The Pulmonary Manifestations of Left Heart Failure*
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The Pulmonary Manifestations of Left Heart Failure* Brian K. Gehlbach, MD; and Eugene Geppert, MD Determining whether a patient’s symptoms are the result of heart or lung disease requires an understanding of the influence of pulmonary venous hypertension on lung function. Herein, we describe the effects of acute and chronic elevations of pulmonary venous pressure on the mechanical and gas-exchanging properties of the lung. The mechanisms responsible for various symptoms of congestive heart failure are described, and the significance of sleep-disordered breathing in patients with heart disease is considered. While the initial clinical evaluation of patients with dyspnea is imprecise, measurement of B-type natriuretic peptide levels may prove useful in this setting. (CHEST 2004; 125:669–682) Key words: Cheyne-Stokes respiration; congestive heart failure; differential diagnosis; dyspnea; pulmonary edema; respiratory function tests; sleep apnea syndromes Abbreviations: CHF ϭ congestive heart failure; CSR-CSA ϭ Cheyne-Stokes respiration with central sleep apnea; CPAP ϭ continuous positive airway pressure; Dlco ϭ diffusing capacity of the lung for carbon monoxide; DM ϭ membrane conductance; FRC ϭ functional residual capacity; OSA ϭ obstructive sleep apnea; TLC ϭ total lung ϭ ˙ ˙ ϭ capacity; VC capillary volume; Ve/Vco2 ventilatory equivalent for carbon dioxide early 5 million Americans have congestive heart For a detailed review of the pathophysiology of N failure (CHF), with 400,000 new cases diag- high-pressure pulmonary edema, the reader is re- nosed each year.1 Unfortunately, despite the consid- ferred to several excellent recent reviews.2–4 erable progress that has been made in understanding the pathophysiology of pulmonary edema, the pul- monary complications of this condition continue to The Pathophysiology of Pulmonary challenge the bedside clinician. This review presents Congestion a physiologic basis for understanding the pulmonary Clinicians who are experienced in the care of manifestations of left heart failure (eg, left ventricu- patients with chronic CHF are familiar with the lar failure and/or mitral valve disease). Particular body’s remarkable ability to adapt to a chronically emphasis is placed on the effects on the lung of both elevated pulmonary capillary wedge pressure. How acute and chronic pulmonary venous hypertension, is it that a previously healthy individual develops while congenital heart disease is not considered. We pulmonary edema when the pulmonary capillary reviewed the MEDLINE database for articles rele- wedge pressure reaches 25 mm Hg, whereas a vant to the pathophysiology and clinical conse- patient with longstanding CHF is ambulatory at a quences of pulmonary venous hypertension and pul- filling pressure of 40 mm Hg (Fig 1)? The answer monary edema. Additional sources were culled from lies in a variety of adaptations that occur in an the references in these articles. For simplicity in this individual who has been exposed to chronically article, we have used the convention of referring to elevated filling pressures. left-sided CHF (eg, left ventricular failure and/or Under normal conditions, there is a linear increase mitral valve disease) as CHF. in pulmonary blood flow from the apex to the base of the lung. Elevated left atrial pressure results in *From the Section of Pulmonary and Critical Care, Department of Medicine, University of Chicago, Chicago, IL. pulmonary venous hypertension and a more uniform Manuscript received November 13, 2002; revision accepted June distribution of blood flow.5 This redistribution is 3, 2003. accomplished through capillary distension and re- Reproduction of this article is prohibited without written permis- 6 sion from the American College of Chest Physicians (e-mail: cruitment. Modest elevations in pulmonary capillary [email protected]). wedge pressure are accommodated in this manner, Correspondence to: Brian K. Gehlbach, MD, Section of Pulmo- without the development of pulmonary edema. nary and Critical Care, University of Chicago Hospitals, 5841 S Maryland Ave, MC 6026, Chicago, IL 60637; e-mail: bgehlbac@ At higher filling pressures, fluid may begin to cross medicine.bsd.uchicago.edu the microvascular barrier. Ernest Starling first de- www.chestjournal.org CHEST / 125/2/FEBRUARY, 2004 669 Table 1—Safety Factors Protecting the Lungs Against Alveolar Edema Accumulation* Factors Description Alveolar barrier properties Extremely low alveolar epithelial opposing edema barrier permeability Low alveolar surface tension (surfactant) Active transport by alveolar epithelial cells Microvascular barrier properties Low permeability to protein opposing increased pressure filtration Washdown of perimicrovascular