Cardiac and Pulmonary Physiology

Question 1

What clinical conditions can lead to a low preload? How does systemic resistance affect cardiac filling pressures?

Answer 1

• Hypovolemia from hemorrhage or fluid loss.
• Venodilation that occurs from general anesthesia.
• Tension pneumothorax and peicardial tamponade, because of the increased pressure around the organs.

SVR = 80 x (MAP - CVP)/CO

MAP = SVR x CO

Basically, the more resistance the less cardiac output and filling pressure can occur.

Question 2

What model can be used to describe changes in pulmonary circulation? How do small and large lung volumes affect pulmonary vascular resistance?

Answer 2

The best model is the distension of capillaries and the recruitment of new capillaries. In large lung volumes, intra-alveolar vessels can be compressed, whereas extra-alveolar vessels have low resistance. The opposite is true of small lung volumes. Increased PVR at small lung volumes helps divert blood flow from collapsed alveoli, such as during one-lung ventilation.

Question 3

How can the A-a gradient, Pa02 to FI02 ratio, or isoshunt diagram be used to quantitate abnormalities of oxygenation? Which measurement of oxygenation depends the most on FI02?

Answer 3

Normal A-a gradients (alveolar to arterial oxygen) are 5 to 10 mmHg, but increase with age.

A-a gradient = PA02 - Pa02

An isoshunt is only available as a PA catheter, so the A-a gradient is the second best way because it is simple and can be calculated at room air. The P/F ratio (Pa02/Fi02) is a measurement of oxygenation that remains more consistent at higher Fi02.

Question 4

What are the determinants of arterial carbon dioxide partial pressure? What clinical conditions can cause hypercapnia during general anesthesia?

Answer 4

PaCO2 is a balance of production and removal.

PaCO2 = k x VCO2/VA

k = 0.863
VCO2 = carbon dioxide production

VA = alveolar ventilation

Causes of hypercapnia during general anesthesia:

1. Fever
2. Malignant hyperthermia
3. Systemic absorption during laparoxcopy procedures
4. Thyroid storm
5. Tourniquet release
6. Administration of sodium bicarbonate

Question 5

What is the physiological basis for the hypercapnic ventilatory response and the hypoxic ventilatory response? What is the typical time course of these responses?

Answer 5

Decreased minute ventilation is the most important and common cause of hypercapnia, mabye due to decreased tidal volume, breathing frequency, or both. Less CO2 is being released and more is being retained in systemic circulation. If alveolar ventilation decreases by 1/2, PaCO2 should double. This takes several minutes as a new stead state develops.

Arterial hypoxemia is is defined as a low partial pressure of oxygen in arterial blood (Pa02 ~<60mmHg) is commonlyl used. Whereas hypoxia reflects circulatory factors too. Arterial hypoxemia is most significant when anoxia is threatened, like apnea, and the difference between the two may be less than a minute.

Question 6

Cardiac index?

Answer 6

CO divided by body surface area

Question 7

Ejection fraction

Answer 7

The percentage of ventricular blood volume that is pumped by the heart in a single contraction

Question 8

Afterload

Answer 8

The resistance to ejection of blood from the left bentricle with each contraction