Capillary PO₂ does not adequately reflect arterial PO₂ in hypoxemic COPD patients

Figures & data

Table 1 Demographic data, ABG, and lung function parameters

N=102	Median	Min	Max	IQR
Male/female (n)	51/51 (50%)
Age (years)	67.5	44	84	62–73
PY (years)a	50.0	0.0	160.0	35.0–65.0
BMI (kg/m^2)	25.5	13.7	57.3	21.4–31.0
apH	7.43	7.31	7.53	7.40–7.45
PaCO2 (mmHg)	45.9	31.5	85.8	41.6–52.1
aHCO3^− (mmol/L)	28.5	20.8	46.3	26.7–30.6
SaO2b (%)	89.1	67.9	98.3	85.6–91.4
SpO2b (%)	88.0	72.0	94.0	85.0–90.0
FEV1 (%)	30	10	62	23–37
FEV1 (L)	0.76	0.31	1.89	0.61–0.94
FEV1/IVC (%)	48	23	69	41–56
TLC (%)c	113	45	210	93–124
TLC (L)c	6.17	2.68	13.18	5.25–7.57
RV (%)d	204	54	506	158–247
KCO (%)e	59	13	133	42–84
TLCO (%)e	36	5	79	28–48

Notes:

a n=98;

b n=100;

c n=87;

d n=86;

e n=43.

Abbreviations: ABG, arterial blood gas analysis; _aHCO₃⁻, arterial standard bicarbonate; BMI, body mass index; _apH, arterial pH; DLCO, diffusing capacity of the lung for carbon monoxide; FEV₁, forced expiratory volume in 1 s; IQR, interquartile range; IVC, inspiratory vital capacity; KCO, transfer coefficient of the lung for carbon monoxide (= Krogh factor); P_aCO₂, partial pressure of arterial carbon dioxide; PY, pack years; RV, residual volume; S_aO₂, arterial oxygen saturation; S_pO₂, peripheral oxygen saturation; TLC, total lung capacity; TLCO, transfer factor of the lung for carbon monoxide.

Table 2 Comparison of P_aO₂ and P_cO₂ in all patients (primary outcome)

n	Measurement 1			Measurement 2			Difference		LOA (95% CI)
	Variable	Mean	SD	Variable	Mean	SD	Mean	SD	Lower	Upper
102	PaO2	56.28	9.08	PcO2	50.29	5.94	5.99	6.05	−5.88 (−7.92 to −3.84)	17.85 (15.81 to 19.89)

Note: All values are expressed in mmHg.

Abbreviations: LOA, limits of agreement; P_aO₂, partial pressure of arterial oxygen; P_cO₂, partial pressure of capillary oxygen.

Figure 1 Bland–Altman comparison of P_aO₂ and P_cO₂ for P_cO₂ values ≤60 mmHg (primary outcome, n=102).

Abbreviations: P_aO₂, partial pressure of arterial oxygen (in mmHg); P_cO₂, partial pressure of capillary oxygen.

Table 3 Comparison of blood gas measurements and peripheral saturation measurements for P_cO₂ ≤55 mmHg (secondary outcome)

n	Measurement 1			Measurement 2			Difference		LOA (95% CI)
	Variable	Mean	SD	Variable	Mean	SD	Mean	SD	Lower	Upper
73	PaO2	52.88	7.34	PcO2	47.55	4.69	5.33	5.52	−5.48 (−7.69 to −3.27)	16.15 (13.94 to 18.36)
73	PaCO2	47.90	7.61	PcCO2	49.06	7.59	−1.16	4.06	−9.11 (−10.73 to −7.48)	6.79 (5.17 to 8.42)
73	apH	7.43	0.04	cpH	7.43	0.04	0.00	0.02	−0.04 (−0.05 to −0.03)	0.04 (0.03 to 0.05)
73	aHCO3^−	29.75	4.00	cHCO3^−	30.06	4.05	−0.31	0.69	−1.65 (−1.93 to −1.38)	1.04 (0.76 to 1.31)
72	SaO2	86.73	5.11	ScO2	85.27	4.86	1.47	6.63	−5.65 (−7.12 to −4.19)	8.58 (7.12 to 10.05)
72	SaO2	86.73	5.11	SpO2	86.25	3.97	0.48	2.87	−5.14 (−6.29 to −3.98)	6.10 (4.94 to 7.25)
72	SpO2	86.25	3.97	ScO2	85.27	4.86	0.98	2.77	−4.44 (−5.55 to −3.32)	6.40 (5.29 to 7.52)

Note: All values are expressed in mmHg.

