Dynamic left ventricular outflow tract obstruction: underestimated cause of hypotension and hemodynamic instability

Abstract

Left ventricular outflow tract obstruction, which is typically associated with hypertrophic cardiomyopathy, is the third most frequent cause of unexplained hypotension. This underestimated problem may temporarily accompany various diseases (it is found in even <1% of patients with no tangible cardiac disease) and clinical situations (hypovolemia, general anesthesia). It is currently assumed that left ventricular outflow tract obstruction is a dynamic phenomenon, the occurrence of which requires the coexistence of predisposing anatomic factors and a physiological condition that induces it. The diagnosis of left ventricular outflow tract obstruction should entail immediate implementation of the therapy to eliminate the factors that can potentially intensify the obstruction. Echocardiography is the basic modality in the diagnosis and treatment of left ventricular outflow tract obstruction. This paper presents four patients in whom the immediate implementation of bedside echocardiography enabled a rapid diagnosis of left ventricular outflow tract obstruction and implementation of proper treatment.

Introduction

For many years, left ventricular outflow tract obstruction (LVOTO) has been associated with systolic anterior motion of the mitral valve (SAM), which is observed in 30–60% of patients with hypertrophic cardiomyopathy (HCM)^(1–3). The epidemiological studies and clinical observations revealed, however, that LVOTO is not associated only with HCM. It also occurs in numerous diseases and may even be found in patients without a noticeable cardiac disease (<1% cases)^(4–6). Left ventricular outflow tract obstruction may be transient and accompany certain clinical situations (hypovolemia, general anesthesia). It is one of the more common causes of unexplained hypotension⁽¹⁾. The diagnosis of LVOTO should entail immediate implementation of the therapy to eliminate the factors that can potentially intensify the obstruction.

This paper presents four patients in whom the immediate implementation of bedside echocardiography according to a simplified protocol enabled a rapid diagnosis of LVOTO and implementation of proper treatment.

Case presentations

Case 1: A male patient (aged 35) was operated due to the Fallot syndrome (a complete correction was made: moving the aorta above the left ventricle, ventricular septal defect patch closure, pulmonary biological valve implantation and tricuspid valve annuloplasty). On the day of the surgery, 2 hours after the patient left the operating room, a sudden hypotension developed (BP 60/40 mm Hg). Inotropic agents (dobutamine, adrenaline and noradrenaline) were administered. Fluid therapy was conducted in accordance with the model used in the department: within the first 4 hours following the procedure, the patient received 1,500 ml of crystalloids and 1,000 ml of colloids. Despite increasing the doses of the medicines, the patient's condition did not improve. In the 6^th hour after the procedure, the following hemodynamic parameters were observed: HR 90/min, BP 70/50 mm Hg, CVP 12–14 mm Hg, diuresis 50 ml/h, post-operative drainage 100 ml/h and increasing metabolic acidosis in blood gases. Subsequently, a bedside echocardiographic examination was conducted which revealed: normal size of the left ventricle (end-diastolic diameter 4 cm), hyperkinetic systolic activity of the left ventricle with ejection fraction of 70%, enlarged right ventricle (end-diastolic diameter 5 cm) with severe hypokinesia of its free wall, presence of SAM and clinically relevant LVOTO with maximal systolic gradient of 40 mm Hg and features of significant hypovolemia (maximal IVC 1.2 cm, IVC CI 76%). With these findings in mind, the inotropic medications were gradually discontinued and further fluid therapy was conducted under ultrasound control of dynamic IVC parameters. The condition of the patient improved in the next several hours.

Case 2: A 76-year-old female reported to the admission room following an episode of a short fainting. The patient had been chronically treated due to arterial hypertension and stable ischemic heart disease. On the day preceding the admission, the patient had been vomiting, which was probably associated with a dietary problem. On physical examination, the following were observed: tachycardia of 90/min, BP 100/60 mm Hg, quiet aortic systolic murmur and features of dehydration (dry mucus membranes with off-white deposit). ECG revealed sinus tachycardia with no morphological abnormalities. In the emergency room, echocardiography was performed which revealed: small left ventricular end-diastolic dimension (3.5 cm), short-axis left ventricular end-diastolic area (LVEDA) 4.5 cm², massive hypertrophy of the septal basal segment (end-diastolic dimension 2.2 cm) with the IVS bulging into the LVOT, normal segmental and global contractility of the left ventricle with ejection fraction of 60%, LVOTO with maximal gradient of 70 mm Hg and narrow, excessively reactive inferior vena cava (maximal IVC 1 cm and IVC CI 92%). Dynamic left ventricular outflow tract obstruction was diagnosed. It was probably provoked by hypovolemia with massive hypertrophy of the left ventricular muscle. Following intravenous fluid delivery (1,500 ml), the patient's condition improved and a check-up LVOT maximal gradient decreased to 35 mm Hg.

