A Case Review Obesity and Effective Ventilation Strategies

A Case Review
Obesity and Effective
Ventilation Strategies
Dhanya Renjith SRNA
USC Program of Nurse Anesthesia
Keck School Of Medicine Los Angeles

Objectives
Discuss the impact of obesity on ventilation
during anesthesia
Identify the role of ventilator settings during
intra operative management of obese patients

Case Scenario
42 year old female patient undergoing laparoscopic
assisted Vaginal Hysterectomy
5 feet 2 inches tall and weighs 205
pounds/BMI=37.4 (IBW=50 kg/ ABW=67.3 kg)
Stable vital signs – Room Air SpO 2 =97%
No intrinsic or extrinsic lung disease
Android distribution of fat
Pre oxygenated with 100% oxygen, with PEEP of 5
cm of H 2 O for 5 minutes; shoulder ramps used
during intubation
Easy intubation

Ventilator settings
Volume control mode- Tidal Volume 600 /
Respiratory rate 12 / I: E 1:2
SpO 2 98%, PIP 18 cm of H2O, Plateau pressure
15 cm of H 2 O; stable vital signs
CO 2 insufflation- PIP increases to 34 cm of H 2 O
– SpO 2 down to 95 %.

How will you ventilate

Respiratory Physiology in
Obesity
Decreased compliance and increased resistance
Shallow and rapid breathing - Increased work of
breathing
Decreased FRC/Decreased Vital capacity / Decreased
Total lung capacity
FRC less than closing capacity
Anesthesia- 50% reduction of FRC in obese patients
as compared to 20% in non obese
Increased oxygen consumption and CO 2 production
even at rest

Respiratory Physiology in
Obesity
Increased cardiac output and minute ventilation
Increased oxygen consumption – PaO 2 in
morbidly obese patients is less than predicted
for similarly aged non obese patients
OSA (5% of obese patients)- hypoxemia ,
hypercapnia, pulmonary and systemic
vasoconstriction, polycythemia

During Anesthesia
Supine position
High elevation of diaphragm (due to increased
visceral and abdominal fat )
Ventilation perfusion mismatch
Right to left shunting
Hypoxemia

During Anesthesia
(continued)
Decreased compliance and increased resistance -
Higher inflation pressures required for
ventilation
LMA vs. ETT
Risks-aspiration / hypoventilation/hypoxemia
Precludes the use of LMA
Study that favors LMA for minor surgeries in
obese

CT scan – normal and obese
patient under anesthesia

Effective ventilation vs. prevention
of injury
Ventilation – SpO 2 , PaO 2 , PaCO 2 , ETCO 2
Repetitive opening and closing of unstable lung
units – cyclic stretch-------- INJURY
Effects of injury- Neutrophil infiltration , rupture
of alveolar bronchial attachment, formation of
reactive oxygen / nitrogen intermediates, cell
death ( necrosis and apoptosis), alteration in
alveolar capillary barrier
Up regulation of pro inflammatory mediators
like TNF-α and interleukin 8 in rabbits given
large TV and moderate hyperoxia

Modes and settings
Volume Control/Pressure Control
Pressure /volume / rate
Peak inspiratory pressure / mean peak pressure
Plateau pressure
I:E ratio
Use of PEEP

Peak pressure vs. plateau pressure

Volume Control Vs. Pressure
Control
BJA article – prospective study on 38 patients with
BMI >35 – laparoscopic surgery
Pressure control vs. Volume control
Pressure Control – better tidal volume , PaO 2 ,lesser
A-a gradient, lower PaCO 2
Peak inspiratory flow was higher in pressure control
(52 liter /sec vs. 41 liter/sec)
Tidal volume at ½ inspiratory time 67 % in pressure
control ventilation and 53% in volume control
ventilation
Downsides – In laparoscopic surgeries with
trendelenberg position- High risk for endobronchial
intubation
And what happens when the surgeon deflates the
abdomen

Volume control

Pressure control

PEEP
In a meta-analysis of eight RCTs involving 330
surgical patients, positive pressure ventilation with
PEEP resulted in favorable effects on day 1
postoperatively in terms of higher PaO 2 /FIO 2 and
lesser atelectatic areas, compared with mechanical
ventilation without PEEP (or zero-PEEP, ZEEP)
In morbidly obese patients 10 cm of H 2 O PEEP has
been shown to reduce respiratory elastance and to
improve oxygen exchange
PEEP shown to increase esophageal sphincter
pressures- reduces the risk of regurgitation
Pressure Control with PEEP – studies

Recruitment Maneuvers
Recruitment maneuvers – Peak inspiratory
pressure of 40 cm Hg for 7-8 sec- increases
lung volumes up to vital capacity
ARS (alveolar recruitment strategy)- stepwise
increase of PEEP/inspiratory pressures (e.g.,
0/10, 5/15, 10/20, 15/25 cm H 2 O) over 8 to 10
respiratory cycles
Applying a continuous positive airway pressure
(CPAP) over 10-30 s

Protective ventilation
strategies
Higher surgical risk (major abdominal, thoracic
and cardiac surgery)-a protective ventilation
strategy (V T 4-6 ml/kg Predicted Body Weight,
PEEP with or without Recruitment Maneuver) -
associated with a reduced expression of
alveolar/ systemic inflammatory markers
ARDS net study - Higher tidal volume vs. lower
tidal volume

Back to the case
Volume control mode- Tidal Volume 600 /
Respiratory rate 12 / I: E=1:2
SPO 2 98% , PIP -18, Plateau pressure 15- Vital
signs stable
CO 2 insufflation- PIP 34 – SPO 2 -95 %
Switched to pressure control with pressure
setting – 24 /RR-12
TV -700s , rate 12 . SpO 2 98-99 and stable vital
signs .

Summary
Was the pressure control mode appropriate
Alternatives
A systematic approach is required

High peak pressure – Identify the cause
Man Vs. Machine
Man-bronchospasm
Lung/pleura/chest wall
patient-ventilator
dysynchrony
Machine-ventilator settings
Circuit-kink, secretions
ETT- Right main stem

Treat the cause / Change the settings
Increase I:E ratio
Pressure control
PEEP
Sigh mode
Recruitment maneuver
Consider changing the position the patient
No improvement – Communicate with the
surgeon- Time for laparotomy

References:
Morgan GE, Mikhail MS, Murray MJ (2006) Clinical
Anesthesiology (4 th ed.). Newyork, NY: McGraw-Hill
Corrie KR et al. The effect of obesity and anesthetic
maintenance regimen on postoperative pulmonary
complications. Anes and Analg. 2011;113(1)4-6
Cadi P et al.Pressure-controlled ventilation improves
oxygenation during laparoscopic obesity surgery
compared with volume-controlled ventilation. Br J Of
Anes. 2008; 100 (5):709–716
Barash, P.G, Cullen, B.F & Stoelting R.F. (2009). Clinical
Anesthesia(6 th ed.) Philadelphia PA: Lippincott Williams
and Wilkins

References:
Pelosi P and Greforetti C. Perioperative management of
obese patients. Best Practice & Research Clinical
Anaesthesiology;(2010)211–225
Hans GA et al. Pressure-controlled Ventilation Does Not
Improve Gas Exchange in Morbidly Obese Patients
Undergoing Abdominal Surgery. Obes Surg(2008);18:71–
76

Questions