TY  - CHAP
M1  - Book, Section
TI  - Respiratory Physiology & Anesthesia
A1  - Butterworth IV, John F.
A1  - Mackey, David C.
A1  - Wasnick, John D.
Y1  - 2022
N1  - 
T2  - Morgan & Mikhail’s Clinical Anesthesiology, 7e
AB  - KEY  CONCEPTS  The  trachea  serves  as  a  conduit  for  ventilation  and  for  clearance  of  tracheal  and  bronchial  secretions  and  has  an  average  length  of  10  to  13  cm.  The  trachea  bifurcates  at  the  carina  into  the  right  and  left  mainstem  bronchi.  The  right  mainstem  bronchus  lies  in  a  more  linear  arrangement  with  the  trachea,  whereas  the  left  mainstem  bronchus  lies  in  a  more  angular  orientation  with  the  trachea.  The  periodic  exchange  of  alveolar  gas  with  the  fresh  gas  from  the  upper  airway  reoxygenates  desaturated  blood  and  eliminates  carbon  dioxide  (CO2).  This  exchange  is  normally  brought  about  by  small  cyclic  pressure  gradients  established  within  the  airways.  During  spontaneous  ventilation,  these  gradients  are  secondary  to  variations  in  intrathoracic  pressure;  during  mechanical  ventilation,  they  are  produced  by  intermittent  positive  pressure  in  the  upper  airway.  The  lung  volume  at  the  end  of  a  normal  exhalation  is  called  functional  residual  capacity  (FRC).  At  this  volume,  the  inward  elastic  recoil  of  the  lung  approximates  the  outward  elastic  recoil  of  the  chest  (including  resting  diaphragmatic  tone).  Closing  capacity  is  normally  well  below  FRC,  but  it  rises  steadily  with  age.  This  increase  is  probably  responsible  for  the  normal  age-related  decline  in  arterial  O2  tension.  Whereas  both  forced  expiratory  volume  in  the  first  second  of  exhalation  (FEV1)  and  forced  vital  capacity  (FVC)  are  effort  dependent,  forced  midexpiratory  flow  (FEF25–75%)  is  more  effort  independent  and  may  be  a  more  reliable  measure  of  obstruction.  Changes  in  lung  mechanics  due  to  general  anesthesia  occur  shortly  after  induction.  The  supine  position  reduces  the  FRC  by  0.8  to  1.0  L,  and  induction  of  general  anesthesia  further  reduces  the  FRC  by  0.4  to  0.5  L.  FRC  reduction  is  a  consequence  of  alveolar  collapse,  and  compression  atelectasis  is  due  to  loss  of  inspiratory  muscle  tone,  change  in  chest  wall  rigidity,  and  upward  shift  of  the  diaphragm.  Local  factors  are  more  important  than  the  autonomic  system  in  influencing  pulmonary  vascular  tone.  Hypoxia  is  a  powerful  stimulus  for  pulmonary  vasoconstriction  (the  opposite  of  its  systemic  effect).  Because  alveolar  ventilation  (V̇A)  is  normally  about  4  L/min  and  pulmonary  capillary  perfusion  (Q̇)  is  5  L/min,  the  overall  ratio  is  about  0.8.  Shunting  denotes  the  process  whereby  desaturated,  mixed  venous  blood  from  the  right  heart  returns  to  the  left  heart  without  being  oxygenated  in  the  lungs.  The  overall  effect  of  shunting  is  to  decrease  (dilute)  arterial  O2  content;  this  type  of  shunt  is  referred  to  as  right-to-left.  General  anesthesia  commonly  increases  venous  admixture  to  5%  to  10%,  probably  as  a  result  of  atelectasis  and  airway  collapse  in  dependent  areas  of  the  lung.  Note  that  large  increases  in  PaCO2  (>75  mm  Hg)  readily  produce  hypoxia  (PaO2  <60  mm  Hg)  at  room  air  but  not  at  high  inspired  O2  concentrations.  The  binding  of  O2  to  hemoglobin  seems  to  be  the  principal  rate-limiting  factor  in  the  transfer  of  O2  from  alveolar  gas  to  blood.  The  greater  the  shunt,  the  less  likely  the  possibility  that  an  increase  in  the  fraction  of  inspired  oxygen  (FiO2)  will  correct  hypoxemia.  A  rightward  shift  in  the  oxygen–hemoglobin  dissociation  curve  lowers  O2  ...
SN  - 
PB  - McGraw-Hill Education
CY  - New York, NY
Y2  - 2024/04/17
UR  - accessanesthesiology.mhmedical.com/content.aspx?aid=1190606540
ER  -