End-tidal to arterial PCO2 ratio: a bedside meter of the overall gas exchanger performance

Intensive Care Medicine Experimental

Table 5 Changes in CT scan, respiratory mechanics and gas exchange variables in response to PEEP increase from 5 to 15 cmH₂O among P_ETCO₂/PaCO₂ ratio quartiles

P_ETCO₂/PaCO₂quartiles Δ	I	II	III	IV	p value
Ranges	[0.416,0.709]	[0.709,0.796]	[0.796,0.886]	[0.886,1.16]
Number of patients	N = 52	N = 51	N = 48	N = 49
Δ Lung volume, mL	579 [417; 775]	602 [436; 719]	588 [467; 735]	586 [443; 823]	0.98
Δ Lung gas, mL	546 [451; 816]	598 [413; 711]	551 [456; 700]	564 [395; 810]	0.99
Δ Lung tissue, g	10.9 [− 42; 53]	26 [− 5.1; 53]	20 [6.4; 43.4]	22.6 [− 23; 56]	0.87
Δ Not aerated tissue, mL	− 170 [− 282; − 78]	− 87.7 [− 160; − 42.3]	− 70.1 [− 124.4; − 40.3]	− 23.5 [− 91.5; 0.3]	< 0.001
Δ Poorly aerated tissue, mL	7.3 [− 50; 171]	− 42.4 [− 97.1; 14.1]	− 37.2 [− 105.7; 15]	− 73 [− 112; − 17]	0.02
Δ Normally aerated tissue, mL	140 [82; 241]	153 [68; 226]	143 [94; 197]	134 [87; 199]	0.96
Δ Hyperinflated tissue, mL	1.5 [0.17; 4]	0.85 [0.13; 1.8]	0.5 [0.04; 3.44]	0.5 [0.11; 5.4]	0.92
Δ Q_s/Q_t	− 0.13 [− 0.2; − 0.08]	− 0.08 [− 0.13; − 0.04]	− 0.08 [− 0.14; − 0.01]	− 0.07 [− 0.13; − 0.03]	0.01
Δ V_d/V_t	0.01 [0.0; 0.03]	0.02 [− 0.01; 0.04]	0.01 [0.0; 0.04]	0.03 [0.02; 0.06]	0.03
Δ PaO₂, mmHg	32 [18.5; 46]	23 [10.2; 58.4]	21.2 [4.3; 36]	16 [4.4; 28]	0.01
Δ E_RS, cmH₂O/L	0.07 [− 4.0; 3.4]	− 0.2 [− 2.2; 4.1]	0.3 [− 4.1; 2.4]	0.0 [− 2.5; 2.2]	0.85

Differences were computed as the variable at PEEP 15 cmH₂O minus the variable at PEEP 5 cmH₂O. Variables are expressed in medians and IQR. P for Mann–Whitney test. Q_s/Q_t: shunt fraction; V_d/V_t: dead space fraction; P/F: PaO₂/FiO₂ ratio; E_RS: respiratory system elastance; SvO₂: central venous oxygen saturation; P_ETCO₂: end-tidal CO₂