Hemodynamic Instability: Definition, Classification, Mechanisms, Causes, Symptoms, Diagnosis and Treatment

It is defined as any instability in blood pressure that can lead to inadequate arterial blood flow to the organs.

Hemodynamic instability as a clinical state is, for practical purposes, a perfusion failure represented by clinical features of circulatory shock and / or advanced heart failure , or simply one or more measurements that may indicate out-of-range values ​​but not necessarily pathological.

Physical signs of acute circulatory failure are primary references for shock, including hypotension , abnormal heart rate, cold extremities, peripheral cyanosis, and mottling, along with bedside measurements of right-sided filling pressure and decreased urine flow.

Regional perfusion failure, such as mesenteric thrombosis or acute vascular obstruction of an extremity due to arterial or venous occlusion, has sometimes been considered as ‘regional shock’ perhaps because it can ultimately lead to systemic perfusion failure and, hence, circulatory shock.

Hemodynamic stability can be explained simply as stable blood flow. If a person is hemodynamically stable, it means that they have a stable heart pump and good blood circulation.

It is also a state in which physiological and mechanical support is required to ensure that there is adequate cardiac input and output or arterial pressure.

Abnormal hemodynamic parameters include heart rate, blood pressure, cardiac output, central venous pressure, and pulmonary arterial pressure.

Classification of hemodynamic instability

Four categorical states of shock have the common denominator of decreased effectiveness of systemic blood flow but with different mechanisms. Critical reductions in intravascular volume produce hypovolemic shock due to loss of blood or fluid.

Cardiogenic shock is due to pump failure; its prototype is an acute myocardial infarction. Distributive discharge includes septic shock, in which we have high flows that bypass the capillary exchange bed, presumably due to arteriovenular bypass or increased venous capacitance.

Distributive shock also follows loss of automatic controls as in the case of spinal cord cross section, or drug-induced expansion of the capacitance bed by ganglion drugs, or decrease in arterial resistance caused by alpha-blocking agents. adrenergic.

The fourth category is that of obstructive shock that is due to a general obstruction of blood flow.

Prototypes of obstructive shock include pulmonary embolism, dissecting aortic aneurysm, a ball valve thrombus, or combined obstructive and cardiogenic shock in the case of pericardial tamponade.

In each case, there is a decrease in tissue perfusion, although the mechanisms are quite discrete.

In addition, hypovolemia has a high probability of complicating circulatory shock from other causes, in part due to adrenergic primed ventricular vasoconstriction with transudation of fluid from capillaries into the interstitial space.

Hemodynamic mechanisms

To understand the sites in the circulatory system that explain hemodynamic stability and, by implication, hemodynamic instability, we identified eight specifics and they include:

  1. Venous return to the right side of the heart or preload.
  2. The myocardium and myocardial contractile function, including heart rate and rhythm, which are determinants of stroke volume and thus rhythm and rhythm-dependent cardiac output.
  3. Precapillary arteriolar resistance that operates as an afterload on the heart.
  4. The capillary exchange circuit that is the site of substrate exchange, including contingent fluid changes to capillary hydrostatic pressure.
  5. Capillary venular resistance which is an important controller of capillary hydrostatic pressure.
  6. Venous capacitance that in some states of shock expands to accumulate large volumes of blood, representing a critical decrease in venous return or preload and, therefore, in cardiac output.

Causes of hemodynamic instability

Hemodynamic instability can occur for many reasons. The critically ill patient may begin with poor vascular tone, a dysfunctional autonomic feedback loop, and / or low cardiovascular reserve.

The causes of hemodynamic instability in the early postoperative period are:

Commons

  • Patient-ventilator dyssynchrony.
  • Hypovolemia.
  • Low systemic vascular resistance.
  • Left ventricular systolic dysfunction.
  • Diastolic dysfunction of the left ventricle.
  • Right ventricular dysfunction.
  • Pericardial compression (tamponade).
  • Alteration of rhythm.

Rare

  • Severe mitral regurgitation.
  • Another valve disease.
  • Dynamic obstruction of the left ventricular outflow tract.
  • Dynamic hyperinflation of the lung.
  • Pneumothoraxic tension.
  • Massive hemothorax.

The patient’s illness and how the care is structured can lead to an imbalance in the supply and demand of oxygen, creating a situation in which any care activity (demand) overloads the supply, creating instability.

As we age, the responsiveness of our autonomic nervous system to gravity-related fluid changes is diminished, putting us at greater risk for cardiac instability.

This response may be related in part to an altered baroreflex function that causes an inappropriate autonomic response.

Critically ill patients with a history of diabetes may be at increased risk for hemodynamic instability. Autonomic dysfunction increases as diabetic complications worsen.

Another reason for hemodynamic instability in critically ill patients can be deduced from research on gravity and space flight.

Cardiovascular instability during a position change is often seen after patients spend long periods of time in a stationary position.

When people change their gravitational reference from a lying position to a sitting position, the body goes through a series of physiological adaptations to maintain cardiovascular homeostasis.

Critically ill patients may experience a similar adaptation when turning laterally, hanging on their legs, or standing up.

With changes in the gravitational plane (position change), stretch receptors read the change in plasma volume, the inner ear responds, and information is sent to the autonomic nervous system to adapt accordingly.

Signs and symptoms

The following are some signs and symptoms of hemodynamic instability:

  1. Hypotension
  2. Abnormal heart rate
  3. Short of breath.
  4. Lung congestion
  5. Cold extremities.
  6. Peripheral cyanosis.
  7. Decreased urine output
  8. Alternative consciousness (restlessness, loss of consciousness, confusion).
  9. Chest pain

Severe hemodynamic instability is usually obvious, but the identification of subtle alterations in cardiovascular function can be difficult.

