PRINCIPLES OF ECHOCARDIOGRAPHY
In echocardiography the heart and great vessels are insonated with ultrasound, which
is sound above human audible range (20,000 Hz). The ultrasound is sent into thoracic
cavity and partially reflected by the cardiac structures. From these reflections,
distance, velocity and density of the objects under examination are derived. These
waves are characterized by their wave length, frequency and velocity. In TEE, ultrasound
beam is a continuous or intermittent chain of sound waves emitted by a transducer
consists of piezoelectric crystals e.g. barium titanate, lead metaniobate or lead
zirconate titanate. The transducers work by piezo-electric effect (from Greek word,
Piezein, to press tight), where by certain crystals deform when strong electric fields
are imposed upon them and reciprocally, will generate a voltage when deformed mechanically.
Commonly, a short ultrasound signal is emitted from the piezo electric crystal, which
is directed to the area to be imaged. After ultrasound wave formation, the crystal
‘listens’ for the returning echoes for a given period and then pauses before repeating
the cycle. The reflected ultrasound wave return to those piezo-electric crystals,
are converted into electrical signals, which may be processed and displayed in the
screen. The amplitude or strength of the returning signal provides information about
the characteristics of the tissue examined.[3]
In the OR as well as in ICU, TEE is commonly used to assess cardiac anatomy and function,
evaluation of thoracic aorta, detection of intra cardiac defects, evaluation of pericardial
effusions and tamponade, detection of intracardiac air, clot or masses (may be responsible
for arrhythmia and haemodynamic instability, assessment of ventricular function and
not the last, the evaluation of myocardial ischaemia. These are the few of the problems
an anaesthesiologist/intensivist encounter during management of a patient that can
be rapidly diagnosed with the aid of TEE.
TERMINOLOGIES ONE SHOULD KNOW
The transducer emits a main beam up to a certain distance from the face known as near
field length. Beyond the near field length the beam diverges into a conical pattern
(far field), with an angle determined by the ratio of the wave length to the transducer
diameter [Figure 12]. These are the secondary beams (side lobe) emitted by the transducer that are angled
obliquely to the centre of main beam. Side lobes are much weaker than the main beam,
but when encounter strong reflectors they can generate fade targets (side lobe artifacts)
and thereby produce multiple images from a single target.
Mechanical sector scanner
For making of a 2-D image, transducer beam is swept over a region of interest in a
fan-like fashion which is called a ‘mechanical sector scanner’.
Resolution
‘Resolution’ is a measure of systems’ ability to distinguish echoes from different
sources that are close to each other in time, space or return signal strength (known
as temporal, spatial and contrast resolution, respectively).
Aliasing
It refers to the distortion or artifact those results when signal reconstructed from
samples is different from the original continuous signal and should be avoided with
proper knob control.
Figure 4: The beam from a transducer showing near field and far field
Figure 5: Transgastric LV short axis view showing mitral valve and left ventricular cavity
Figure 6: Transgastric LV mid papillary view
Figure 7: Mid esophageal 4-chamber view
Figure 8: Mid esophageal LV 2-chamber view
Figure 9: Mid esophageal aortic valve long axis view
Frame rate
It is the frequency rate at which an imaging device produces unique consecutive images
called ‘frames’. It is expressed as frames/second or Hertz (Hz). In moving subjects
(blood/heart) and during quantitative measurement, whether based on the Doppler Effect
or 2D B-mode data, sufficient frame rate is important to avoid under sampling.
Nyquist limit
It gives us a theoretical limit to what rate we have to sample a signal that contains
data at a certain maximal limit. Sampling below this limit can give rise to inadequate
sampling or corruption of the image known as Aliases.
Type of echo images
A mode (amplitude mode) image
It measures only the depth of the tissue (single dimensional) is of limited clinical
usefulness.
M mode (motion mode)
M mode has an advantage of imaging the moving structures (heart wall, valve leaflets).
In M mode imaging, the amplitude of returning echoes is plotted as the brightness
along vertical lines drawn for each transmitted echo pulse. The echo pulses are repeated
at rates of thousands per second and a raster of vertical lines, scanned along the
same path, is drawn from right to left at speeds compatible to ECG recording (25–100
mm/s).
