Physiology

The most important hemodynamic parameter that must be estimated is the adequacy of oxygen delivery to the tissues. This can be assessed in several ways: blood pressure, heart rate, urine output, warm, dry extremities (vs. cold, clammy extremities), and capillary refill (greater than 3 seconds is considered abnormal). These are the old-fashioned, time-honored methods of assessing the adequacy of tissue perfusion which every physician must be skilled at. Frequently, a good assessment of a patient's condition can be made by these parameters and the diagnosis can be fine-tuned using more specific parameters which can be obtained from the Swan-Ganz catheter and central venous pressure monitoring lines.

Shock can be defined as a condition of inadequate tissue perfusion and is due to one of four possible etiologies: 1) cardiogenic shock, 2) hypovolemic shock, 3) septic shock, and 4) neurogenic shock. Another type of shock, tamponade, is actually a subset of cardiogenic shock although it is not a dysfunction intrinsic to the myocardium but rather an external force acting on the myocardium. The Swan-Ganz catheter may be a valuable aid in differentiating these types of shock. This can be done by an evaluation of the cardiac output, cardiac index, systemic vascular resistance, pulmonary vascular resistance, CVP, pulmonary artery pressure, and pulmonary capillary wedge pressure (from which can be estimated the left atrial pressure and the left ventricular end diastolic pressure).

The mixed venous oxygen saturation is also a useful parameter in distinguishing the types of shock. The cardiac output is the stroke volume times the heart rate and is described in liters per minute. The cardiac index is the cardiac output divided by the surface area in meters squared. Normal cardiac output is in the range of 4-5 liters per minute. The cardiac index is in the range of 2-3 liters per minute per meters squared. The "filling pressures" is a term frequently used on the Cardiothoracic Surgery service and refers to the pressure within the chambers of the heart. This can be estimated by the CVP which mirrors the right atrial filling pressure and is normally in the range of 2-5 mmHg. The right ventricular pressure is more difficult to measure and is in the range of 30/0 mmHg. The pulmonary artery pressure is in the range of 30/10 mmHg and is measured by the Swan-Ganz distal port. The left atrial pressure is in the range of 8-10 mmHg and can be estimated by the pulmonary capillary wedge pressure. When the distal end of the pulmonary artery catheter is wedged in the pulmonary artery, it reflects the pressure in the pulmonary veins, which in turn reflects the pressure in the left atrium.

The left ventricular pressure is in the range of 120/5 mmHg. The left ventricular end diastolic pressure can be estimated by the left atrial pressure, which in turn can be estimated by the pulmonary capillary wedge pressure.

The systemic vascular resistance is an important parameter. It is one of the primary factors related to afterload. Elevated systemic vascular resistance may occur in the presence of hypovolemic shock where catecholamine surge causes profound vasoconstriction and elevation of the systemic vascular resistance to try to support the blood pressure. In certain instances in the postoperative cardiac surgery patient, pharmacologically decreasing the systemic vascular resistance may be very helpful in improving cardiac output since the heart will have to contract against less resistance. Of course if the problem is volume-related peripheral vasoconstriction, then simply volume loading the patient will decrease the systemic vascular resistance physiologically rather than pharmacologically.

The systemic vascular resistance can be calculated by the following formula: [(MAP-CVP)/C.O.] x 80 = dynes cm-5. The mean arterial pressure can be calculated as two times the diastolic plus the systolic all divided by three. If one does not use the conversion factor 80, then the value obtained is in Woods units, which was used in the past more frequently. Pulmonary vascular resistance is an assessment of the resistance in the pulmonary vasculature. This is calculated as: [(MPP-wedge)/C.O.] x 80. MPP is the the mean pulmonary artery pressure and is calculated as is MAP. This is more commonly expressed in Woods units than the systemic vascular resistance is.

The filling pressures may have a direct role in cardiac output according to the Starling curve. Initially cardiac output goes up dramatically with increasing left atrial pressure; however, at a critical point the cardiac output starts to plateau and when the pressure is so high that distention occurs, then cardiac output falls. The improvement of cardiac output with increasing left atrial pressure is less pronounced with ischemic or compromised hearts. In tamponade, volume loading is one of the key therapeutic modalities available prior to surgery.

Inotropes shift the Starling curve in favor of improved cardiac output with relation to filling pressures; this is done by improving contractility. Unloading the ventricle by decreasing afterload also improves cardiac output. The Swan-Ganz catheter is an extremely useful device for measuring filling pressures of the right atrium, pulmonary artery pressure, pulmonary capillary wedge pressure, cardiac output, systemic vascular resistance and pulmonary vascular resistance. It has three ports. One is a proximal infusion port, the other is a proximal pressure port, and the third is a PA distal pressure port. There also exists a balloon inflation device and a thermistor for measuring cardiac output via the thermodilution method.

When placing the Swan-Ganz, there is a typical pressure trace as the balloon transverses the right atrium, the right ventricle, the pulmonary artery, and then is wedged into position. Typically, the balloon is inflated when the catheter is inserted 15 cm and is then passed through the right heart into the pulmonary artery and wedged into position. Normally, it wedges in the right pulmonary artery although it may go into the left pulmonary artery on occasion. It is important to

keep the balloon inflated when directing the catheter through the right heart lest it bang too forcefully against the right heart endocardium, possibly causing perforation or becoming entrapped in the trabeculations.

Right bundle branch block is a frequent finding during passage of the catheter and one must be extremely cautious, especially in the presence of left bundle branch block lest a complete heart block occur. PVCs and ventricular tachycardia are also common.

