Spontaneous Circulation

Spontaneous Circulation focuses on advanced ECG interpretation, cardiac pharmacology, hemodynamic assessment and resuscitation, and managing acute coronary syndrome. It is devoted to translating the best evidence-based treatments from critical care, resuscitation, and trauma for bedside use in the emergency department.

Monday, April 7, 2014

The Narrow Gate

The patient first felt winded one night after doing the dishes. She was breathing so hard by morning that she was barely able to get out of bed. And a cough had started. “The flu” went through her mind. She stayed home from work to rest, but all day she just couldn’t catch her breath. The cough got worse and was making her chest hurt, and she felt her heart racing. She was exhausted by evening, but knew she wasn’t going to be able to sleep. The temperature was below 0°F outside, but she bundled up and drove herself the three miles to the emergency department. Barely able to speak by the time she stepped up to the triage desk, she was only able to get out “You need to slow my heart down.” The nurse knew that look.

The patient sat down hard in the wheelchair rescuing her from the burden of standing. “Medical Resus” was paged overhead, and the nurse rushed the patient back.

A (airway) was clearly intact as the patient was placed on the cart. Everyone knew that B was going to be problem. The monitoring cables were on before most people noticed, and there was an uncomfortable pause for several seconds while everyone in the Resus Bay waited for the first SpO2. 68%. Heart rate 146 bpm. That’s all it took. An IV was placed, and the patient was intubated in the next few minutes. Attention turned to the cause of respiratory failure. An ECG and chest radiograph was obtained, and are shown in Figures 1 and 2.

Figure 1. Presenting ECG.

Figure 2. Chest radiograph.

The ECG demonstrated atrial fibrillation with rapid ventricular response with ST-segment depression in the lateral leads concerning for myocardial ischemia. Bilateral pulmonary infiltrates show on the chest x­ray, which also shows a double density along the left heart border representing the bulging of the left atrial appendage. Key items on the differential at this point would include decompensated heart failure, acute coronary syndrome, infectious pneumonia, sepsis, and ARDS. A few minutes with the ultrasound revealed this key image. (Click here to see the video.)

A remarkable hypertrophied and calcified mass is on the ventricular side of the mitral valve, with severely impaired opening of the mitral valve during diastole. The anterior leaflet billows while remaining fixed at the commissure, visually described as a “hockey stick.” The patient’s presentation and this ultrasound are diagnostic of severe mitral stenosis. It was once much more common, but the incidence of mitral stenosis has been gradually declining in the United States. The patient has decades of being asymptomatic after the stenosis is initiated. Patients have several more decades once symptoms do occur before they become debilitating. Ten-year survival once this happens, however, approaches 10 percent. (Table 1.)

Most cases of mitral stenosis by far can be attributed to rheumatic disease. The immune response against Group A streptococcal antigens cross-react with structural glycoproteins of heart valves, especially the mitral valve, with leads to an ongoing auto-inflammatory response and destruction of the native valve. The hallmark of rheumatic mitral stenosis is commissural fusion, scarring and thickening of the valve, and shortening and thickening of the tendinous cords. Radiation therapy has quickly become a significant secondary cause of mitral stenosis.

Distinguishing between rheumatic heart and radiation stenosis can be difficult, even after a careful history. Commissural fusion does not seem to exist in radiation-associated mitral stenosis. Radiation also tends not to damage the papillary muscles and chordae tendineae. Radiation, however, tends to cause significantly more valvular calcification. Dyspnea is the most common feature of clinically significant mitral valve stenosis. Exacerbation of symptoms can be precipitated with increased cardiac demand (exercise, stress, critical illness) and atrial fibrillation with rapid ventricular rate. Restriction of the cardiac output causes fatigue, and cough and hemoptysis can be present. Evidence of elevated right and left heart pressures such as pulmonary and lower extremity edema are also often present. The physical exam, however, is not specific for mitral stenosis, and even the classic low­pitched rumbling mid­diastolic murmur is lost as the mitral stenosis progresses. (Table 2.)

A stenotic mitral valve significantly changes cardiac hemodynamics once the valve area falls to less than 2 cm2 (normal 4­6 cm2). High velocities and a large transmitral pressure gradient are needed because the entire cardiac output must pass through the stenosis. These higher gradients are generated by increased left atrial pressure in systole and diastole, and are transmitted upstream to the pulmonary vasculature and right side of the heart, causing pulmonary edema, pulmonary hypertension, and elevated central venous pressures. The left ventricular end diastolic pressure remains low from the reduced preload, and cardiac output is therefore depressed despite these higher pressures. The systemic vasculature compensates with higher resistance. The transmitral pressure gradient is a useful indicator of severity:

● Mild: <5 mm Hg
● Moderate: 5-10 mm Hg
● Severe: >10 mm Hg

Ultrasound is the primary means of evaluating mitral stenosis, and several techniques can determine the valve area and transmitral pressure gradient:

● 2­D planimetry. The opening area of the valve can be measured in the parasternal short axis view at the level of the mitral. The gain and view must be carefully optimized to avoid underestimating the valve area.
● Pressure half-time (PHT). The prolonged emptying time between the left atrium and left ventricle can be exploited because the time that pressure takes to half of its initial maximum value has been shown to be empirically related to the valve area (MVA=220/PHT). The continuous wave Doppler velocity is measured at the level of the mitral valve in an apical four-chamber view. The pressure gradient is related to the maximum velocity by a simplification of Bernoulli’s equation (p=4V2). The time it takes for the A pressure half-time of 220 msec corresponds to a valve area of 1 sq cm.
● Continuity equation. Uses mitral and aortic flow in addition to the cross-section area of the left ventricular outflow tract to derive the mitral valve area. The area is measured by planimetry and flow measured by Doppler assessment of velocity time integral (VTI). This method is less commonly used because of the accumulation in measurement errors.
● Proximal isovelocity surface area (PISA) method. Utilizes a fluid dynamic feature where accelerating flow forms hemispheres of isovelocities. This utilizes the flow convergence region proximal to the high velocity trans mitral jet in diastole. It is highly accurate but difficult to perform in transthoracic views and best utilized during transesophageal echocardiography.

The transmitral gradients are highly heart rate-dependent, and patients who develop rapid atrial fibrillation can quickly deteriorate. There is reduced filling time, which demands much higher filling pressures, which in turn causes significant pulmonary edema and occasional hemoptysis from pulmonary venous rupture. Treating mitral stenosis is directed at controlling the heart rate and maintaining sinus rhythm (e.g., digoxin, diltiazem, beta-blockade). Left ventricle contractility is generally preserved in mitral stenosis. β-blockade does result in decreased right ventricular contractility, which in pulmonary hypertension can further compromise the cardiac output and systemic blood pressure.

The loss in right ventricle contractility, however, is more than offset by the beneficial effects of reducing the heart rate. Loop diuretics should be used for diuresis and their venodilation effects (mediated by angiotensin II and prostaglandins) to reduce pulmonary vascular congestion. Left atrial dilation and stasis substantially increases the risk of thromboembolism, especially in atrial fibrillation, and therefore anticoagulation is critical. Surgical intervention is ultimately required, and involves carefully choosing balloon mitral valvuloplasty, mitral valve commissurotomy, or mitral valve replacement.