Abbreviations: _apH, arterial pH; _aHCO₃⁻, arterial standard bicarbonate; _cHCO₃⁻, capillary standard bicarbonate; _cpH, capillary pH; LOA, limits of agreement; P_aCO₂, partial pressure of arterial carbon dioxide; P_cCO₂, partial pressure of capillary carbon dioxide; P_aO₂, partial pressure of arterial oxygen; P_cO₂, partial pressure of capillary oxygen; S_aO₂, arterial oxygen saturation; S_cO₂, capillary oxygen saturation; S_pO₂, peripheral oxygen saturation.

Figure 2 Bland–Altman comparison of P_aO₂ and P_cO₂ for P_cO₂ values ≤55 mmHg (secondary outcome, n=73).

Abbreviations: P_aO₂, partial pressure of arterial oxygen (in mmHg); P_cO₂, partial pressure of capillary oxygen.

Table 4 Comparison of blood gas measurements and peripheral saturation measurements in all patients (secondary outcome)

n	Measurement 1			Measurement 2			Difference		LOA (95%CI)
	Variable	Mean	SD	Variable	Mean	SD	Mean	SD	Lower	Upper
102	PaO2	47.04	8.32	PcO2	48.01	8.23	−0.97	3.75	−8.31 (−9.5 to −7.05)	6.37 (5.11 to 7.63)
102	apH	7.43	0.04	cpH	7.43	0.04	0.00	0.02	−0.04 (−0.05 to −0.03)	0.04 (0.03 to 0.05)
102	aHCO3^−	29.22	3.82	cHCO3^−	29.52	3.86	−0.30	0.70	−1.67 (−1.91 to −1.43)	1.08 (0.84 to 1.31)
100	SaO2	88.29	5.13	ScO2	87.00	5.03	1.29	3.27	−5.11 (−6.22 to −4.00)	7.69 (6.58 to 8.80)
100	SaO2	88.29	5.13	SpO2	87.33	3.93	0.96	2.67	−4.28 (−5.19 to −3.37)	6.20 (5.29 to 7.11)
100	SpO2	87.33	3.93	ScO2	87.00	5.03	0.33	2.73	−5.02 (−5.95 to −4.09)	5.68 (4.75 to 6.61)

Note: All values are expressed in mmHg.

Abbreviations: _aHCO₃⁻, arterial standard bicarbonate; _apH, arterial pH; _cHCO₃⁻, capillary standard bicarbonate; _cpH, capillary pH; LOA, limits of agreement; P_aCO₂, partial pressure of arterial carbon dioxide; P_cCO₂, partial pressure of capillary carbon dioxide; S_aO₂, arterial oxygen saturation; S_cO₂, capillary oxygen saturation; S_pO₂, peripheral oxygen saturation.

Table 5 Comparison of P_aO₂ and P_cO₂ with respect to P_cO₂-based indication of LTOT

	PaO2 ≤55 mmHg (n=52)	PaO2 >55 mmHg (n=50)
PcO2 ≤55 mmHg (n=73)	52 (51.0%)	21 (20.6%)
PcO2 >55 mmHg (n=29)	0 (0%)	29 (28.4%)

Abbreviations: P_aO₂, partial pressure of arterial oxygen; P_cO₂, partial pressure of capillary oxygen.

Table 6 Assessment of pain associated with blood gas analyses

n=102	Mean	Min	Max	SD
ABGVAS	23.86	0	86.0	22.72
CBGVAS	13.98	0	70.0	16.19

Abbreviations: ABG_VAS, pain (rated on the 100-mm visual analog scale) associated with arterial blood gas analysis; CBG_VAS, pain associated with capillary blood gas analysis (on 100-mm visual analog scale); max, maximum value; min, minimum value.