Another important parameter that the Swan-Ganz catheter can evaluate is the mixed venous saturation (SVO2). This is the saturation of the venous blood retrieved from the pulmonary artery. Recall that the PvO2 in the pulmonary artery is about 40 mmHg which corresponds to 75% saturation. This is normal, i.e. 25% less than the normal PaO2 which is 100%. An adequate mixed venous saturation is usually in the range of 25% less than the arterial saturation. One may recall that the most desaturated blood in the body is drained from the coronary sinus because of the high oxygen use by the normal heart. Thus venous saturation of the coronary sinus is less than in any other part of the body. Venous saturation in the inferior vena cava is fairly high because the kidneys take up or utilize less oxygen than other organs. The superior vena cava generally has a lower mixed venous saturation than the inferior cava because the brain utilizes more oxygen. These sources of venous blood "mix" in the right atrium and then go through the ventricle up into the pulmonary artery. This is why samples taken through the distal tip of the pulmonary artery catheter are called mixed venous samples.

The mixed venous oxygen saturation is an important hemodynamic parameter which can be used to define trends in a patient's hemodynamic status and clinical outcome. For example, patients with impending tamponade will frequently show a decrease in their mixed venous oxygen saturation long before changes in heart rate or blood pressure occur. The reason is that the mixed venous condition indicates poor tissue oxygenation resulting from poor cardiac output related to the tamponade. This may not yet be reflected by the blood pressure or heart rate. A similar situation occurs in a failing myocardium from poor contractility secondary to ischemia, edema or other causes.

There are only three factors which will cause decreased mixed venous saturation and should be known by every surgery resident managing patients in the CSICU. These include:

1) Decreased cardiac output: This bespeaks of low tissue perfusion resulting in increased extraction of oxygen from the available blood flow, resulting in decreased mixed venous oxygen saturation. This results from poor inotropy, low volume or slow heart rate. It can be confirmed by other hemodynamic parameters including cardiac output. Management involves inotropes, volume or increasing the heart rate.

2) Decrease in oxygen content of the blood: The formula for oxygen content is [Hg] x 1.38 x % saturation. As one can see from the formula, oxygen content can be decreased by both decreased hemoglobin concentration, i.e. anemia or low hematocrit, or can be effected by the saturation of the blood, as for example, the

patient who is not being ventilated with adequate FIO2 during the initial perioperative period. Either of these will decrease the oxygen content and, thus, the saturation of the venous blood. This occurs because of increased extraction from the available oxygenated red blood cells by the oxygen hungry tissues.

3) Increased metabolic demand by the peripheral tissues: This occurs classically in cases of shivering in the immediate postoperative period. One often sees a patient shivering simultaneous with the mixed venous saturation dropping precipitously. The problem can be at least partially corrected by paralyzing the patient to prevent the shivering reaction.

Thus in all the major types of shock seen in the immediate postoperative period after cardiac surgery, the SVO2 will drop. One particular instance, however, of shock in which the SVO2 does not decrease is sepsis. This is a very important point in differentiating sepsis from other conditions of shock. In conditions of sepsis, AV shunts are opened in which oxygenated blood bypasses the tissues and goes directly into the venous circuit resulting in an increase in SVO2.

Table 10.1 shows the differentiation of hypovolemic, cardiogenic, and septic shock based on the blood pressure, CVP, wedge pressure, cardiac output, SVO2 and the SVR. Additionally, tamponade is shown which basically mimics cardiogenic shock with the exception that the filling pressures are even more markedly elevated with tamponade than in the latter and tend to equilibrate at a higher level.

Pulmonary embolus is also shown in Table 10.1. This is a rare event after cardiopulmonary bypass; however, it does occur as early as a week postoperatively. Since the patient has been fully systemically heparinized, it is extremely rare to develop a pulmonary embolus in the immediate perioperative period. However, a week postoperatively, we personally have seen several patients develop massive pulmonary emboli with a high mortality. It is not a condition to be taken lightly. Pulmonary emboli too will decrease the SVO2 because of inadequate oxygenation of the available hemoglobin. The patient dies after massive pulmonary embolus for two reasons: asphyxiation from inadequate blood oxygenation and decreased cardiac output with poor volume loading of left heart because of obstruction of the pulmonary artery.

Cardiac output measurements can be made by several methods. These include:

1) Indicator dilution method with indocyanine green.

2) Thermodilution technique with the Swan-Ganz catheter.

Table 10.1. Hemodynamic parameters related to type of shock

Type of shock

BP

P CVP

Wedge

C.O.

SVO2 SVR

Hypovoliemic

J

î J

J

J

1 î i 1

Cardiogenic

I

î î

î

J

1 î i 1

Septic

J

î J

J

îî

î 1 1 1 i i

Tamponade

I

î îî

îî

J

i 1

PE

J

î î

-

J-

1 î i 1

3) Fick method which is basically evaluation of the rate of oxygen consumption. This is a function of the rate of blood flow times the rate of oxygen pickup by the red blood cells. Hence, cardiac output equals oxygen consumption divided by AV oxygen difference times ten.

Herbal Remedies For Acid Reflux

Herbal Remedies For Acid Reflux

Gastroesophageal reflux disease is the medical term for what we know as acid reflux. Acid reflux occurs when the stomach releases its liquid back into the esophagus, causing inflammation and damage to the esophageal lining. The regurgitated acid most often consists of a few compoundsbr acid, bile, and pepsin.

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