|Year : 2020 | Volume
| Issue : 3 | Page : 227-231
Effect of General Endotracheal Anesthesia and Mechanical Ventilation on the Echocardiographic Measurements in Severe Aortic Stenosis
Muralidhar Kanchi1, Rudresh Manjunath1, Pooja Natarajan1, Kumar Belani2
1 Department of Anesthesiology and Intensive Care, Narayana Institute of Cardiac Sciences, Narayana Health City, Bengaluru, Karnataka, India
2 Department of Anesthesiology, M Health Fairview, University of Minnesota, Minneapolis, MN, USA
|Date of Submission||09-Apr-2020|
|Date of Decision||27-Jun-2020|
|Date of Acceptance||26-Jul-2020|
|Date of Web Publication||18-Dec-2020|
Dr. Muralidhar Kanchi
Department of Anesthesia and Intensive Care, Narayana Institute of Cardiac Sciences, Narayana Hrudayalaya Health City, Bommasandra Industrial Area, Anekal Taluk, Bengaluru - 560 099, Karnataka
Source of Support: None, Conflict of Interest: None
Introduction: We measured the peak pressure gradient (pPG) and mean pressure gradient (mPG) obtained by transesophageal echocardiography (TEE) after induction of anesthesia and compared it with the preoperative pPG and mPG by transthoracic echocardiography (TTE) in adults with aortic stenosis (AS). We also compared the aortic valve area (AVA) measurements as obtained preoperatively by TTE versus those by TEE following induction of general endotracheal anesthesia (GETA) during the inspiratory phase, expiratory phase of the ventilatory cycle and with incremental increases in tidal volume. Materials and Methods: All patients had preoperative TTE within 1 month of surgery and was reviewed 1 day before the surgery. After anesthetic induction, precardiopulmonary bypass (CPB) TEE evaluation was done to measure mPG and pPG across AV, under steady-state conditions. Three different controlled tidal volumes: 8 ml, 10 ml, and 12 ml per kg body weight were utilized during the TEE measurements. Results: A total of 90 adults underwent aortic valve replacement from 2017 to 2018. The preoperative pPG and mPG across the AV by TTE was 96.7 ± 23.27 mmHg and 60.7 ± 18.1 mmHg, respectively. Compared to preoperative TTE, pre-CPB TEE pressure gradient during both phases of ventilation under GETA was significantly lower. The pPG and mPG were higher during inspiration as compared to those in the expiratory cycle during mechanical ventilation under GETA (pPG during inspiration = 66.63 ± 22.15 mmHg; mPG during inspiration = 38.24 ± 13.65 mmHg; pPG during expiration = 52.49 ± 19.10 mmHg; mPG during expiration = 30.76 ± 12.66 mmHg). There were no significant changes in AVA between TTE/TEE and inspiration/expiration. Conclusions: The findings of this study demonstrate that the TEE pre-CPB PGs underestimated the severity of AS; hence, the severity of AS must be interpreted with caution during GETA and mechanical ventilation (MV). In addition, PGs must be done at similar points in the respiratory cycle.
Keywords: Aortic stenosis, echocardiography, general endotracheal anesthesia, mechanical ventilation
|How to cite this article:|
Kanchi M, Manjunath R, Natarajan P, Belani K. Effect of General Endotracheal Anesthesia and Mechanical Ventilation on the Echocardiographic Measurements in Severe Aortic Stenosis. J Indian Acad Echocardiogr Cardiovasc Imaging 2020;4:227-31
|How to cite this URL:|
Kanchi M, Manjunath R, Natarajan P, Belani K. Effect of General Endotracheal Anesthesia and Mechanical Ventilation on the Echocardiographic Measurements in Severe Aortic Stenosis. J Indian Acad Echocardiogr Cardiovasc Imaging [serial online] 2020 [cited 2021 Apr 13];4:227-31. Available from: https://www.jiaecho.org/text.asp?2020/4/3/227/303930
| Introduction|| |
Grading of aortic stenosis (AS) before aortic valve replacement (AVR) is often performed using transthoracic echocardiography (TTE) before surgery. During AVR, general endotracheal anesthesia (GETA) and mechanical ventilation (MV) may influence the transoesophageal echocardiographic (TEE) measurement of the pressure gradients (PGs) across the aortic valve leading to underestimation of the severity of AS. The American College of Cardiology (ACC)/American Heart Association (AHA) guidelines for the care of patients with valvular heart disease provide an algorithm for AVR decision-making. This algorithm uses measurements to grade the severity of AS by Doppler-derived mean pressure gradient (mPG), aortic valve area (AVA) by the continuity equation, and peak velocity across the AV. In addition, the decision for performing AVR is influenced by the presence of symptoms and estimation of left ventricular function as estimated by the left ventricular ejection fraction. Patients undergoing cardiac surgery present to the operating room following an extensive cardiac workup. However, pre cardiopulmonary bypass (pre-CPB) assessment of the aortic valve using transesophageal echocardiography (TEE) can affect surgical decision-making based on new findings during elective surgery. According to a large retrospective review of 12,566 patients undergoing cardiac surgery, 3,835 patients underwent isolated coronary artery