Case 3: A male patient (aged 58) was operated due to severe degenerative mitral insufficiency. The surgical procedure included square excision of a part of the mitral leaflet (within which prolapse was initially present) and mitral annuloplasty with a rigid ring with 32 mm in diameter. In the operating room, BT suddenly dropped (70/40 mm Hg) when extracorporeal circulation had been discontinued. Intraoperative transesophageal echocardiography was performed which revealed: normal segmental and global contractility of the left ventricle with ejection fraction of 60%, minute mitral insufficiency and the presence of severe SAM that caused left ventricular outflow tract obstruction (maximal gradient 90 mm Hg). The patient received fluids (1,000 ml of crystalloids) and inotropic support was discontinued (low doses of dobutamine). Since no significant improvement was seen, extracorporeal circulation was started again and the cardiac surgeon decided about reimplantation of the mitral ring. The rigid ring implanted before was replaced with a flexible physiological ring, one size bigger (34 cm). A checkup transesophageal echo examination confirmed a satisfactory effect of the repeated surgery and resolution of LVOTO. The postoperative period proceeded without complications.

Case 4: A 54-year-old female patient was brought to our hospital with a suspicion of acute coronary syndrome with ST-segment elevation, for further diagnosis and intervention. Additional examinations performed prior to admission to hospital (ECG, high-sensitivity troponin assay) confirmed the initial diagnosis. As the patient was being transported by an ambulance, her cardiac status was stable (HR 80/min, BP 120/80 mm Hg); she received intravenous nitroglycerin infusion, but there was no need to administer positive inotropic agents. When the patient was brought to hospital, a sudden BP drop (to 70/40 mm Hg) was observed with tachycardia of 100/min and a short loss of consciousness. Bedside echocardiography, conducted as an emergency by a resident on duty in the emergency room, revealed: segmental left ventricle contractility disorders (akinesia of the apex and anterior wall) with retained global contractility (ejection fraction 50%), SAM/LVOTO with LVOT maximal gradient of 60 mm Hg and mild mitral insufficiency. Nitroglycerin was immediately discontinued and replaced with fluids. Moreover, a continuous infusion of noradrenalin was started. When hemodynamic stabilization was obtained, the patient was transferred to the hemodynamics laboratory where acute obstruction of the interventricular branch of the left coronary artery was diagnosed. Simultaneously, angioplasty of the infarctrelated artery was performed. No recurrence of LVOTO was observed directly after the procedure and during further hospitalization.

Discussion

Left ventricular outflow tract obstruction is a dynamic phenomenon and a relatively frequent, but still too rarely identified cause of hypotension and loss of consciousness^(1–3). The most common cause of dynamic obstruction/tightening of the left ventricular outflow tract is so-called SAM (systolic anterior motion), i.e. a “forward” mitral valve systolic motion towards the LVOT (left ventricular outflow tract)⁽³⁾. SAM was described for the first time in the 1960s in patients with asymmetrical hypertrophic cardiomyopathy (HCM), and it was initially believed to be one of the pathognomonic signs of this disease entity⁽²⁾.

It is currently assumed that LVOTO is a dynamic phenomenon, the occurrence of which requires the coexistence of two elements: 1) predisposing anatomic factors and 2) a physiological condition that induces such a phenomenon. It is thus obvious that SAM and LVOTO may be transient in so-called high-risk patients (anatomically predisposed) due to a change of the fluid status (e.g. dehydration, surgical procedures under general anesthesia etc.)^{(1, 3–5)}.

Left ventricular outflow tract obstruction (due to SAM) leads to the extension of the systolic ejection phase and a decrease in ejection volume. It can also lead to coaptation of mitral leaflets and, as a result, to significant mitral insufficiency, which further impairs cardiac output⁽³⁾. The clinical consequences of sudden SAM/LVOTO is abrupt hypotension and low cardiac output syndrome that do not respond to standard treatment or even exacerbate following the administration of positive inotropic agents and vasodilators^{(1, 3)}. In such cases, a hemodynamic improvement is brought about by fluid therapy and administration of negative inotropes and vasoconstrictors (which is contrary to the first expected reaction).

Echocardiography plays the major role in diagnosis as well as in estimation of the severity and consequences of LVOTO, which is confirmed by the cases presented above.

In the first case (tetralogy of Fallot), we observed a typical combination of unfavorable predisposing factors (enlargement of the right ventricle with a shift of the septum to the left) and hypovolemia as well as vascular dilation, which were transient (associated with general anesthesia). SAM/LVOTO – the consequence of such a condition, was additionally intensified by the implementation of positive inotropic agents. It was bedside echocardiography that finally enabled the correct diagnosis to be established and proper treatment to be started. Moreover, ultrasound assessment of IVC dynamic parameters enabled the degree of vascular bed filling to be monitored in a reliable way and actual fluid requirement to be determined.