B mode (brightness mode) or 2-D mode
In this mode, as ultrasound waves penetrate tissues of different acoustic impedance
along the path of transmission, some are reflected back to the transducer (echo signals)
and some continue to penetrate deeper. The returned echo signals are processed and
combined to generate an image. This mode allows the measurement of a plane of tissue
(both depth and width) in real time and thereby makes it easier to appreciate the
relationship between various anatomic structures.
Doppler echocardiography
This allows evaluation of blood flow patterns, direction and velocity; thus permits
documentation and quantification of valvular insufficiency or stenosis, presence of
cardiac shunts, estimation of blood flow and cardiac output. Calculation of blood
flow velocity is possible when the flow is parallel to the angle of ultrasound beam.
Two types of Doppler echocardiography are in clinical use
Pulsed Wave Doppler
PW allows us to measure blood velocities at a single point (designated as sample volume).
It requires the ultrasound probe to send out a pulsed signal to a certain depth (chosen
by the operator) and then to listen for the reflected frequency shift from that particular
depth. Subsequently, the computer calculates the velocity of flow at the chosen point.
Continuous Wave Doppler
CW allows us to measure blood velocities along an entire line of interrogation. It
requires the probe to continuously send out pulses of ultrasound along a line and
continuously listen for the multitude of reflected frequency shifts that are coming
back. Because of this, we are able to use continuous wave Doppler to pick up very
high velocity flows, ones which PW cannot accurately measure. The major disadvantage
with CW Doppler is that sampling of blood flow velocity and direction occurs all along
the ultrasound beam, not in a specific area.
Color flow Doppler echocardiography
Doppler colour flow imaging is colour-coded for direction and velocity of blood flow.
Most systems code blood flow towards the transducer as red and flow away as blue.
Differences in relative velocity of flow can be accentuated, and the presence of multiple
velocities and directions of flow (turbulences) can be indicated by different maps
which utilize variations in brightness and colour.
Contraindications and complications
The expansion of TEE carries with it not only the benefits of rapid and highly effective
investigation but also the risk associated with the procedure itself and therefore
must be performed only by qualified physicians. The possible contraindications and
complications associated with this technology are described [Table 3].
Table 3:
Contraindications and complications
|
Contraindications
|
Complications
|
|
Absolute
|
Dental injury
|
|
Oesophageal strictures, tumours, diverticula
|
Vocal cord damage
|
|
Scleroderma of esophagus
|
Thermal injury
|
|
Acute upper gastrointestinal bleeding
|
Gastrointestinal tract (GI) tract bleeding
|
|
Perforated viscus
|
Oesophageal perforation/bleeding
|
|
Recent upper gastrointestinal surgery
|
Arrhythmia
|
|
Oesophageal varices
|
Laryngeal palsy
|
|
Relative
|
Dysphagia
|
|
Atlanto axial joint disease
|
Accidental tracheal extubation
|
|
Prior irradiation to the chest
|
Airway obstruction and increased ventilatory pressure
|
|
Hiatal hernia
|
Distraction from anaesthetic care
|
Steps for insertion of a TEE probe in an anaesthetized individual
-
Mouth is examined for abnormalities and loose teeth
-
An informed consent from the patient is obtained prior to the procedure
-
Decompression of stomach by a nasogastric tube
-
A bite-guard is inserted to prevent injury to the probe by the patient’s teeth
-
Lubrication of the probe with jelly
-
Displace the mandible anteriorly and advance the probe gently in the midline; if blind
insertion of the probe is not easy, a laryngoscope may be used to expose the posterior
pharynx and permit direct passage of the probe into the esophagus. Avoid undue force
at any stage during insertion of the probe
-
The tip of the transducer is allowed to return to the neutral position before advancing
or withdrawing the probe
-
Clean and decontaminate of the probe after each use
-
It is ideal to have an electrocardiogram trace on the echocardiographic imaging screen.
