Medical brain notes 7

  Anesthesia and the lung And the Lord God formed man from the dust of the ground and breathed into his nostrils the breath of life; and man became a living soul  (Genesis 2;7) The concept of breath and soul reverberates through many languages in which spirit and breath share overlapping meanings. For example, in English, to  inspire  can have a physiological or psychological connotation, while to  expire  can mean nothing more than to exhale, or it can describe the moment when your spirit leaves you with your last breath. In anesthesia, we deal with both; on the one hand, the breath that needs to be provided for patients who cannot breathe by themselves and, on the other hand, the spirit – in a larger sense – which we subdue with drugs. Small wonder, then, that the linkage of breath and life gives us awesome responsibilities. In our practice no function is more important than ventilation, and no organ more integral to our practice than the lungs. Failure o...

  Purpose of breathing Breathing brings in oxygen necessary for cellular respiration and eliminates the resulting carbon dioxide. If oxygen supply does not meet demand, desperate cells revert to anaerobic metabolism, resulting in lactic acidosis. 1  Our oxygen requirement depends on the metabolic rate, but for a resting individual 3 mL O 2 /kg/min, should suffice. Meanwhile, we generate CO 2  at a rate dependent on the respiratory quotient “ R :” where  V ˙ C O 2  and  V ˙ O 2  are the minute production of carbon dioxide and consump-tion of oxygen, respectively.  R  depends on the energy source (carbohydrates, proteins, fat).  R  approaches 1 in several conditions including pregnancy and patients on total peripheral nutrition (TPN), but we usually peg it at 0.8. Control of breathing Can you commit suicide by simply not breathing or by willing your heart to stop? Even though we have voluntary muscular control over ventilation, we can...

  Purpose of breathing Breathing brings in oxygen necessary for cellular respiration and eliminates the resulting carbon dioxide. If oxygen supply does not meet demand, desperate cells revert to anaerobic metabolism, resulting in lactic acidosis. 1  Our oxygen requirement depends on the metabolic rate, but for a resting individual 3 mL O 2 /kg/min, should suffice. Meanwhile, we generate CO 2  at a rate dependent on the respiratory quotient “ R :” where  V ˙ C O 2  and  V ˙ O 2  are the minute production of carbon dioxide and consump-tion of oxygen, respectively.  R  depends on the energy source (carbohydrates, proteins, fat).  R  approaches 1 in several conditions including pregnancy and patients on total peripheral nutrition (TPN), but we usually peg it at 0.8.

  Control of breathing Can you commit suicide by simply not breathing or by willing your heart to stop? Even though we have voluntary muscular control over ventilation, we cannot stop breathing. We are hard-wired so that, in response to rising carbon dioxide tensions in the medulla, carbon dioxide-sensitive neurons stimulate ventilation to keep the arterial partial pressure of CO 2  (PaCO 2 ) near 40 mmHg. In physiological sleep, they let the PaCO 2  drift up to 45 mmHg, while in pregnancy, they are reset by the controller to maintain 30 mmHg. Within physiological limits, ventilation and PaCO 2  (Fig.  10.1 ) keep a linear relationship. We can restrain the center pharmacologically with opioids or deep inhala-tion anesthesia. In some diseases resulting in high PaCO 2 , the respiratory center fatigues permanently, and these “CO 2  retainers” must then rely on hypoxemia to drive their ventilation. Perhaps surprisingly, oxygenation is not detected in the brain ...

  Mechanics of ventilation Spontaneous ventilation at rest involves generating negative intrathoracic pres-sure (by lowering the diaphragm and expanding the chest wall), causing air to be drawn into the lungs. This requires that the upper airway remains patent. In the presence of an obstruction, e.g., tongue, mass, mechanical, we observe retrac-tions, particularly around the clavicles and the jugular notch and, in children, the intercostal spaces. An early sign is a  tracheal tug,  a little downward movement of the larynx with each inspiration. A reliable sign of airway obstruction, the tra-cheal tug signals the recruitment of accessory muscles to maintain gas exchange. Similarly, pulmonary cripples (advanced emphysema) and patients still partially paralyzed after anesthesia will show a tracheal tug. Hypoxemic patients weakened by drugs or muscle disease require immediate assisted ventilation with bag and mask and, if necessary, establishment of a patent airway. At rest, ...

  The work of breathing The medullary centers control the PaCO 2  by altering the minute ventilation ( V ˙ E  ): V ˙ E =  V T ×  f where  V T =  tidal volume and  f =  respiratory rate. How these parameters change to maintain minute ventilation depends on the work of breathing. Because inhaling requires the work of muscles, it is “costly,” in an energy expenditure sense, to breathe. In general, a few large breaths are more efficient than many small ones because all breaths must move the same amount of deadspace volume (about 150 mL for the average adult, see below). Endotracheal tubes offer much resistance and can greatly increase the work of breathing. The ventilator will ease this burden by doing the inspiratory work for the patient. Just as with all other muscles, disuse leads to reduced strength and stamina. Several investigators continue to study the optimal amount of respiratory muscle loading to prevent muscle atrophy and weaning diffi...