The second case is also a classic example of the combination of anatomic predisposition (ventricular septal hypertrophy in hypertension, sigmoid-shaped interventricular septum) and an inducing physiological condition (absolute hypovolemia as a consequence of emesis). It must be emphasized that SAM usually occurs in patients with severe untreated arterial hypertension⁽⁴⁾. In such cases, echocardiography usually shows: significant concentric hypertrophy of the LV with a marked asymmetrical hypertrophy of the basal part of the interventricular septum that bulges into the LVOT (bulging septum) and small chamber of the left ventricle^{(4, 5)}. Such anatomic changes were also observed in our patient. An ultrasound examination also enabled significant hypovolemia to be confirmed. Hypovolemia, both absolute and relative (which is the effect of vasodilatory properties of anesthetics), causes a decrease in the degree to which the left ventricle is filled and leads to LVOT obstruction and hyperdynamic systolic action of the LV. Moreover, improperly filled LV causes unfavorable changes in its geometry (relative anterior and central dislocation of the papillary muscles). Hyperdynamic activity of the “empty” left ventricle leads to the increase in the velocity in the outflow tract and increases the strength with which the mitral valve moves towards the interventricular septum. In consequence, it leads to LVOTO.

The third case presents SAM/LVOTO during a cardiac surgery. SAM/LVOTO developed probably due to: the technique of the procedure to repair the mitral valve (usage of a rigid ring), general anesthesia and administration of inotropic agents (dobutamine). The literature reports confirm that approximately 5% of mitral valve repair surgeries may be complicated by SAM and LVOTO⁽⁶⁾. In such a group of patients, SAM is determined by: anterior dislocation of the mitral coaptation point and mitral leaflets’ shift towards the LVOT. Significant risk factors of SAM following mitral valve repair surgeries include: the ratio of the length of the anterior mitral leaflet to the posterior one <1.3, annuloplasty with the use of rigid/semi-rigid rings and small size of the LV. Due to the detection of SAM and significant LVOTO, the surgeon decided about an immediate reimplantation of the mitral ring, replacing it with a physiological one with a greater size.

In the fourth patient, LVOTO occurred together with acute anterior myocardial infarction with ST-segment elevation.

The literature reports include instances of SAM in the course of acute myocardial infarctions⁽³⁾. Cardiogenic shock and appearance of a new murmur above the heart usually develop in this group of patients. Therefore, typical mechanical complications of infarction were suspected (rupture of the interventricular septum or the papillary muscle). The detection of raised flow parameters in the LVOT and diagnosis of SAM with LVOTO were possible only in echocardiography. The mechanism underlying SAM in myocardial infarction is a compensatory hyperkinesia of the segments that are not involved in infarction and a geometrical change of the entire LV and LVOT. In the patient presented above, an immediate echocardiographic examination enabled: assessment of the extensiveness of ischemia, ruling out mechanical complications of infarction and identifying direct causes of the clinical condition deterioration (SAM/LVOTO), which guided further treatment.

The clinical cases presented above confirm the essential role of echocardiography in diagnosis and management of SAM/LVOTO. Both literature reports and author's own observations indicate that such a phenomenon is relatively common in various groups of patients (not only in hypertrophic cardiomyopathy). Therefore, it seems justified that echocardiography conducted in patients with unexplained hypotension should also include an evaluation for the presence of LVOTO. SAM/LVOTO can be identified in: 2D and M-mode examinations (fig. 1), which reveal a systolic motion of the mitral leaflet towards the interventricular septum, color Doppler examination (turbulent flow in the LVOT), LVOT mapping with pulsed-wave Doppler (raised flow parameters lead to the exceeded Nyquist limit and cause aliasing phenomenon; fig. 2). Moreover, the application of continuous-wave Doppler enables the measurement of maximal velocity and LVOT gradient (fig. 3).

Fig. 1.

M-mode at the level of the mitral leaflet tips; SAM is visible

Fig. 2.

Pulsed-wave Doppler measurement of flow in the LVOT; aliasing is visible

Fig. 3.

Measurement of the maximal flow velocity in the LVOT with continuous-wave Doppler

Conclusion

1. Left ventricular outflow tract obstruction is an underestimated dynamic phenomenon that might temporarily accompany various diseases.

2. It is assumed that SAM and LVOTO require the coexistence of predisposing anatomic factors and a physiological condition that induces it.

3. Hypotension and low cardiac output syndrome, which are consequences of SAM/LVOTO, do not respond to typical treatment or even aggravate when positive inotropic and vasodilating agents are administered. If the presence of SAM/LVOTO is confirmed, one should implement fluid therapy, negative inotropes and vasoconstrictors.

4. Echocardiography with evaluation of the LVOT in terms of obstruction is essential to diagnose SAM/LVOTO.

Conflict of interests

The authors do not report any financial or personal links with other persons or organizations, which might negatively affect the content of this publication and claim authorship rights to this publication.