DISCUSSION
Respiratory failure is one of the most common conditions in the ICU and
often requires chest imaging for accurate diagnosis38,
39. Physicians need to make an accurate diagnosis, define disease
severity, assess response to treatment and prognosticate, and PU
represents an emerging technique for accomplishing these in a safe,
timely, and cost-efficient manner. Portable chest x-ray has significant
limitations7 and potential risks (e.g. accidental
removal of lines/tubes) while chest CT has notable drawbacks such as
increased radiation, cost, and the need to transport the patient from
the ICU9, 10, 40.
Small studies of PU scoring systems in disease-specific patient
populations and their association to various clinical metrics
exist5, 11, 15–20, as does a large (260 patients)
study proposing a standardized diagnostic system based on lung
ultrasound profiles1. However, to our knowledge a
large prospective study of a simple lung ultrasound scoring system and
its association to clinical outcomes including mortality across a wide
spectrum of clinical disease has not been previously reported. The
current approach was studied in 250 patients from medical/surgical,
neuro, and cardiovascular ICUs, and thus the results are highly
applicable to a diverse ICU population. Furthermore, this 9-point ,
taking on average 2-3 minutes to perform, can be feasibly integrated
into the daily physical of patients in their typical positioning.
The 9 lung zones included in this algorithm reflect a clinical in an
applied setting, rather than a workflow designed for a research
protocol. There are no formally endorsed approaches to a clinical PU ,
though both an 8 and 28-point have been described5. A
clinical PU sequence often involves conceptual lobar anatomy, and thus
the right lateral, caudal area was divided into both an anterior and
posterior zone over the right middle and lower lobes respectively.
Designation of an “extra” zone in the right lung, in addition to being
clinically relevant, can be physiologically rationalized based on the
greater total lung capacity of the right lung41. Of
interest, 3.9% of our patients with aspiration and pneumonia had
findings present only in the right lateral, caudal zone on the anterior
axillary line that would have been missed if that zone was not included
in the . A sensitivity analysis was conducted both with and without this
additional zone included, and similar associations between lung scores
and outcomes were observed with both methods.
Many previous studies have examined the association of the presence of
B-lines to various outcomes. However, in this study, ascending points (0
to 3) were assigned to the continuum of lung abnormality classifications
from the normal, fully aerated A classification (0 points) to the least
aerated consolidation and atelectasis (3 points). Of note, there was no
significant difference in the predictive power of our score when
atelectasis, consolidation, or small consolidations resulted in an extra
point beyond the B3 classification (i.e. the zone was given a total of 4
points rather than 3) versus if each of these entities received 3 points
for simplicity. By assigning 1 point for each zone with effusion
(with patients positioned semi-upright) a feasible and semi-quantitative
assessment of pleural effusion was incorporated into the TLS. The
modification of isolated atelectasis in the lateral caudal lung zone on
the posterior axillary line receiving 1 point instead of 3, resulted in
improved predictive power across metrics and is consistent with the
limited significance this isolated finding has in the clinical
environment.
The association of this PU scoring system to mortality in a large
population of intubated ICU patients with ARF is important from a
clinical standpoint, as various small studies have only demonstrated
associations of specific PU findings with outcomes in specific disease
states15, 17–20. Non-ultrasound based models predict
mortality amongst ICU patients, however they typically rely on a
combination of physiologic variables and comorbid conditions, and few
exist specifically for intubated patients42, 43.
Beyond the observed association with mortality, potential use of the TLS
in predicting ventilator hours and LOS may be useful in guiding family
expectations, patient flow, and quality measures tied to LOS, but this
requires further study.
Some limitations to our study exist. To preserve efficiency, we examined
a limited surface area of the lung and this may have led to
overestimation of a process that only existed under the transducer, or
the inability to visualize a pulmonary process present just outside the
area of examination. Our inter-observer agreement was calculated based
solely on image review, thus dismissing the variability that can exist
in image acquisition. Physicians performing the were blinded to patient
data, but there were opportunities for this blinding to have been
incomplete and possibly influence their assessment (e.g. medication
infusions visible at the bedside, other provider discussions in/around
the patient room). Having a single provider perform the repeat on an
individual patient, while meant to ensure consistency, may have
introduced bias with the examiner knowing the previous location of
findings. Despite these instances of potential “unblinding”, they all
do reflect the reality of PU in the clinical setting. Retrospective
chart review was used to determine a final diagnosis, but, as can be the
case in ARF patients, more than one etiology may have been contributing,
or the diagnosis was not definitive. The number of extubation more than
48 hours after the initial exam was small (n=30) due to ICU workflow and
difficulty coordinating a blinded US with the treatment team. This
prevented us from adequately assessing TLS change from baseline to
extubation.
The differential diagnosis for ARF is often difficult to narrow based on
history, physical , and laboratory data alone. PU is immediately
available at the bedside and can provide helpful diagnostic information
in these patients, especially when combined with cardiac and vascular
US. This simple PU scoring system, which provides for standardized
quantification and communication of PU findings between and providers,
had good agreement amongst providers, was quick to perform, and
correlated well with important clinical outcomes.
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Figures