Tonocap

Simultaneous arterial (a), mixed-venous ( $\overline{\text{v}}$ ), and mesenteric-venous (mes) blood samples were withdrawn at each measurement time point to determine blood gases, hemoglobin, and lactate concentrations (The Alere Epoc blood analysis system; Alere, Waltham, MA). Concurrent mucosal jejunal CO₂ was measured by gas tonometry (Tonocap, Datex-Ohmeda; Tonometrics Division, Instrumentarium) as described elsewhere (4 (link)). O₂ and CO₂ parameters were calculated according to the following formulas: $\text{C}{\text{a}}_{{\text{O}}_2}=(\text{Hg}\times \text{S}{\text{a}}_{{\text{O}}_2}\times 1.34)+(\text{P}{\text{a}}_{{\text{O}}_2}\times 0.003)$
$\text{C}{\text{v}}_{{\text{O}}_2}=(\text{Hg}\times \text{S}{\text{v}}_{{\text{O}}_2}\times 1.34)+(\text{P}{\text{v}}_{{\text{O}}_2}\times 0.003)$
$\text{Da-}{\text{v}}_{{\text{O}}_2}=\text{C}{\text{a}}_{{\text{O}}_2}-\text{C}{\text{v}}_{{\text{O}}_2}$
$\text{D}{\text{o}}_2=\text{C}{\text{a}}_{{\text{O}}_2}\times \text{CO}$
$\stackrel{․}{\mathrm{V}}{\text{o}}_2=\text{C}{\text{a}}_{{\text{O}}_2}-\text{C}{\text{v}}_{{\text{O}}_2})\times \text{CO}$
$\text{E}{\text{R}}_{{\text{O}}_2}=(\text{C}{\text{a}}_{{\text{O}}_2}-\text{C}{\text{v}}_{{\text{O}}_2})/\text{C}{\text{a}}_{{\text{O}}_2}$
$\text{mes-E}{\text{R}}_{{\text{O}}_2}=(\text{C}{\text{a}}_{{\text{O}}_2}-\text{Cvme}{\text{s}}_{{\text{O}}_2})/\text{C}{\text{a}}_{{\text{O}}_2}$
$\text{Mixed venous-to-arterial}{\mathrm{C}\mathrm{O}}_2\text{\hspace{0.17em}difference\hspace{0.17em}}(\text{P}\overline{\text{v}}\text{-}{\text{a}}_{\text{C}{\text{O}}_2})=\text{P}{\overline{\text{v}}}_{\text{C}{\text{O}}_2}-\text{P}{\text{a}}_{\text{C}{\text{O}}_2}$
$\text{Mesenteric venous-to-arterial\hspace{0.17em}}{\mathrm{C}\mathrm{O}}_2\text{\hspace{0.17em}difference\hspace{0.17em}}(\text{Pmes-}{\text{a}}_{\text{C}{\text{O}}_2})=\text{Cvme}{\text{s}}_{\text{C}{\text{O}}_2}-\text{P}{\text{a}}_{\text{C}{\text{O}}_2}$
$\text{Jejunum mucosal-to-arterial\hspace{0.17em}}{\mathrm{C}\mathrm{O}}_2\text{\hspace{0.17em}difference\hspace{0.17em}}(\text{Ptis-}{\text{a}}_{\text{C}{\text{O}}_2})=\text{Pti}{\text{s}}_{\text{C}{\text{O}}_2}-\text{P}{\text{a}}_{\text{C}{\text{O}}_2}$ where $\text{C}{\text{a}}_{{\text{O}}_2}$ and $\text{C}{\text{v}}_{{\text{O}}_2}$ are the arterial and venous O₂ content, respectively; $\text{P}{\text{a}}_{\text{O}}{}_{{}_2}$ and ${\text{Pv}}_{{\text{O}}_2}$ represent the arterial and venous partial pressures, respectively, and CO represents cardiac output. $\text{E}{\text{R}}_{{\text{O}}_2}$ and $\text{mes-E}{\text{R}}_{{\text{O}}_2}$ represent the total and mesenteric O₂ extraction ratio, respectively.

Blood pressure (BP) was continuously measured using a non-invasive volume clamp method (Nexfin, Edwards Lifesciences BMEYE, Amsterdam, the Netherlands). Left ventricular SV was estimated by a pulse contour method (Nexfin CO-trek, Edwards Lifesciences BMEYE, Amsterdam, the Netherlands) [24 (link), 25 (link)] and CO was SV times heart rate (HR). SV index (SVI) was the ratio of SV and body surface area [26 ]. SPV and PPV were calculated from the BP signal:
$100\times \frac{{\text{A}}_{\text{max}}-{\text{A}}_{\text{min}}}{({\text{A}}_{\text{max}}+{\text{A}}_{\text{min}})/2}$
with A_max/min equal to, respectively, systolic arterial pressure (SAP) and pulse pressure (PP; SAP minus diastolic arterial pressure (DAP)). PPV and SPV were calculated for each breath and averaged over 5 consecutive breaths.
Airway flow and pressure were measured using an Alveotest flowmeter (Jaeger, Würzburg, Germany), tidal volume (TV) was the integral of airway flow (expressed in mL per kg predicted body weight) and end-tidal CO₂ (PetCO₂) was measured by capnography (Tonocap, Datex-Ohmeda, Madison, USA). Signals were visually inspected for artefacts and 60-second intervals were used for offline analysis (Matlab R2007b, Mathworks Inc. MA, USA).