Figure 10: Mid esophageal bicaval view
Figure 11: Upper oesophageal descending aortic short axis view
Figure 12: Upper oesophageal Descending aortic long view
Common problems encountered in neurosurgical patients that can be diagnosed by TEE
-
Venous air embolism and paradoxical air embolism
-
Identification of haemodynamic instability during perioperative period
-
Presence of associated cardiac anomaly with meningomyelocoele
-
Cardiac manifestation of intracranial haemorrhage
-
Positioning of ventriculo atrial (VA) shunt and identification of the complications
related to it
-
Positioning of RA aspiration catheter
-
Percutaneous placement of VA shunt.
Venous air embolism and paradoxical air embolism
The advantages of sitting and semi-sitting positions during neurosurgical procedures
are well-known. However, these positions are not free from dreaded risks like high-grade
air embolism and subsequent central nervous dysfunction. The incidence of VAE ranges
from 1.6–76% depending on the presence or absence of an intracardiac shunt and surgery
in sitting/semi-sitting position.[4]
[5]
[6]
[7]
[8] Preoperative examination by TTE with a valsalva maneuver and intravenous echo contrast
can help to diagnose only the presence of an intracardiac shunt with 100% sensitivity
does not add to the management of a case in which VAE occur.[9] Demonstration of air bubbles in the great veins and cardiac chambers by TEE has
to be considered as a warning sign of VAE. Once there is suspicion, necessary steps
like interruption of surgical dissection and quick closure of venous system leak has
to be followed. American Society of Anesthesiologists and Society of Cardiovascular
Anaesthesiologists Task Force on TEE practice guidelines considers use of TEE as an
class I indication to identify VAE during intraperative period.[1]
Identification of the presence of intracardiac shunt by TEE, Bubble contrast study
To know the presence of a patent foramen ovale (PFO)/atrial septal defect (ASD), a
standard 2-D TEE view with special emphasis on the intra-atrial septum from midesophageal
4-chamber and bicaval view can be taken and windows that best displayed the intraatrial
septum with LA and RA can be selected. [Figures 13] and [14] Agitated saline can be injected from a central line. This results in prompt opacification
of RA and RV. In the presence of a intracardiac shunt, the left chambers also demonstrates
the appearance of the micro bubbles. [Figure 15]
Figure 13: Mid oesophageal 4-chamber view showing atrial septal defect
Figure 14: Mid oesophageal bicaval view showing atrial septal defect
Figure 15: Bubble contrast (agitated saline) study to demonstrate PFO, bubbles crossing interatrial
septum and appears in LA
TEE criteria for diagnosis of VAE
-
Grade 0: No air bubble visible, no air embolism
-
Grade 1: Air bubble visible in TEE
-
Grade 2: Air bubble visible in TEE with decrease in end tidal carbon dioxide (EtCO2) ≤3 mm Hg
-
Grade 3: Air bubble visible in TEE with decrease in EtCO2 >3 mm Hg
-
Grade 4: Air bubble visible in TEE with decrease in EtCO2 >3 mm Hg and decrease in mean arterial pressure ≥20% or increase in heart rate >40%
or both
-
Grade 5: Air embolism causing arrhythmia with haemodynamic instability requiring cardiopulmonary
resuscitation.[9]
Every time, the appearance of air bubble on TEE even in patients with intracardiac
shunt does not result in right to left shunting of air bubbles causing fatality. However,
uses of TEE in semi-sitting position diagnose the clinically relevant events secondary
to the presence of air in venous system e.g. a decrease in EtCO2 in capnography.[10]
[11]
Fathi et al. reviewed 4,806 patients considered for neurosurgery in sitting position and 5,416
patients who underwent percutaneous patent foramen ovale (PFO) closure. The overall
rate of venous air embolism was 14–39% and the rate of clinical and TEE defected paradoxical
air embolism (PAE) was 0–14% in their series. The authors recommended screening of
PFO and considering closure in cases in which the sitting position is the preferred
neurosurgical approach. This study however has a major limitation due to lack of level
4 evidence and from using data from observational cohort studies.[5] Three more authors identified TEE as a more sensitive method to diagnose VAE and
PAE than transcranial Doppler (TCD).[12]
[13]
[14]
Two types of air embolism can be detected in 2-D TEE. The single bubble type, can
happen during skin and muscle incisions, craniotomy and brain lesion excision. Electrocoagulation
and application of bone wax can prevent further aggravation and embolism formation.