bypass grafting (CABG); among these, 3.3% of patients had an unplanned aortic or mitral valve procedure added to the surgery based on pre-CPB TEE findings. Similarly, in the same review, of 1,823 patients undergoing mitral valve surgery, 1.0% of patients underwent an unplanned aortic valve procedure based on incidental findings during pre CPB TEE. This underlines the need to investigate the role of TEE measurements more closely for precise surgical decision-making. According to guidelines issued by the American Society of Anesthesiologists (ASA) and the Society of Cardiovascular Anesthesiologists (SCA) Task Force, pre-CPB TEE is essential for intraoperative assessment of all open heart and thoracic aortic surgical procedures. In a retrospective observational study, preoperative TTE findings were compared with pre-CPB TEE and it was found that pre-CPB TEE often underestimates AS severity. The reason for the difference might be that preoperative grading was done in spontaneously breathing patients using TTE, whereas, with TEE under GETA, the loading conditions are likely to be different. Intraoperative pre CPB TEE findings in AS patients can impact surgical decision-making; thus, accurate interpretation of these findings has extreme importance. However, there are no available guidelines or validated cutoffs of parameters to grade AS using TEE under GETA.,
The American Heart Association/American college of cardiology guidelines and the European Association of Echocardiography/American society of echocardiography recommendations for AS assessment is the most commonly used for intraoperative guidance. However, these guidelines were developed using TTE on spontaneous breathing patients and it may be different from patients under GETA. With these concerns in mind, we conducted a prospective study. The aim of the study is to compare the peak and mPG s of the aortic valve, and AVA in spontaneous breathing (preoperative TTE), versus in anesthetized patients (pre-CPB TEE), in different phases of ventilation with variations in tidal volume. Aims of the study should be elaborated. It is already a known fact that the pressure gradients being flow-related are likely to be underestimated after induction of GA. What new findings the author has expected, before the plan of the study needs to be mentioned here.
| Materials and Methods|| |
After approval from the ethical committee and informed consent, a prospective observational study was conducted in a large volume tertiary heart care center on adult patients of either gender undergoing elective AVR for severe AS between January 2017 and December 2018. The exclusion criteria included AS with severe LV dysfunction, AS with moderate or severe aortic regurgitation (AR), low-flow, low-gradient AS, AS associated with other valvular lesions and patients with contra-indications to TEE placement. AS severity was defined by the following echocardiographic parameters: peak velocity, mPG, AVA by continuity equation, dimensionless index (DI) and indexed AVA, as shown below. References quoted here should have been 1 and 4 [Table 1].
|Table 1: Grading the severity of aortic stenosis (modified from Baumgartner et al.|
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Measurement of gradients and valve area by transthoracic echocardiography
TTE was performed using Philips HD15 S5-2 sector array cardiac transducer (Philips company, make 2008, Netherlands), 1 day before surgery. This echocardiographic examination was performed as a hospital policy to review the findings, to confirm the diagnosis and look for any associated lesions which might have been missed in the initial echocardiography and values noted. The peak pressure gradient (pPG), mPG, and AVA values were obtained according to practice guidelines proposed by the American Society of Echocardiography. mPG was obtained by integrating a continuous wave Doppler (CWD) tracing of flow across the aortic valve. Apical (5 chamber view), suprasternal, or right parasternal views were used to align sample volume parallel to blood flow with not greater than 15° angle between the incident beam and the flow of blood. This multiwindow interrogation was done so that we could identify the maximum gradient. AVA was calculated through the continuity equation: AVA (cm2) = ([CSALVOT] [VTILVOT])/VTIAV.
To determine the cross-sectional area (CSA) of the left ventricular outflow tract (LVOT), the LVOT diameter was obtained using the parasternal long axis view in the mid-systolic phase of cardiac cycle with zooming. LVOT diameter was obtained within 0.5-1.0 cm of the valve orifice at the location of the LVOT. CSA was estimated using the formula: CSA = πr2 (r = d/2), where d is the LVOT diameter. Velocity time integral (VTI) of the LVOT was obtained using the apical 5 chamber view using pulsed-wave Doppler (PWD) with a sweep speed of 100 mm/s. The aortic valve gradient, VTI was calculated using CWD. All TTE examinations were conducted by a consultant cardiologist, using single cardiac cycle values of velocity and gradient across the aortic valve.
