End tidal-to-arterial CO₂and O₂gas gradients at low- and high-altitude during dynamic end-tidal forcing

Abstract

We sought to characterize and quantify the performance of a portable dynamic end-tidal forcing (DEF) system in controlling the partial pressure of arterial CO2 (Pa(CO2)) and O2 (Pa(O2)) at low (LA; 344 m) and high altitude (HA; 5,050 m) during an isooxic CO2 test and an isocapnic O2 test, which is commonly used to measure ventilatory and vascular reactivity in humans (n = 9). The isooxic CO2 tests involved step changes in the partial pressure of end-tidal CO2 (PET(CO2)) of -10, -5, 0, +5, and +10 mmHg from baseline. The isocapnic O2 test consisted of a 10-min hypoxic step (PET(O2) = 47 mmHg) from baseline at LA and a 5-min euoxic step (PET(O2) = 100 mmHg) from baseline at HA. At both altitudes, PET(O2) and PET(CO2) were controlled within narrow limits (<1 mmHg from target) during each protocol. During the isooxic CO2 test at LA, PET(CO2) consistently overestimated Pa(CO2) (P < 0.01) at both baseline (2.1 ± 0.5 mmHg) and hypercapnia (+5 mmHg: 2.1 ± 0.7 mmHg; +10 mmHg: 1.9 ± 0.5 mmHg). This P(a)-PET(CO2) gradient was approximately twofold greater at HA (P < 0.05). At baseline at both altitudes, PET(O2) overestimated Pa(O2) by a similar extent (LA: 6.9 ± 2.1 mmHg; HA: 4.5 ± 0.9 mmHg; both P < 0.001). This overestimation persisted during isocapnic hypoxia at LA (6.9 ± 0.6 mmHg) and during isocapnic euoxia at HA (3.8 ± 1.2 mmHg). Step-wise multiple regression analysis, on the basis of the collected data, revealed that it may be possible to predict an individual's arterial blood gases during DEF. Future research is needed to validate these prediction algorithms and determine the implications of end-tidal-to-arterial gradients in the assessment of ventilatory and/or vascular reactivity.

Shear-mediated dilation of the internal carotid artery occurs independent of hypercapnia

“…All cardiorespiratory variables were sampled continuously at 1KHz via an analogue-to-digital The PETO2 and PETCO2 were controlled by a portable dynamic end-tidal forcing system that has been previously described in detail (42,43). Our end-tidal forcing system effectively controls endtidal gases through wide ranges of PETCO2 and PETO2 independent of ventilation (42,43) and has been previously used for similar P ETCO2 manipulations (21).…”

“…Our end-tidal forcing system effectively controls endtidal gases through wide ranges of PETCO2 and PETO2 independent of ventilation (42,43) and has been previously used for similar P ETCO2 manipulations (21).…”

Comparing and characterizing transient and steady‐state tests of the peripheral chemoreflex in humans

“…The experimental set-up was similar between protocols with the exception that during steady-state tests, P ET,O 2 and P ET,CO 2 were controlled in a stepwise fashion using a dynamic end-tidal forcing system (DEF; Tymko et al 2015). For these tests, we only measured and calculated respiratory variables, including V TI , f R ,V I , P ET,CO 2 , P ET,O 2 and S cO 2 .…”

“…Swansen & Bellville 1975;Robbins et al 1982;Steinback & Poulin, 2007;Koehle et al 2009;Tymko et al 2015). This technique is used to deliver a specific hypoxic stimulus while simultaneously controlling CO 2 at a constant level in attempt to 'target' the peripheral chemoreceptors.…”

The effects of graded changes in oxygen and carbon dioxide tension on coronary blood velocity independent of myocardial energy demand

“…Time corrections were applied to the PO2 and PCO2 signal to account for the gas analyzer delay, and values corresponding to the moment of end expiration were identified as the PETO2 and PETCO2. A dynamic end-tidal forcing system was used to clamp and manipulate both PETO2 and PETCO2 throughout the protocol as previously described (44,47,48). After instrumentation, a normoxic CO2 reactivity test was conducted first, followed by a 30-min rest and an isocapnic hypoxia reactivity test in succession; the trials were not randomized to avoid any carryover effects of hypoxia on sympathetic nervous system activation (53).…”