Discussion :
Pulmonary artery catheterization was shown to be harmful and increase
the resource utilization in ICU17. However it can be
argued that, this may be true of mixed intensive care unit populations.
The study may not be applicable to the patients with cardiac disease, in
particular cardiac surgical population. As recently as 2011, Schwann et
al18, demonstrated a similar outcome specific to the
cardiac surgical population. These authors showed that PAC use increased
mortality and organ specific complications. At the same time there has
been a parallel increase in the use of transesophageal echocardiography
in the perioperative cardiac surgical population. The current
guidelines19, recommends TEE use in all open heart
procedures, a distinct difference from the previous version of the
guidelines which limited use to specific populations. While TEE can
offer excellent hemodynamic monitoring it may not be able to measure
vascular pressures as accurately as invasive catheterization. Given the
continuous endeavor to limit the use of invasive vascular devices on
account of multiple risks, it is reasonable to develop echocardiography
based surrogates of estimating static pressures such as the pulmonary
artery systolic pressure, the left ventricular end diastolic or left
atrial pressure. Our investigation is yet another attempt in this
direction but specifically targeted at the surgical population with a
focus on the intra- operative period.
A hundred subjects, predominantly undergoing myocardial
revascularization without cardiopulmonary bypass constituted the study
population. A majority of the population (66%) had normal left
ventricular systolic function. Also, when pulmonary artery wedge
pressures were categorized to be ‘high’ (≥15 mm Hg), only 12% of
patients had numerical values above this limit.
The Doppler evaluation of transmitral flow has been traditionally used
to categorise patients with abnormalities of diastolic function, and, by
extension higher left sided filling pressure4.
According to 2009 ASE diastolic function guidelines, transmitral flow
Doppler was used to classify patients into 3 distinct classes
–E/A<1, E/A 1-2 ,E/A>2 representing either
normal, equivocal or elevated left atrial pressure respectively. This
was particularly pertinent to patients with depressed LV ejection
fraction. In our study 36% of population were having E/A <1,
56% have E/A 1-2 and 8% were having E/A>2 suggesting that
in this population majority were having equivocal E/A values.
In the group with E/A 1-2 , this measure alone could detect only 50% of
patients with elevated PCWP. Specificity was also suboptimal (43.2%).
However a very small number of patients (6/56) in the study cohort were
having simultaneous values of E/A 1-2 and PCWP>15.When the
E/A >2 ,this value was highly specific (95%) in detecting
elevated PCWP but continued to be poorly sensitive(33%). When the
E/A<1, both sensitivity and specificity were markedly poor.
Thus, in accordance with the extant guidelines, transmitral Doppler is
extremely low utility in determining PCWP and only can be used in
conjunction with other measures such as early mitral annular velocities
(Ea), deceleration time or absolute velocity of E >50
cm/sec.
Another measure used to assess the left ventricular filling status is
the ratio of systolic phase to diastolic phase velocities in the
pulmonary vein. With decrease in left atrial compliance the proportion
changes such that the S/D ratio is less than unity. In the population
under study, S/D ratio scored better than the transmitral flow velocity
ratio in so far as the sensitivity was higher at 58.6%; however the
specificity was lower at 76%.
The majority of patients in the population had normal EF and the
measurement has been de-emphasised both in the
current16 and the previous EAE/ASE
guidelines4 for such patients. An extension of this
measurement is the systolic faction (SF). In our study the correlation
coefficient for systolic fraction and PCWP was weak. In patients with
normal EF a threshold value of SF set at 0.39 was considerably sensitive
but was virtually useless in determining truly positive patients. When
specificity criteria were used SF was considerably specific in
determining elevated filling pressures but was poorly sensitive. It
could therefore be of use only in patients with depressed ejection
fraction and therefore a much higher pre- test probability of suffering
high filling pressures in the left heart. The pattern demonstrated in
the study for these three variables (E/A, S/D and the SF) is entirely
consonant with previous descriptions in the
literature20.
However the cutoff for maximising specificity as determined in our study
was higher at 0.6 rather than 0.4 as mentioned in previous standard
guidelines4, on the subject. It is unknown whether
this is on account of the significant change in loading conditions that
might ensue from administration of anaesthetics and positive pressure
ventilation. Also the PCWP value set in our study at 15 mm Hg is not the
universal cutoff value to determine elevated filling pressure and is
sometimes set at 12 mm Hg or higher. The inability to obtain reasonable
sensitivity and specificity simultaneously continued for the other
measures such as deceleration time and the ratio (E/Vp)
of early phase of transmitral flow (E) to mitral propagation velocity
(Vp, ). Similar considerations such as those that apply
to E/A, S/D ratios and the SF apply to these parameters as well. In
determining true positives DT with a threshold of 196 msec was able to
differentiate patients with a PCWP≥15 from those with lower value of
this variable (specificity 80.7% but with a sensitivity of 8% and
accuracy of 72%). This is replicated elsewhere in the
literature21,22.
There has been significant focus on measurement of early mitral annular
velocities measured from either the lateral or the septal mitral annulus
or both. This parameter denoted as Ea is central to determination of the
presence or absence of diastolic function in patients with normal
ejection fraction in the current guideline. Ea velocities have been
significantly correlated to the time constant of left ventricular
relaxation (tau). They are widely considered to be less loading
condition dependent than their blood pool counterpart (E).
The ratio of the two velocities (E/Ea) with a threshold of 14 has been
set as a fairly consistent discriminator of raised vs normal filling
pressure in the recently issued guidelines16 on the
subject, across patients with normal and reduced EF. The important
caveat remains that clinical practice guidelines are in themselves not
validated and are not therefore extrapolatable.
In our dataset of patients with predominantly preserved EF, the E/Ea
was, by far, the most consistent discriminator of raised vs normal
filling pressure with areas under the receiver operator characteristic
curve of 0.79, 95% confidence interval of (0.67-0.92). The thresholds
to diagnose an elevated PCWP with emphasis on specificity was 11.55
which is lower than most of the literature described cutoff values. Even
with a sensitivity exceeding 91% a cutoff set at 9.2 was determined to
have a specificity of 64% with an accuracy of 67%. It is important to
note that both thresholds lie in the conventional interval considered to
be indeterminate23.
There are at least three studies with a focus on the cardiac surgical
population on semi invasive estimation of PCWP using echo based
assessment; one of the studies only used the E/Ea ratio and was unable
to demonstrate adequate correlation between the echo and the invasive
pressure surrogate24. Yet another study which was not
restricted to the intra- operative period alone but also studied
preoperative transthoracic echocardiogram derived estimates found that
while the pre-operative results were largely consonant with the
traditional discourse on the subject the institution of general
anaesthesia and the subsequently obtained transesophageal values were
grossly discordant. E/Ea lost considerable discriminative power.
Further, the TTE values also performed poorly suggesting that this had
less to do with the processes involved in extrapolation from TTE to TEE
and vice versa10. In the post cardiopulmonary bypass
period the E velocity and the S velocity also increased steeply for yet
unknown reasons. With continued focus on the perioperative use of the
E/Ea our study showed better predictive value for the E/Ea than
previously reported in the perioperative literature, area under receiver
operating characteristic curve (AUROC) of 0.79 vs. 0.68 as previously
reported, Cowie B et.al25.
Thus, our study in consonance with the global literature replicates the
poor predictive value of transmitral and pulmonary vein Doppler in
assessment of high filling pressures. It suggests that the best possible
estimate for elevated filling pressure may be made from an assessment of
the E/Ea; this is higher than previously reported and should be tested
in a larger, prospective population with separate group analyses for
patients with normal and reduced ejection fractions respectively.