Rob Bartlett
Keywords
July 19, 2010
I.
PEEP: PaO2, Lung Volumes, LV effect
a. Positive end expiratory pressure – continuous pressure exerted throughout
the respiratory cycle (CPAP) – 3 indications
i. Prevent derecruitment, by returning the functional residual volume to
the physiologic range.
ii. Protect the lungs against injury by preventing phasic opening and
closing of atelectatic units
iii. Assist cardiac performance, during heart failure, by increasing mean
intrathoracic pressure
b. Lung Volumes
i. FRC (functional residual capacity) – about 2.5 liters of volume left at
the end of normal tidal breathing that serves as our O2 reservoir
during apnea (capacity composed of two volumes)
1. Residual volume – about 1 liter
2. Expiratory reserve volume
ii. FRC decreased under general anesthesia due to low tidal volume
ventilation and reduced sigh breaths to prevent atelectasis.
1. PEEP maintains small airway tone and prevents pt from
reaching closing capacity thereby Increasing FRC
2. Decreased atelectasis improves the Low V/Q mismatch
(shunt with venous admixture) and raises PO2
c. Cardiac Effects
i. Increased intrathoracic pressure may decrease venous return and
right heart preload
ii. Left Ventricle - Afterload is decreased by acting as external
compression of the LV making it easier to achieve a given systemic
pressure.
d. Complications of PEEP
i. Alveolar overdistention – Heterogenicity of alveoli means Nondependent zone/healthy alveoli tend not to collapse at end expiration
and added peep may over distend alveoli collapsing capillaries
creating increased Dead space and large V/Q mismatch (wasted
ventilation)
ii. Cardiac –
1. Decrease Venous Return - PEEP increases intrathoracic
pressure and may decrease venous return to right heart
leading to decreased cardiac output
a. Most pronounced with extreme PEEP and
hypovolemia
2. Elevated Pulmonary vascular resistance – PEEP increases
pressure on intrapulmonary vasculature and increases PVR
leading to increased RV afterload.
a. Most pronounced with acute increases PVR (ie new
PE) or extreme PEEP
b. If RV failure and over distention may reduce LV
compliance secondary to interventricular
interdependance (shared septum)
iii. Neuro - May reduce CPP by two mechanisms
1. Reduction of cardiac output
2. Increased intrathoracic pressure can be transmitted to SVC
and decrease venous drainage from jugular veins.
References:
http://www.ccmtutorials.com/rs/peep/page10.htm, accessed on 9/17/2010.
Marino, Paul. The ICU Book. 3rd edition 2007. pg481-486.