Evaluation of carbon dioxide rebreathing during exercise assisted by noninvasive ventilation with plateau exhalation valve

Figures & data

Figure 1 Experimental setup.

Notes: The facemask was attached in series with capnostat5 (mainstream CO₂ sensor), pneumotachograph, PEV (flow of the PEV was directly recorded by a pneumotachograph), ventilator tubing, and BiPAP machine. Airway pressure and flow and the volume of CO₂ delivered from the circuit at each breath were measured at the distal end of the ventilator circuit. Furthermore, to evaluate the leak flow of the PEV, another pneumotachograph was attached to the PEV.
Abbreviations: BiPAP, bi-level positive airway pressure; PEV, plateau exhalation valve.

Table 1 Characteristics of study subjects

Male:Female (n)	18:0
Age (years)	69.3±1.4
Height (cm)	165.6±1.2
Weight (kg)	60.2±1.9
BMI (kg/m^2)	22.0±0.6
FEV1 (L)	0.77±0.04
FEV1 (% predicted)	29.5±1.6
FEV1/FVC (%)	35.0±1.5
FVC (L)	2.22±0.11
FVC (% predicted)	65.5±2.95
MVV (L/min)	33.4±1.6
MVV (% predicted)	32.8±1.8
Peak watt (W)	56.3±16.1
Peak VO2/kg (mL/min/kg)	13.4±2.9
Peak VO2 (% predicted)	47.1±12.0
Peak VE (L/min)	28.2±6.5
Resting RR (breath/min)	18.7±3.0
Peak RR (breath/min)	29.2±5.8
Peak tidal volume (L)	0.99±0.25
Peak HR (beat/min)	115.1±11.6
PaCO2 (mmHg)	40.7±4.4

Note: Results are reported as mean ± standard deviation after testing for normal distribution (Kolmogorov–Smirnov test).

Abbreviations: BMI, body mass index; FEV₁, forced expiratory volume in 1 s; FVC, forced vital capacity; HR, heart rate; MVV, maximal voluntary ventilation; RR, respiratory rate; VE, minute ventilation; VO₂, oxygen uptake; PaCO₂, partial pressure of CO₂.

Table 2 Comparison of breath characteristics at rest and peak exercise in COPD patients

	COPD patients
	At rest	Peak-exercise
IPAP (cmH2O)	14.1±1.2	13.6±2.0
EPAP (cmH2O)	4.1±0.2	4.5±0.4$
Vte/Te (L/s)	0.39±0.15	0.82±0.27$
Tidal FiCO2 (%)	0.48±0.19	1.8±0.6$
FetCO2 (%)	2.6±0.7	5.5±0.6$
Ex/Te (L/s)	0.41±0.02	0.39±0.03$
Ti (s)	0.67±0.12	0.98±0.22$
Te (s)	2.59±0.75	1.53±0.29$
Vte (L)	0.92±0.27	1.21±0.31$
Ex vol (L)	1.06±0.31	0.61±0.13$
Ti/Ttot (%)	0.28±0.05	0.31±0.05
RR (breath/min)	17.9±4.9	27.9±4.9$
SpO2 (%)	98.0±1.2	88.8±2.0$
HR (beat/min)	88.5±10.6	119.7±8.8$
Systolic blood pressure (mmHg)	120.3±11.9	174.1±8.5$
Diastolic blood pressure (mmHg)	71.5±11.2	87.2±8.5$

Notes: Results are reported as mean ± standard deviation after testing for normal distribution (Kolmogorov–Smirnov test).

$ P<0.01 peak-exercise vs at rest.

Abbreviations: COPD, chronic obstructive pulmonary disease; EPAP, expiratory positive airway pressure; Ex vol, the volume vented out from the exhalation valve during expiration; Ex/Te, the mean flow vented out from the plateau exhalation valve during expiration; FetCO₂, fractional concentration of end-tidal CO₂; HR, heart rate; IPAP, inspiratory positive airway pressure; RR, respiratory rate; SpO₂, oxygen saturation; Te, expiratory time; Ti, inspiratory time; Ti/Ttot, inspiratory duty cycle; Tidal FiCO₂, mean inspiratory fraction of CO₂; Vte, expiratory tidal volume; Vte/Te, mean expiratory flow.

Figure 2 The changes of Ex/Te varied with Vte/Te throughout rest and exercise periods in one representative subject.

Abbreviations: Ex/Te, mean flow vented out from the plateau exhalation valve during expiration; Vte/Te, mean expiratory flow.

Figure 3 The changes of tidal FiCO₂ varied with Vte/Te throughout rest and exercise periods in two representative subjects (A, B).

Notes: (■) Data points are mean values of the expiratory tidal flow and fraction of inspired CO₂ of the next breath for respective subjects. (-) represents the curve that fits the scatter plot.

Abbreviations: FiCO₂, fractional concentration of inspired CO₂; Vte/Te, mean expiratory flow.

Figure 4 Typical example of analysis of nonlinear curve fitness of mean expiratory flow-tidal FiCO₂ curve.()()Notes: The filled square (■) on the fitted curve (thick line) represents V_k which divides the curve to yield the best fit of two lines (A and B, thin lines). Closed circle (○) represents the calculated inflection point.

Abbreviations: FiCO₂, fractional concentration of inspired CO₂; Vte/Te, mean expiratory flow.