Clinical evaluation of hemodynamically unstable patients and diagnosis

The clinical examination of hemodynamically unstable patients provides timely, low-risk, and potentially useful diagnostic information.

This review will examine the evidence supporting the use of clinical examination findings to guide treatment decisions and predict outcomes in patients with hemodynamic instability.

The clinical examination remains an important initial step in the diagnosis and risk stratification of patients.

Despite the limitations of current techniques, the availability, low risk, and the ability to perform repetitive testing ensure that clinical examination of the hemodynamically unstable patient continues to be a useful tool for the intensivist until more useful tests are validated in this population. of patients.

Hemodynamic instability can be diagnosed by monitoring vital signs such as respiratory rate, pulse, blood pressure, urine output, organ perfusion, finger temperature gradient, and capillary fill time.

Complications such as bleeding, thrombosis, thrombophlebitis, pulmonary embolism, and arterial spasm can sometimes occur. The pulse is the first sign that indicates hemodynamic instability. But it is neither sensitive nor specific to confirm it.

Respiratory rate is another sign for finding hemodynamic instability. The change in respiratory rate can be used to find a response to the treatments offered.

Blood pressure and mean arterial pressure are appropriate indicators for instability. Extreme temperatures can also indicate severe instability. Oliguria clearly represents low cardiac output.

Therefore, critically ill patients must be carefully monitored to avoid complications due to hemodynamic instability.

Important diagnostic clues can be obtained from the patient’s history and intraoperative course. Operating notes, angiograms, and echocardiograms should be reviewed.

The examination should focus on the cardiovascular system, particularly the presence of new murmurs. Specific diagnoses can be suggested based on the electrocardiogram (ECG), central venous pressure, and blood pressure waveforms.

If the EKG tracing is abnormal, 12-lead and atrial EKGs should be obtained.

Respiratory problems can cause hemodynamic instability and a careful examination of the respiratory system should be performed, including monitoring the circuit and setting the ventilator.

Chest drain bottles should be inspected for blood loss and bubbling. Important trends can be identified in the chart of 24-hour intensive care units.

Other blood tests, such as arterial blood gases, complete blood count, clotting status, and troponin may be ordered, depending on the circumstances. Chest X-rays should be reviewed.

Treatment

A step-by-step approach to the diagnosis and initial treatment of the hemodynamically unstable postoperative cardiac patient:

Step 1 : Confirm the presence of hemodynamic instability.

Check level and zero all transducers. Re-level and re-zero all pressure transducers if necessary. Check all infusion pumps and the integrity of all infusion lines.

Step 2 : Does the patient have an immediate life-threatening problem (i.e. mean arterial pressure (MAP) <50 mmHg)?

Step 3 : Clinically evaluate the patient.

Perform a specific physical exam, focusing on the cardiac and respiratory systems, ventilator, and chest drains.

Review the 24 hour chart. Get relevant research: blood gases, superior vena caval (SsvcO2), electrocardiogram (atrial and 12-lead), and chest x-ray. Review the old notes and intraoperative course. Inform the surgeon

Step 4 : Consider the following interventions:

Paralyze and sedate the patient and ventilate with 100% oxygen. If indicated, disconnect the head from the ventilator and ventilate by hand with a manual resuscitator.

Move the heart rate to 90 beats / min using the dual chamber pacemaker [DDD] or the A + V asynchronous pacing mode of [DOO] at maximum output.

Give a fluid exposure of 500 ml of a crystalloid (for example, normal saline or Plasma-Lyte). Begin or increase inotropic support.

Step 5 : If the diagnosis remains uncertain, insert a pulmonary artery catheter and / or perform an echocardiogram.

If there is a clinical suspicion of low or high cardiac output, a pulmonary artery catheter should be inserted. If a pulmonary artery catheter is in place and cardiac output is low, an echocardiogram should be performed.

If the patient remains hypotensive and the cause is unclear, an echocardiogram should be performed. If a specific diagnosis (eg tamponade) is clinically suspected, an echocardiogram should be performed.

If hemodynamic status remains inadequate despite intra-aortic balloon pump and high-dose inotropic drug therapy combination, and cardiac dysfunction is potentially reversible, ventricular assist device placement should be considered.

Hemodynamic monitoring

Hypovolemia is usually characterized by hypotension in association with low atrial pressures.

With compensated left ventricular systolic dysfunction, hemodynamic findings may be normal. Signs of decompensation include elevated pulmonary artery wedge pressure, pulmonary arterial hypertension, low cardiac output, and low venous oximetry.

A patient with a central venous pressure below 8 mmHg or a pulmonary artery wedge pressure below 10 mmHg, in the presence of hypotension, is invariably sensitive to fluids.

However, there are no absolute values ​​of central venous pressure and pulmonary arterial wedge pressure that predict fluid responsiveness, and in many circumstances patients with much higher filling pressures also benefit from fluid administration.

High central venous pressure can be caused by right ventricular dysfunction or high intrathoracic or intrapericardial pressure. A central venous pressure greater than 15 mmHg is sometimes required to optimize cardiac output in patients with right ventricular dysfunction.

In contrast, in patients with left ventricular dysfunction, central venous pressure may be low despite circulatory overload. In severe left ventricular hypertrophy, the pulmonary artery wedge pressure may need to be above 18 mmHg to optimize preload.

In contrast to absolute values ​​of central venous pressure and pulmonary artery wedge pressure, respiratory fluctuations in central venous pressure and blood pressure waveform (paradoxical pulse) are predictive of responsiveness. fluid.

A patient with a respiratory oscillation in the arterial waveform is invariably sensitive to fluids, regardless of atrial pressure.