The other one is stormy bubble type, which can happen during dura and muscle incision.
This is prevented by electrocoagulation, reflection of the dura and suturing of the
affected muscle.[15]
Identification of associated cardiac anomaly and its relevance
Meningomyelocoele is one of the common problems encountered in neonate. Conotruncal
anomaly, septal defects, coartation of aorta, hypoplastic left heart syndrome apart
from patent ductus arteriosus and PFO can be associated with Meningomyelocoele and
have anaesthetic implications. In a series of 105 neonates aged from 1–20 days who
were posted for Meningomyelocoele surgery, congenital heart disease was detected in
39% of patients in preoperative TEE examination. However, the cardiac examination
was abnormal only in 13% cases. This report implies that as clinical examination to
detect heart disease is insensitive in neonates, routine screening echocardiogram
has an important implications for intracardiac shunting, urinary tract instrumentation
(antibiotic prophylaxis), post-operative pulmonary hypertension and neurosurgical
procedure (venous air embolism).[16]
[17]
Utility of TEE for identification of cardiovascular instability in the operating room
and ICU
Cardiac manifestations of intracranial haemorrhage are an accepted phenomenon that
affect in patient outcomes and poses a challenge for neuro-intensivists. The cardiac
manifestations may vary from subtle electrocardiography (ECG) changes to myocardial
infarction, congestive cardiac failure and ventricular dysfunction and Takatsubo cardiomyopathy.
Takatsubo cardiomyopathy is one such condition which has a similar clinical picture
like acute coronary syndrome (ACS) and distinguished from above by echocardiography
findings. The treatment of Takatsubo cardiomyopathy is conservative whereas the later
needs aggressive therapy including surgical intervention. The TEE finding of Takatsubo
cardiomyopathy includes low ejection fraction, akinesia of mid and apical segments
of left ventricle with or without a thrombus.[18]
[19]
[20]
[21]
Aneurysmal subarachnoid haemorrhage often present with haemodynamic instability and
pulmonary oedema requiring vasoactive drugs.[22] This situation is associated with severe ventricular dysfunction. There may be chamber
dilation depending upon the duration of problem, regional wall motion abnormalities,
global reduction in systolic function and decrease in ejection fraction (EF).[23] Global reduction in systolic function is estimated from fractional area change (FAC),
which is the proportional change in the area of left ventricle (LV) short axis (TG
LVSX view) during systole and calculated from the following formula:
Where EDA = end diastolic area and ESA = end systolic area and the normal range is
36–64%. EF represents the proportion of diastolic volume that is ejected during ventricular
contraction and is calculated as:
Where ESV = end systolic volume, EDV = end diastolic volume and the normal range is
55–75%.[24]
Neurogenic stunned myocardium presenting with left ventricle non-compaction can also
occur in consequence to hypertensive hydrocephalous. This condition is associated
with severe ventricular dysfunction and life-threatening ventricular ectopics during
intraoperative period.[25] TEE can demonstrate transient non-compaction aspect of the left ventricular wall.
In a normal person trabeculation of the ventricle is normal, as prominent, discrete
muscular bundle >2 mm. In non-compaction, there is extremely prominent trabeculation.
Echocardiographically, it is diagnosed when trabeculations are more than twice the
thickness of the underlying ventricular wall.[26] Timely diagnosis and treatment can save the patient.
