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CHAPTER 48: Ventilator Waveforms: Clinical Interpretation 411

the patient and facilitates reintubation over the tube changer if required. • Lunn WW, Sheller JR. Flow volume loops in the evaluation of upper
Careful consultation with anesthesia and otolaryngology services prior airway obstruction. Otolaryngol Clin North Am. 1995;28(4):721-729.
to making a decision to extubate is important, and all necessary equip-
ment and personnel should be at the bedside in marginal cases. • Toumpanakis D, Kastis GA, Zacharatos P, et al. Inspiratory
Some studies have suggested that quantification of the cuff leak, resistive breathing induces acute lung injury. Am J Respir Crit Care
either by volume or by percentage of total tidal volume, may predict Med. 2010;182(9):1129-1136.
the occurrence of postextubation stridor in patients not intubated for • Vieira F, Allen SM, Stocks RM, et al. Deep neck infection.
UAO. Because the rate of reintubation in such cases is frequently low, Otolaryngol Clin N Am. 2008;41(3):459-483.
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it remains unclear whether this practice should be applied to all endotra-
cheal tube removals, particularly if it results in more patients remaining
intubated for longer periods, or on a case-by-case basis (eg, the patient
intubated for a known UAO). REFERENCES
Complete references available online at www.mhprofessional.com/hall
COMPLICATIONS OF UPPER AIRWAY OBSTRUCTION

Short of cardiac arrest, the most feared complication of UAO is anoxic
brain injury. Negative-pressure pulmonary edema may also occur.
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This condition typically occurs in patients with severe UAO, in whom CHAPTER Ventilator Waveforms:
several mechanisms act synergistically to promote its development. Clinical Interpretation
First, the patient generates extremely negative pleural pressures during 48
inspiration attempting to overcome the UAO. This promotes venous Gregory A. Schmidt
return and a shift of the interventricular septum to the left, decreasing
left ventricular preload. At the same time, left ventricular afterload is
dramatically increased by the fall in intrathoracic pressure, as well as
by catecholamine-induced systemic hypertension. These events cause a KEY POINTS
transfer of blood volume from the systemic to the pulmonary circula- • Pressure and flow waveforms reveal a wealth of information
tion. Here, pulmonary capillary transmural pressure is elevated because regarding the patient’s physiologic derangement.
of reduced interstitial pressure and possibly elevated capillary pressure,
the latter from increased blood volume and pulmonary vascular tone • Distinguishing the contributions of resistive and elastic pressures
from hypercapnia and hypoxia. This rise in capillary transmural pres- allows tailoring and monitoring of therapy.
sure causes pulmonary edema. On occasion, the edema fluid in such • AutoPEEP should be sought in all mechanically ventilated patients.
patients has been noted to be pink or blood-tinged, possibly from red • Ventilator waveforms show how adequately the physician has
blood cell leakage caused by high transmural pressures across disrupted accommodated the ventilator to the patient.
alveolar-capillary membranes. A recent animal model suggests that
pulmonary edema following upper airway obstruction may be due in • Patient effort confounds interpretation of pressures and flows.
part to acute lung injury, helping to explain cases in which the clinical • Attention to ventilator waveforms can improve the accuracy of
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resolution of pulmonary edema and hypoxemia was prolonged. hemodynamic interpretation and is essential for judging the valid-
ity of dynamic predictors of fluid-responsiveness.

KEY REFERENCES
Intensive care ventilators generate tidal ventilation by applying to the
• Bowen T, Cicardi M, Farkas H, et al. 2010 International consensus endotracheal tube or mask a pressure higher than the alveolar pressure.
algorithm for the diagnosis, therapy and management of heredi- This is true whether the mode of ventilation is volume-preset (volume
tary angioedema. Allergy, Asthma, & Clin Immunol. 2010;6(1):24. assist-control [ACV], synchronized intermittent mandatory ventilation
• Cicardi M, Levy RJ, McNeil DL, et al. Ecallantide for the treat- [SIMV]); pressure-preset (pressure support ventilation [PSV], pressure-
ment of acute attacks in hereditary angioedema. N Engl J Med. control ventilation [PCV]); or more complex modes (pressure-regulated
2010;363(6):523-531. volume control [PRVC], proportional assist ventilation [PAV], air-
• Fan T, Wang G, Mao B, et al. Prophylactic administration of paren- way pressure release ventilation [APRV], volume support ventilation
teral steroids for preventing airway complications after extubation [VSV]). The capability to display waveforms turns modern ventilators
in adults: meta-analysis of randomized placebo controlled trials. into sophisticated probes of the patients’ respiratory mechanics and of
BMJ. 2008;337:a1841. patient-ventilator interaction. Respiratory system mechanics and wave-
• Haymore BR, Yoon J, Mikita CP, et al. Risk of angioedema with form analysis should be integrated into routine ventilator management
angiotensin receptor blockers in patients with prior angioedema of the critically ill patient. The fundamental aims are to (1) determine
associated with angiotensin-converting enzyme inhibitors: a meta- the nature of the mechanical derangement of the respiratory system;
analysis. Ann Allergy Asthma Immunol. 2008;101(5):495-499. (2) assay the response to therapy and time; (3) reveal autoPEEP; and
(4) determine the patient-ventilator interaction to guide adjustment of
• Johnson RF, Stewart MG, Wright CC. An evidence-based review ventilator settings. In addition, respiratory muscle activity must be con-
of the treatment of peritonsillar abscess. Otolaryngol Head Neck sidered when measuring hemodynamic pressures such as the pulmo-
Surg. 2003;128(3):332-343. nary artery occlusion pressure (wedge pressure, Ppw) or the right atrial
• Kehmani RG, Randolph A, Markovitz B. Corticosteroids for the pre- pressure (Pra), since these pressures are determined at end-expiration
vention and treatment of post-extubation stridor in neonates, chil- or when judging the validity of dynamic fluid-responsiveness predictors
dren and adults. Cochrane Database Syst Rev. 2009;(3):CD001000. (such as pulse- or stroke-volume variation), since these depend on a pas-
• Low K, Lau KK, Holmes P, et al. Abnormal vocal cord function sively ventilated patient. The timepoint of end-expiration, as well as the
in difficult-to-treat asthma. Am J Respir Crit Care Med. 2011; presence of inspiratory and expiratory effort (both of which can greatly
184(1):50-56. confound interpretation of hemodynamic pressures) can be readily dis-
cerned by analyzing ventilator waveforms.

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