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Respiratory Assessment and Monitoring 349

Research vignette, Continued
recruitment manoeuvre at five time points. Seventy-three samples For analysis, data from the arterial blood sample SaO 2 was consid-
were included for analysis from 16 patients. The SaO 2 values ranged ered the ‘gold standard’, and compared to the forehead and the
from 73–99.6%. The forehead sensor provided measurements that finger sensor SpO 2 values. Each patient was used as their own
deviated more from arterial measures than the finger sensor (mean control for the five different measures: baseline, SRM at maximum
absolute deviations 3.4%, 1.1% respectively, P = 0.02). The greater PEEP, end of SRM, 30 and 60 minutes after SRM. A repeated mea-
variability in forehead measures taken at maximum PEEP was sures T test was used to assess for systematic differences on each
reflected in the unusually large precision estimates of 4.24% associ- of the five measurement points. Bland-Altman analysis was used
ated with these measures. No absolute differences from arterial to illustrate differences between forehead or finger sensors and the
measures taken at any other time points were significantly differ- gold standard SaO 2 . This analysis is an alternative to correlation
ent. The finger sensor is as accurate as the forehead sensor in coefficients which can be misleading, as correlation measures the
detecting changes in arterial oxygen saturation in adults with strength of relation between two variables but not the agreement
acute respiratory distress syndrome and it may be better at levels between them. Bland-Altman analysis is based on graphic tech-
of high PEEP such as during recruitment manoeuvres. niques and simple calculations (see Further reading). This paper
presents easily comprehensible figures demonstrating the differ-
Critique ences between forehead and finger sensors, and arterial blood
Critical care nurses often have to manage different monitoring oxygen saturation.
equipment, and patient safety is reliant on the function and preci- The study authors examined for measurement bias (systematic
sion of devices. This study compared forehead and finger sensors measurement differences between finger or forehead sensor and
in pulse oximetry in the ICU. Pulse oximetry is standard equipment ‘gold standard’). A small but statistically significant difference was
for assessing respiratory status and finger sensors are the most noted when comparing finger sensor SpO 2 and SaO 2 ; however this
common probe to measure oxygen saturation SpO 2 in critically ill was less than 1% and not considered clinically significant. Of note,
adults. Use of forehead sensors is a new technique believed to be there was a significant difference between the forehead and finger
less vulnerable to peripheral vasoconstriction and motion artifact sensor at maximum PEEP (40 cmH 2O); the forehead sensor devi-
than the finger sensor. The authors described that comparisons ated more in measurement from the SaO 2 than the finger sensor.
between these two sensors were previously conducted in studies There was also drop-out of signal from one patient with the finger
during anaesthesia, mechanical ventilation and low cardiac index. sensor at maximum PEEP level; the authors discussed that this
In the present study the probes were tested in patients with ARDS indicates that the equipment may not be reliable under all circum-
while a stepwise recruitment manoeuvre (SRM) was performed. stances. When comparing forehead and finger sensor saturation at
The SRM using high PEEP may cause a reduced cardiac output (CO) more routine PEEP levels, the differences were within an accept-
and damped arterial waves due to the subsequent high intratho- able range. Patients with any compromise in heart rate, blood pres-
racic pressure. For this reason it was relevant to compare the finger sure, arrhythmia or SpO 2 (<85%) during SRM were withdrawn.
sensor that influenced arterial waves with a forehead sensor that There was no discussion whether some patients had any of these
would remain unaffected.
complications; the authors described that 7 samples were not
This single-site prospective consecutive study included 16 included in the analysis due to low reliability of signals at maximum
mechanically ventilated (MV) adult patients with early ARDS; ven- PEEP.
tilation was pressure-controlled with different levels of PEEP. All
patients had a radial arterial line with invasive blood pressure For the primary outcome measure, finger sensors were more accu-
monitoring, and a central venous catheter. Excluded patients were rate than the forehead sensors at high PEEP levels during SRM. The
those with pneumothorax, intercostal catheter with air leak, bro- study demonstrated that the hypothesis – the newer forehead
chospasm, acute pulmonary oedema, raised intracranial pressure, sensor could measure oxygen saturation better – was not sup-
arrhythmia or mean arterial pressure (MAP) below 60 mmHg. The ported. The study did have a small sample, and these findings
authors did not indicate that any next of kin declined participation therefore need to be evaluated in a larger trial. This study can be
for the patient, and there was no explanation as to why only 16 seen as a pilot study; a common and appropriate way of creating
patients were included in the study. The demographic data dem- evidence for a larger trial. Also note that these results only relate
onstrated a good mix of patients in the ICU of different age, gender, to a certain brand of equipment; this could have been discussed
and diagnosis, although there was no detail about the sampling further.
procedure. It was not clear whether measurements of SaO 2 and Overall, this small but well-conducted study is an important con-
SpO 2 were gathered by the same data collector. Each patient was tribution to understanding the precision and reliability of new
tested on five different occasions (73 measures in total; seven were equipment, and reflects clinical practice in ICU. This study comple-
excluded due to equipment failure). Post-hoc sample size calcula- ments information provided in this chapter, and highlights
tions indicated that with a probability of 85% the study would potential measurement bias with equipment. Nurses need to
detect a treatment difference at 5% significance level if the true be confident in clinical information provided by monitoring
difference between the means was 2%; this makes the study find- equipment, to ensure that the assessment and monitoring of a
ings trustworthy. patient is not compromised nor their safety threatened.