Addressing Assumptions for the Use of Non-invasive Cardiac Output Measurement Techniques During Exercise in COPD

Figures & data

Table 1. Patient characteristics

Variable	COPD (n = 13)
Age (years)	66 ± 1
Height (cm)	168 ± 3
Weight (kg)	74 ± 5
BMI (kg/m^2)	26 ± 2
FEV1 (% predicted)	58 ± 3
RV (% predicted)	149 ± 10
TLC (L)	7.0 ± 0.4
RV/TLC	51 ± 2
FRC (% predicted)	152 ± 10
IC (L)	2.5 ± 0.1
D LCO (% predicted)	55 ± 4
D LCO / VA (ml/mmHg/min/L)	5.5±3.3
PaO2 (mmHg)	76.1 ± 2.7
Pa CO2 (mmHg)	42.5 ± 1.5
Hct (%)	48.1 ± 1.1
pH	7.43 ± 0.01
Peak exercise data
VO2 (LO2·min^-1)	1.2 ± 0.4
Power (watt)	88 ± 29
VE (L·min^-1)	60 ± 17
HR (bpm)	131 ± 29
SpO2	94.5 ± 0.5
PET CO2	32.4 ± 1.7

BMI = body mass index; FEV₁ = forced expiratory volume in one second; RV = residual volume; TLC = total lung capacity; FRC = forced residual volume; IC = inspiratory capacity; D_LCO = diffusing capacity of the lung for carbon monoxide; PaO₂ (mmHg) = O₂ content in arterial blood PaCO₂ (mmHg) = CO₂ content in arterial blood;SpO₂ (%): pulse oximetry derived oxyhemoglobin saturation.

Table 2. Cardiorespiratory responses to cycle exercise

	(% Wpeak)
	Baseline	20	35	50
Power output (watts)	0	16.5 ± 0.5^*	29.5 ± 0.8^*	41.2 ± 1.2^*
(L min^-1)	0.31 ± 0.01	0.56 ± 0.01 ^*	0.67 ± 0.02 ^*	0.79 ± 0.02 ^*
HR bpm	80 ± 5	89 ± 5	97 ± 7	103 ± 6 *
(L min^-1)	16.0 ± 0.9	26.1 ± 1.4 ^*	30.5 ± 1.6 ^*	36.4 ± 1.7 ^*
VT (ml)	1.0 ± 0.1	1.1 ± 0.1	1.3 ± 0.1 *	1.4 ± 0.1 *
Fb (breaths/min)	18.9 ± 1.2	24.3 ± 1.1 *	30.5 ± 1.6 *	28.4 ± 1.6 *
IC (L)	2.6 ± 0.2	2.5 ± 0.2	2.5 ± 0.2	2.3 ± 0.2
SpO2 (%)	94.8 ± 0.5	94.7 ± 0.5	94.8 ± 0.7	94.6 ± 0.5

*Indicates P ≤ 0.05 from baseline.

= oxygen uptake; = ventilation; SpO2 (%): pulse oximetry-derived oxyhemoglobin saturation.

Figure 1. Cardiac output in COPD derived using various non-invasive techniques. Results consistently show significantly lower measured using N₂O-RB as compared to the PaCO₂-derived CO₂-RB or the bioimpedance approach, at rest as well as for all exercise intensities (*p < 0.05).

End-tidal CO₂-derived rebreathing values are also significantly lower than those calculated using direct PaCO₂ under all conditions (^§p < 0.05); End-tidal CO₂-derived rebreathing values are however not statistically different from those obtained using N₂O-RB; CO₂-RB(PaCO₂): Cardiac output () derived from non-inert CO₂ gas rebreathing with PaCO₂ correction; CO₂-RB(PETCO₂): Cardiac output () derived from non-inert CO₂-RB gas rebreathing without PaCO₂ correction; N₂O-RB: Cardiac output () derived from inert N₂O gas rebreathing; Impedance: Cardiac output () determined by impedance cardiography.

Figure 2. Cardiac output derived using various non-invasive techniques in healthy subjects measured in our laboratory. Comparison of techniques at rest or at each exercise intensities reveals no statistically significant differences.

CO₂-RB(PETCO₂): Cardiac output () derived from non-inert CO₂-RB gas rebreathing without PaCO₂ correction; N₂O-RB: Cardiac output () derived from inert N₂O gas rebreathing. Impedance: Cardiac output () determined by impedance cardiography.