Feasibility of critical care ergometry: Exercise data of patients on mechanical ventilation analyzed as nine-panel plots

Abstract

Nine-panel plots are standard displays of cardiopulmonary exercise data, used in cardiac and pulmonary medicine to investigate the nature of exercise limitation. We explored whether this approach could be used to analyze the data of critically ill patients on mechanical ventilation, capable of exercising actively. Patients followed an incremental exercise protocol using a bedside cycle ergometer. Respiratory gases were analyzed using indirect calorimetry, and blood gases were sampled from arterial catheters. Data of seven patients were combined into nine-panel plots. Systematic analysis clarified the nature of exercise limitation in six cases. Resting metabolic rate was increased in all patients, with a median oxygen uptake ( ${\dot{V}O}_2$ ) of 5.52 (IQR 4.29-6.31) ml/kg/min. Unloaded cycling increased the ${\dot{V}O}_2$ by 19.8% to 6.61 (IQR 5.99-7.08) ml/kg/min. Adding load to the ergometer increased the ${\dot{V}O}_2$ by another 20.0% to reach ${\dot{V}O}_{2\text{peak}}$ at a median of 7.14 (IQR 6.67-10.75) ml/kg/min, corresponding to a median extrinsic workload of 7 W. This was accompanied by increased CO₂ production, respiratory minute volume, heart rate, and oxygen pulse. Three patients increased their ${\dot{V}O}_2$ to >40% of predicted ${\dot{V}O}_{2max}$ , two patients passed the anaerobic threshold. Dead space ventilation was 44%, decreasing to 42% and accompanied by lower ventilatory equivalents during exercise. Exercise produced no net change in alveolo-arterial PO₂ difference. We concluded that diagnostic ergometry in mechanically ventilated patients was feasible. Analysis of the data as nine-panel plots provided insight into individual limitations to exercise.

Keywords: circulation; critical care; exercise physiology; mechanical ventilation; oxygen consumption.

FIGURE 1

Respiratory gas analysis of seven patients during five phases of exercise. Panels (a–d) correspond to panels 1, 2, 4, and 5 of the nine‐panel plots proposed by Wasserman (5th Edition, 2012), respectively. Panel (a) shows O₂ uptake (${\dot{\mathrm{V}}\mathrm{O}}_2$) and CO₂ production (${\dot{\mathrm{V}}\text{CO}}_2$), panel (b) shows heart rate (HR) and O₂‐pulse, panel (c) shows the ventilatory equivalent for O₂ (EqO₂) and CO₂ (EqCO₂), and panel (d) shows the expiratory minute volume ($\dot{\mathrm{V}}\mathrm{E}$).${\dot{\mathrm{V}}\mathrm{O}}_{2\text{peak}}$ did not always coincide with the end of exercise

FIGURE 2

Proposed layout of nine‐panel plot for patients on mechanical ventilation. Exercise data of a mechanically ventilated patient, presented as a nine‐panel plot. The exercise test was stopped because of high systolic blood pressure. Light brown areas indicate unloaded cycling, purple areas indicate loaded cycling, vertical grey lines indicates the anaerobic threshold (at a ${\dot{\mathrm{V}}\mathrm{O}}_2$ of 800 ml/min for this patient); the tangent in panel 3 is an aid to determine the anaerobic threshold (V‐slope method). Panels 1 to 7 are identical to the nine‐panel plot proposed by Wasserman (5th Edition, 2012), with the exception of SpO₂ (moved from panel 7 to panel 6), P_EO₂ and P_ECO₂ (added to panel 7), and P_AO₂ (added to panel 7). Panels 8 and 9 are new additions. Abbreviations: EqCO₂, ventilatory equivalent for CO₂ ($\dot{\mathrm{V}}\mathrm{E}$/${\dot{\mathrm{V}}\text{CO}}_2$); EqO₂, ventilatory equivalent for O₂ ($\dot{\mathrm{V}}\mathrm{E}$/${\dot{\mathrm{V}}\mathrm{O}}_2$); HR, heart rate; P_aO₂, arterial oxygen pressure; P_aCO₂, arterial carbon dioxide pressure; P_AO₂, ideal alveolar oxygen pressure; P_ECO₂, mixed expired carbon dioxide pressure; P_EO₂, mixed expired oxygen pressure; P_ETCO₂, end‐tidal carbon dioxide pressure; P_ETO₂, end‐tidal oxygen pressure; PO₂, partial pressure of oxygen; PCO₂, partial pressure of carbon dioxide; RER, respiratory exchange ratio (${\dot{\mathrm{V}}\mathrm{O}}_2$/${\dot{\mathrm{V}}\text{CO}}_2$); SBP, systolic blood pressure; SpO₂, peripheral capillary oxygen saturation; T_E, expiratory time; T_I, inspiratory time (s); ${\dot{\mathrm{V}}\text{CO}}_2$, carbon dioxide production; $\dot{\mathrm{V}}\mathrm{E}$, minute ventilation; ${\dot{\mathrm{V}}\mathrm{O}}_2$, oxygen uptake; VT, tidal volume