Toriyama et al.17 studied the effect of CO2 bathing in 83 limbs with critical ischemia and achieved limb salvage in 83% without surgery. They concluded that peripheral vasodilation from CO2 bathing resulted from an increased parasympathetic and decreased sympathetic activity. In the current study, treatment with transdermal CO2 in a localized area produced a sustained, remote vasodilation, and a lowering of systemic blood pressure.
These findings share some similarity with the hemodynamic changes that occur following an acute bout of exercise, in which both neural and vascular components contribute to a sustained decrease in vascular resistance and blood pressure that persists after cessation of exercise18. In the current study, the period of sustained vasodilation seen in response to transdermal CO2 was heightened in diabetic patients.
Interestingly, in hypertensive individuals, the period of post-exercise hypotension is of greater magnitude and duration as compared to that of normotensive individuals 18, 19. Paradoxically, the current findings in diabetic patients exposed to transdermal CO2 as well as previous findings in hypertensive patients post-exercise, imply that sensitivity to signals that mediate these cardiovascular responses increases in patients with pre-existing cardiovascular dysfunction19.
A sustained decrease in systolic blood pressure occurs post-exercise and here, following application of transdermal CO2, suggesting that neural mechanisms contribute to the observed reduction in systemic vascular resistance. The roles of efferent sympathetic nerve activity18-20, afferent nerve activity from muscle 21-24, and the baroreceptor reflex20, 23 in mediating post-exercise hypotension remain controversial.
Neural mechanism(s) could contribute to changes in skin SPP and systolic blood pressure induced by exposure to transdermal CO2. Future studies will need to monitor heart rate, heart rate variability, and sympathetic nerve activity during and after transdermal CO2 in order to more fully assess the role of the autonomic nervous system in mediating the sustained increases in SPP and systolic blood pressure reported in this initial study.
Vascular conductance increases in both active muscle and inactive vascular beds following a bout of dynamic exercise 25, 26, suggesting that circulating factors contribute to this period of sustained systemic vasodilation. Vasodilation occuring independently of neural regulation constitutes more than 50% of the increase in systemic vascular conductance that occurs post-exercise; however, the mechanisms that underlie the post-exercise vasodilation have remained elusive.
Studies that have employed blockade of nitric oxide or evaluation of circulating nitric oxide metabolites have shown that the post-exercise vasodilatory response does not rely on circulating nitric oxide availability27, 28. A recent study by New and colleagues28 indicates that the nadir of post-exercise hypotension coincides with the peak of appearance of lipid hydroperoxides in venous blood, suggesting that reactive oxygen species with known vasodilatory properties29-32 contribute to the exercise-induced decrease in systemic vascular resistance. In the current study, transdermal CO2 was applied to the thumb, and a significant increase in SPP was measured at the toe.
Thus, a similar circulating vasodilatory stimulus may contribute to the remote, sustained vasodilation created by local transdermal application of CO2. Further investigations will need to focus on the identification of the mechanisms involved in both the local and remote factors that contribute to the sustained hemodynamic changes produced by exposure of the skin to CO2.
Recently, studies have documented that episodes of brief, non-damaging ischemia occurring in a tissue can induce systemic protection against ischemia-reperfusion injury in a remote organ. This phenomenon, termed remote ischemic conditioning, has been demonstrated to confer protection against ischemic events in the myocardium33-35, brain36, and kidney37, 38.
Although shown to be effective in various clinical and pre-clinical models 34, 35, 38-40, the mechanism(s) of remote protection have not been clearly identified. Both neural and humoral mechanisms have been proposed to contribute to the protection against ischemic damage afforded by remote ischemic conditioning38, 39, 41-43.
Basalay et al.41 have shown that when remote ischemic conditioning is applied before induction of myocardial ischemia, sensory nerves and recruitment of a parasympathetic neural pathway are involved in reduction of infarct size. In contrast, application of remote ischemic conditioning after myocardial ischemia also afforded protection against infarction, but was not altered by vagotomy or peripheral denervation41.
Remote ischemic conditioning has also been demonstrated to improve perfusion of transplanted kidneys, suggesting that remote conditioning confers protection that does not rely on intact neural circuitry38. Recently, Michelsen and colleagues42 have demonstrated that dialysate of human plasma from subjects who underwent either ischemic preconditioning or exercise preconditioning reduced infarct size in rabbit hearts, indicating that release of a humoral factor, possibly acting on opioid receptors, contributes to the cardioprotective effects of ischemic and exercise preconditioning.
Other reports in the literature have also shown evidence of a humoral substances that mediate protection against ischemia when remote ischemic conditioning is applied; however, these substance(s) remain to be identified. Application of transdermal CO2 produces a remote vasodilation that may be mediated through release of a circulating humoral agent.
Future investigations will need to focus on assessment of plasma samples during and following transdermal CO2 application.
This pilot study demonstrated an increase in measures of remote skin microvascular function with D’OXYVA, a simple commercially-available device to deliver transdermal CO2. The effects of the treatment were evident at all periods up to and including the last test period, 240 minutes post-exposure.
Although the sample size was small in this study, a clear increase in SPP and SPP/SBP ratio and a decrease in SBP and DBP continued for 4 hours post-treatment. The differences in skin perfusion and blood pressure responses detected between diabetic and non-diabetic subjects will require further examination in larger studies.
Click below to access Prof. Judy Delp’s Presentation on Transdermal Delivery of Carbon Dioxide Boosts Microcirculation.