Measurement of gradients and valve area by transesophageal echocardiography
During the pre CPB TEE, using Philips CX50 echocardiography machine, probe X7-2t (Philips ultrasound, make 2014, CX cart, Bothell, WA, USA) was used after anesthetic induction, before sternotomy with the chest not yet open. The deep trans-gastric AV long axis view was used to align the sample volume as parallel to blood flow as possible using PWD of a single cardiac cycle using a sweep speed of 100 mm/s. The pPG and mPG was ascertained by manually observing the ventilator timings of inspiration and expiration. The measurements were obtained with a tidal volume of 8 ml/kg, 10 ml/kg and 12 ml/kg with 3 min of time interval between the increments in tidal volume. LVOT diameter was measured in the mid-esophageal aortic valve long axis view in the mid-systolic phase of the cardiac cycle. All the TEE examinations were done by a consultant anesthesiologist with at least 5 years of experience in cardiac anesthesia.
A standard technique of anesthesia, CPB and surgery were followed. All patients received oral alprazolam 0.25 mg the night before surgery. On the day of surgery, after anesthetic induction and skeletal muscle relaxation with pancuronium, endotracheal intubation was performed. The patients were mechanically ventilated initially with a tidal volume (Vt) of 8 ml/kg body weight and the respiratory rate was adjusted to maintain ETCO2 between 35–40 torr. A TEE probe was inserted and during steady-state conditions (heart rate of 60–90/min; systolic BP between 100 and 140 mmHg and mean arterial pressure of >65 mmHg) the aortic valve was assessed pre-CPB both during inspiration and exhalation. Following this, the measurements were repeated at Vt of 10 ml/kg and then Vt of 12 ml/kg and the respiratory frequency was altered to achieve an ETCO2 between 35 and 40 torr. The mean systolic blood pressure of the patients measured before TTE was 167 mmHg and postanesthesia during TEE was 125 mmHg.
Data were analyzed using SPPS (IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY, USA: IBM Corp.). Continuous variables are presented in terms of mean and standard deviation, whereas categorical variables are presented in number and percentage. An independent sample t-test was used to compare the continuous variables between the two groups, and a paired sample t-test was used to compare the pre- and post-procedure values. A value of P < 0.05 was considered statistically significant.
| Results|| |
During the study period, there were a total of 90 eligible patients with severe AS, of which 54 were male and 36 were female with a mean age of 58.3 ± 13.2 (standard deviation [SD]) years with a range of.(mention the age range with median value/SD) 72 patients underwent AVR and 18 underwent CABG + AVR. Patient characteristics and procedure types are summarized in [Table 2]. Preoperative pPG and mPG across the AV (TTE) were 96 ± 26.5 (mean ± SD) mmHg and 60.7 ± 18.1 (mean ± SD) mmHg [Table 3]. The pPG and mPG were significantly lower during pre–CPB TEE as compared to preoperative TTE (preoperative TTE pPG of 96.7 ± 23.27 mmHg vs. pre-CPB TEE pPG of 66.63 ± 22.15 mmHg and preoperative TTE mPG 60.7 ± 18.1 mmHg pre-CPB TEE mPG of 38.24 ± 13.65 mmHg). The pPG and mPG were significantly higher during the inspiratory phase as compared to expiration during all three Vt changes [Table 4]. Importantly, the AVA was not significantly different when measured by TEE (0.67 ± 0.23 cm2) versus TTE (0.65 ± 0.34 cm2). Doppler velocity index (DVI) was not measured in this study in this study based on the findings of Whitener et al., who demonstrated no change in the DI with TTE as compared to TEE. [Table 5]. Furthermore, of note, the increase in Vt from 8 ml/kg to 10 ml/kg and then to 12 ml/kg under GETA did not influence the PG and AVA in patients with severe AS.