Cardiovascular collapse can occur during spinal surgery due to acute blood loss following
major vascular injury. At times, the bleeding is occult, when it is into the abdomen
or retroperitoneum.[27]
[28] This condition can be easily identified by TEE, the details are described elsewhere
in this review. Myocardial ischaemia as a cause of severe haemodynamic disturbances
associated with ST segment abnormality can occur during aneurysm clipping.[29] As several other conditions are also responsible for ST segment changes, myocardial
ischemia can be identified by appearance of new regional wall motion abnormalities
(RWMA) in TEE (TG LV SX view) during continuous monitoring. TEE can also act as an
ischaemia monitor when ECG is not interpretable (e.g. Bundle branch block or the presence
of pace maker).[1]
Pericardial tamponade and heart failure can occur secondary to ventriculo-pleural
shunt mal function.[30]
[31] TEE findings in tamponade vary with rapidity of fluid collection in the pericardial
space. The most important 2-D TEE finding is right ventricular collapse during diastole.
There may be right atrial invasination during late diastole. Collection of fluid/blood
can be identified in the pericardial cavity.[3] Pulmonary embolism (PE) is a frequent cause of morbidity and mortality in neurosurgical
practice.[32] Pulmonary angiography is a gold standard to identify such situation. However, TEE
is a rapid bed side test that is both sensitive and specific. The diagnosis of PE
by TEE is often indirect. It is very uncommon to visualize a thrombus unless it is
in the main pulmonary artery or proximal aspects of right or left pulmonary artery
as rest of the segments are usually not visualized by TEE. In ICU patients, the identification
of new onset of RV dysfunction should raise the possibility of PE. There may be increased
in size of pulmonary arteries, right ventricular dysfunction of varying degrees, flattening
of interventricular septum, volume overload of RV, dilation of right atrium (RA),
functional tricuspid regurgitation and increase in size of inferior venacava.[3] In the absence of these findings the diagnosis of PE is unlikely. Initiation of
thrombolytic therapy can be done if not contraindicated and patient could be followed
up with another TEE examination, which can evaluate the efficacy of treatment as well.
Severe haemodynamic instability can happen due to inadequate volume status during
the perioperative period. This has many causes apart from the neurosurgical problem
and represents considerable threat to the life of the patient. TEE is ideal to identify
the problem and guide appropriate therapy both in the operating room and intensive
care unit. [Table 4]
[24]
[33]
Table 4:
TEE findings of some common haemodynamic problem
|
Problem
|
TEE finding
|
Haemodynamic findings
|
|
Contractility
|
EDA
|
ESA
|
FAC
|
BP
|
CI
|
RAP
|
|
EDA: End diastolic area, ESA: End systolic area, FAC: Fractional area change, CI:
Cardiac index, RAP: Right atrial pressure, LV: Left ventricle, SVR: Systemic vascular
resistance, BP: Blood pressure, TEE: Transoesophageal echocardiography
|
|
Hypovolemia
|
Vigorous
|
↓
|
↓
|
↔
|
↓
|
↓
|
↓
|
|
Reduced LV compliance
|
Vigorous
|
↓
|
↓
|
↔
|
↓
|
↓
|
↔
|
|
Low SVR
|
Vigorous
|
↔
|
↓
|
↑
|
↓
|
↑
|
↑
|
|
Systolic Dysfunction
|
Vigorous
|
↑
|
↑
|
↓
|
↓
|
↓
|
↑
|
TEE guided volume management can be done with assessment of preload, contractility
and afterload.
Preload: A number of ways have been proposed to assess LV volume by 2-D TEE. They
can be used to do one-time assessment/continuous monitoring method to assess fluid
responsiveness. The various measures are left ventricular end diastolic volume (LVEDV),
left ventricular end diastolic area (LVEDA), superior venacava (SVC) collapsibility,
inferior venacava (IVC) size and response to fluid challenge.[1]
[3] The criteria for diagnosing hypovolemia include LV end diastolic diameter (LVEDD)
less than 25 mm, systolic obliteration of LV cavity (kissing papillary muscle) and
a LV end diastolic area (LVEDA) of less than 55 cm2. These measures are obtained from the TG mid-papillay SX view of LV.[3] LVEDA measurements have been validated to track the fluid status changes. Within
the limits, fluid administration results in an increase in EDA; this is associated
with an increase in stroke volume in case of hypovolemia.
