In newborns PO2 measured at the skin surface (PcO2) correlates
with arterial PO
2
(PaO2) almost in a one to one relationship at
a sensor temperature of 43 to 44 °C, whereby the accuracy
of PcO
2
compared to PaO2 is best up to PaO2 of 80 mmHg
(10.67 kPa), above which it increasingly tends to read lower
than PaO
2
(especially in adults). As target PaO2 levels in
newborns are usually below 90 mmHg (12 kPa), a correction
of PcO
2
values measured at a sensor temperature of 43 to
44 °C is normally not necessary. TcPO
2
designates an estimate
of PaO
2
and corresponds to the measured PcO2.
Note: On the SDM and throughout this manual (unless
explicitly stated otherwise) ‘tcPO
2
’ is displayed/labeled as ‘PO2’.
Good to know!
Warming the skin tissue beneath the sensor to a constant
temperature improves accuracy as it a) increases capillary
blood flow/induces local arterialization, b) stabilizes
metabolism, and c) improves gas diffusion through skin
tissue. With increasing sensor temperature the application
duration (‘Site Time’) must be evaluated carefully and
adjusted accordingly to reduce the risk of burns. Special
attention must be given to patients with sensitive skin
at the sensor site (e.g. preterm or geriatric patients, burn
victims, patients with skin diseases) and/or very low skin
tissue perfusion beneath the sensor site (e.g. hypothermic
patients, patients with vasoconstrictions, low blood pressure,
or circulatory centralization (shock)).
Please refer to Technical Manual for the SDM (HB-005752)
and the references cited therein for additional information on
transcutaneous blood gas monitoring.
Limitations of tcPCO2 and tcPO
2
The following clinical situations or factors may limit the
correlation between transcutaneous and arterial blood gas
tensions:
• Hypo-perfused skin tissue beneath the sensor site due
to low cardiac index, circulatory centralization (shock),
hypothermia (e.g. during surgery), use of vasoactive drugs,
arterial occlusive diseases, mechanical pressure exercised
on measurement site, or inadequate (too low) sensor
temperature.
• Arterio-venous shunts, e.g. ductus arteriosus (PO
2
specific).
• Hyperoxemia (PaO
2
> 100 mmHg (13.3 kPa)) (PO2 specific).
• Inadequate measurement site (placement over large
superficial veins, on areas with skin edema (e.g. oedema
neonatorum), skin breakdown, and other skin anomalies).
• Improper sensor application resulting in an inadequate, not
hermetically sealed contact between the sensor surface and
the patient’s skin causing the CO
2
and O2 gases diffusing out
of the skin to intermix with ambient air.
• Exposure of the sensor to high ambient light levels (PO
2
specific).
CAUTION: Compared to the corresponding arterial blood
gases PCO
2
readings are typically too high and PO2 readings
typically too low if the measurement site is hypo-perfused.
CAUTION: The SDMS is not a blood gas device. Keep
the above mentioned limitations in mind when interpreting
PCO
2
and PO2 values displayed by the SDM.