|Table 3: Peak and mean pressure gradients across the aortic valve and aortic valve area as measured by transthoracic echocardiography (patients awake) versus transesophageal echocardiography (patients anesthetized and mechanically ventilated)|
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|Table 4: Peak and mean gradients across the aortic valve as measured by transesophageal echocardiography during the precardiopulmonary bypass period|
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|Table 5: Precardiopulmonary bypass aortic valve area during general endotracheal anesthesia during different phases of the ventilatory cycle|
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| Discussion|| |
Our results suggest that anesthesia and positive-pressure ventilation (PPV) alter the hemodynamic profile in patients with AS, resulting in disparity in the pPG and mPG between the preoperative TTE and pre-CPB TEE calculations. Reasons for this may be as follows: (1) suboptimal beam echo alignment to the direction of blood flow during TTE versus TEE and operator variability; (2) errors associated with the measurement of LVOT diameter for AVA calculation; (3) inability to replicate “street” conditions during pre-CPB TEE; and (4) changes induced by PPV. It is worthwhile to note that in an anesthetized, neuromuscular blocked or mechanically ventilated patient, the LV stroke volume increases with inspiration and decreases with expiration. During intermittent PPV, an increase in lung volume during inspiration compresses the lung tissue and displaces blood contained in the pulmonary venous reservoir into the left heart chambers. This leads to an increase in LV preload and stroke volume. Similarly, an increase in intrathoracic pressure reduces LV afterload. This phenomenon is extensively documented in critical care medicine and is referred to as “stroke volume variation” and is used as an index of a dynamic parameter of preload. The opposite is true in a conscious patient who is breathing spontaneously. The study revealed that the pre-CPB pPG and mPG were significantly lower when compared to the preoperative TTE gradient calculated during spontaneous breathing and thus underestimating the severity of AS under GETA. These differences in the pressure gradients were significantly higher during the inspiratory phase than during expiration. The variation in the tidal volume from 8 to 12 ml/kg did not affect the pressure gradient. The postulation for this finding is that during the time of measurement, the patients were optimally filled and were not hypovolemic and hence any influence of the increase in the tidal volume in the range of 8–12 ml/kg did not have a significant effect stroke volume and therefore, on the pressure gradients measured. Ideally, the pulse pressure and stroke volume variation need to be normalized to remove any effect of change in loading conditions associated with general anesthesia and intermittent PPV which is practically challenging.
One way to interpret the severity of AS is to use a DI or DVI that is load-independent., However, Whitener et al. could not demonstrate that DI was better than peak velocity, mPG or AVA at matching AS gradients between intraoperative pre-CPB TEE and preoperative TTE. DI, when measured with pre-CPB TEE, overestimated the severity of AS when compared to TTE. However, pPG and mPG measured with pre-CPB TEE showed lower values compared with TTE. Uda et al. studied 319 patients, concluded that DI measurements are far better with intra-operative pre-CPB TEE. They also reported that pre-CPB TEE underestimates pPG and mPG.
Another factor to explain the differences in pressure gradients between the preoperative TTE obtained under spontaneous ventilation conditions versus the TEE measured pressure gradients during general anesthesia may be attributed to the lower transvalvular flow with mechanical ventilation along with the effects of general anesthesia.
Schwartzenberg et al. retrospectively evaluated 100 patients with moderate or severe AS and concluded that preoperative TTE is better for the detection of AS when compared to pre-CPB TTE. They also conducted a study and demonstrated that TTE when done with sedation downgrades AS severity in 17.4% of their patients. Nanditha et al. did a study including 60 patients, wherein they concluded DI and continuity equation are good indicators for measurement of AVA, but pPG and mPG show lower values when compared to awake patients.
It is pertinent to estimate the AVA using three-dimensional (3D) echo and examine the morphology of the aortic leaflets using 2D and 3D-echo. Furukawa et al. compared 2D-TEE and 3D-TEE for assessment of AVA and showed that geometric AVA is smaller with 3D-TEE with improved accuracy in measurement due to reduced image acquisition time. A study done by Whitener et al. also concluded that hemodynamic standardizations or adjustments of pre-CPB TEE of mPG and AVA values may be necessary in anesthetized patients before assigning an AS grade using these parameters.
The limitations of this study are as follows: (i) the inspiratory and expiratory phase of respiration was timed fairly accurately using the ventilatory waveform on the mechanical ventilator screen along with end-tidal carbon-di-oxide (EtCO2) as an adjunct and the respiratory waveform on the multichannel hemodynamic monitor, but respirograph was not used, (ii) all the measurements of gradients during phasic changes with reference to ventilatory cycle was done as a single reading and not as an average of 3–5 respiratory cycles, (iii) 3D TEE data set for all patients was not available and hence not included in the analysis, (iv) DVI value was not included in this study based on the findings of Whitener et al. who demonstrated no change in the DI with TTE as compared to TEE.
| Conclusions|| |
The findings of this study demonstrated that the severity of AS is underestimated by TEE under GETA during the pre-CPB period. AVA was not significantly different in pre-CPB TEE, during both phases of ventilation and during changes in the tidal volume. In addition, this study demonstrated that there was a significant increase in the pPG and mPG across the aortic valve during the inspiratory phase of the respiratory cycle as compared to the expiratory phase. However, alteration in the tidal volume between 8 ml/kg and 12 ml/kg did not significantly influence the pPG and mPG across the aortic valve in patients with severe AS under GETA. We suggest that the measurement of PGs must be done at similar points during the respiratory cycle and the severity of AS must be interpreted with caution during GETA and MV.
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]