, Brunstetter, & Tarver, 2018). In contrast, humans have established a permanent presence in lower orbit on the ISS, and much research has been conducted to understand the adaptation of physiological systems to microgravity, or rather to implement countermeasures to minimize the debilitating impacts of this adaptation (Mekjavic, Amoaku, Mlinar, & Jaki Mekjavic, 2020). Recent observations of visual impairment in astronauts (Mader et al., 2011) have rendered the unresolved effect of microgravity on the visual apparatus a high priority in space life sciences research.…”

“…Countermeasures to mitigate potential long‐term sequelae are yet to be identified. Our recent studies suggest that the daily exercise conducted by astronauts to mitigate musculoskeletal atrophy within the hypercapnic environment of the ISS causes significant elevations in intraocular pressure (ocular hypertension), which might contribute to the aetiology of SANS (Mekjavic et al., 2020).…”

“…Before the ISS, astronauts were younger and were not required to conduct the levels of exercise seen today. A recent study (Mekjavic et al., 2020) has examined the IOP response to hand‐grip exercise in subjects aged 50–65 years. In contrast to a younger group (Mlinar T., Jaki Mekjavic P. & Mekjavic I.B., unpublished observations), their exercise‐induced increase in IOP was much greater.…”

The aetiology of spaceflight‐associated neuro‐ocular syndrome might be explained by a neural mechanism regulating intraocular pressure

“…unpublished; Mekjavic et al . 2020), perhaps due to the known age‐related modifications in the biomechanical properties of the lamina cribrosa.…”