Contractility: The usual methods used for the measurement for global LV systolic function
are EF, FAC and segmental wall motion abnormality. Assessment of ventricular systolic
function and their changes over time is of enormous help in therapeutic decision making
(e.g. RWMA can indicate acute ischaemia where as RV dysfunction points more towards
the presence of PE or COPD).
Afterload: It is estimated to be low when there is low mean arterial pressure, normal
left ventricular end diastolic diameter (LVEDD) and collapsing LV at end systole as
demonstrated by 2-D TEE.
Reduced LV compliance is associated with significant diastolic dysfunction have important
implication for volume status. Firstly, EDA at which the LV is optimally filled will
be low; and secondly, the required filling pressure will be high. Low systemic vascular
resistance (SVR) often follows after significant blood loss, and a consequence of
drug therapy. Sepsis is another reason for low SVR in patients with long-term ICU
stay. The patients with low SVR often respond to an inotrope like nor epinephrine
or phenylephrine when haemoglobin and volume status is adequate. LV systolic dysfunction
is usually secondary to Takatsubo cardiomyopathy associated with normal left ventricular
internal dimension but depressed LV ejection fraction. These patients usually respond
to a negative inotrope (beta blocker), fluids and intra-aortic balloon pump. Chronic
LV dysfunction is rare in neurosurgical setting unless the patient have a long standing
history of myocardial infarction/cardiomyopathy. These patients have features of reduction
in global LV function and ventricular dilatation along with a normal cardiac output
and systemic blood pressure and are more susceptible to hypovolemia during the perioperative
period. Acute systolic dysfunction also occur secondary to ischaemia following prolonged
hypotension due to massive volume loss. These patients do not have any ventricular
dilation, whereas systemic blood pressure and cardiac output may be severely depressed.
Such patients are at high risk of haemodynamic instability during perioperative period
and usually respond to volume and inotropes.
Placement of a ventriculoatrial shunt (VA shunt) is one of the commonest neurosurgical
procedure for the treatment of hydrocephalus. Current positioning of the distal end
of catheter in RA is of paramount importance for monitoring shunt patency and reducing
the incidence of VA shunt related morbidity. TEE is a helpful aid to guide the placement
of this shunt, advancement of guidewire and shunt catheter to SVC can be guided by
TEE.[34]
[35] This procedure is however not free from complications like thrombosis of intracardiac
end of catheter, thromboembolism and tricuspid stenosis. Routine echocardiography
at regular intervals can diagnose this problem and prevent the subsequent sequel to
thromboembolism and tricuspid stenosis.[36]
[37] TEE can also offer a rapid and accurate assessment of VA shunt function especially
detection of a leak of cerebrospinal fluid to RA through the shunting device.[38] Tonn reported a case of bilateral basal infiltrate and pleural effusion in a patient
with VA shunt. TEE established the diagnosis of a thrombus in right atrium which was
removed and a shunt revision was performed.[39] Percutaneous approach of placement of VA shunt is gaining importance to make the
procedure less invasive there by a reduced hospital length of stay and less complication.
TEE can monitor the exact position of atrial catheter tip and its function.[40] Hydatid cyst of brain though uncommon is not a rare entity. This may associated
with a cyst in heart which can be diagnosed by echocardiography in the same setting.
A timely diagnosis can prevent the cardiac complications.[41]
Future direction
There is no randomized clinical trials evaluating the impact of TEE in neurosurgical
and neuroanaesthesia settings An ideal trial in this area can be done which may show
the beneficial effect of the change in TEE directed problem management and initiation
of various anti ischemic and thrombolytic measures apart from guided volume therapy
on the post-operative outcome compared to the current standard monitors. TEE is a
monitor that can change the process of perioperative management in neurosurgical patients
but may not change the surgery per se. This technology should be compared with other routine monitors (pulmonary artery
catheter and non-invasive cardiac output monitor) to evaluate the reliability, interpretability
and change in decision making which has a direct effect on the patient outcome in
terms of hospital length of stay, decrease in post-operative morbidity and mortality.
One can go for cost-effective and cost-effective analysis to make an effective impact
in neuroanaesthesia practice.
