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.