protein osmotic pressure Plasma protein concentration Interstitial exclusion volume Alveolohilar interstitial pressure gradient Perimicrovascular interstitial compliance Coagulation of edema fluid Liquid clearance pathways Lung lymphatic system Loose peribronchovascular connective tissue sumps Resorption into blood vessels Mediastinal drainage Pleural space Figure 1. Chest radiograph of a 28-year-old man in whom an idiopathic dilated cardiomyopathy was diagnosed after presenting Expectoration with weight gain and breathlessness. This chest radiograph was *Reproduced with permission by Flick and Matthay.2 obtained when the patient’s pulmonary capillary wedge pressure was 40 mm Hg, and prior to a 40-lb diuresis. Pulmonary edema is notably absent, although cardiomegaly and distension of the left upper lobe vein suggest the presence of heart disease. suited for gas exchange. The thick portion of the alveolar-capillary unit contains an interstitium with a gel-like protein component. A rise in capillary hydro- static pressure favors the formation of edema first in this interstitial compartment, removed from the scribed the basic forces regulating fluid flux across critical gas-exchanging regions. As fluid accumulates this barrier in 1895, stating that fluid exchange in the lung represents a balance between capillary hydro- static forces favoring pulmonary edema and intersti- tial oncotic pressures opposing it. Although it was originally thought that the lung was “dry” in health, it is now recognized that Starling forces favor the transudation of fluid under normal conditions.7 For- tunately, there are a number of mechanisms that prevent pulmonary edema from occurring. Table 1 provides a comprehensive list of these protective mechanisms, or “safety factors,” several of which are described in more detail below.2 The first of these safety factors is the dual nature of the alveolar- capillary unit, which is composed of a thin side and a thick side (Fig 2). The thin side consists of a capillary closely apposed to the alveolar airspace. Here, the Figure 2. Electron micrograph of a normal alveolar-capillary unit. On the thin side of the alveolar-capillary unit (THIN), the capillary endothelium and alveolar epithelium are basal laminae of the alveolar epithelium and capillary endothe- attenuated, the basement membranes are fused, and lium are fused. The thick side of the barrier (THICK) contains an complex epithelial junctions exist. The low perme- interstitial matrix that separates the alveolar epithelium from the capillary endothelium. (Human lung surgical specimen, ability of this region, coupled with the short distance transmission electron microscopy.) EP ϭ alveolar epithelium; necessary for diffusion, make the thin side well- EN ϭ capillary endothelium. 670 Reviews in the interstitial compartment, there is a rise in its scopic level, many such specimens demonstrate hydrostatic pressure and a lowering of its oncotic alveolar fibrosis, while electron micrographs show pressure, both of which serve to oppose further fluid thickening of the capillary endothelial and alveolar flux. epithelial cell basement membranes. These Once fluid forms in the interstitium, it is trans- changes are thought to reduce the permeability of ported along a negative pressure gradient to the the alveolar-capillary membrane to water, and thus interlobular septae, then to the peribronchovascular prevent the formation of pulmonary edema. Pul- 8–11 space, and finally to the hila. Edema fluid also monary arteries exhibit intimal fibrosis and medial collects in the pleural space, which accommodates hypertrophy, with extension of the muscular layer up to one quarter of all excess lung water in animal into small arterioles (ie, muscularization). Pulmo- 12 models of pulmonary edema. Lymphatic vessels nary veins are abnormally thick-walled, and lym- that are responsible for fluid clearance are contained phatic vessels are dilated and occasionally muscu- within the connective tissue of the interlobular septa, larized. Finally, hemosiderosis is present in a the peribronchovascular sheath, and the pleura. The substantial number of cases, likely from erythro- lymphatic system is highly recruitable, able to in- crease the clearance of lung water by more than cyte egress across the alveolar-capillary membrane 10-fold over time.13 as a consequence of microvascular trauma. Together, the interstitial and pleural spaces act as The natural history of severe mitral stenosis a sump for excess lung water.3 Following the accu- provides an excellent example of how pulmonary mulation of sufficient interstitial fluid, the alveoli vascular remodeling can cause variability in the begin to flood. Once this occurs, the alveolar epithe- clinical presentation of pulmonary venous hyper- lium and distal airways actively transport