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Laryngeal Damage From Intubation
To the Editor: I applaud the efforts of Castella et aI to evaluate the advanta~es of early intercricothyroidotom): Although their belief that this technique is preferable over prolonged translaryngeal intubation may be correct, confirming this hyp4Jthesis by a carefully planned and performed randomized study is essential.
Gene L. Colice, AI.D., F.C.C.~, U:terans Administration Medical Center, White River Junction, VT
Diagnosing Pulmonary Embolism To the Editor: I was interested in the letter by Dr. Kufs (Chest 1989; 96:953) referring to the article on arterial blood ~s analysis and anwographically-proven pulmonary embolism (PE) by Cvitanic and Marino (Chest 1989; 95:48-51). While Dr. Kufs is quite correct in cautioning us regarding correctly assessing the sensitivity of various tests for pulmonary embolism, it is certainly worth emphasing that clinically significant pulmonary embolism is highly unlikely to be present in the absence of evidence of abnormal ~s exchange, ie, either hypoxemia or normoxemia in the presence of hypocapnia. In this regard, it is useful to consider a relatively simple test requiring only minimal extra effort over and above the measurement of arterial blood gases, (ie, the VdNt ratio). This has been demonstrated in a prospective study (Am Rev Respir Dis 1986; 133:679-85) to be as useful as the radioisotope scan and may obviate the need for pulmonary anwography in some patients. In that study, the diagnostic sensitivity of a VdlVt value >0.4 in the presen<..'e of a normal spirogram as a positive test of PE, and of a VdNt <0.4 excluding the diagnosis of PE, was 100 percent: specificity was 94 percent. Bayesian analysis of those data indicated that the probability of a correct diagnosis of PE using these criteria in a similar population would be 90.5 percent; for exluding PE, 96.7 percent. These data clearly indicated that, as a diagnostic test in PE, VdlVt measurement is at least <.'Omparable in terms of sensitivity and specificity to radiosiotope lung scannin~ and would certainly, if routinely applied, reduce the requirement fc)r angiography in a proportion of patients suspected to have PE. It should be noted that only relatively simple measurements are involved: simultaneous arterial blood ~as analysis, expired gas collection and analysis, and spirometry. N. K. Burki, M.D., Ph.D., f:C.C.~, University of Kentucky Chadler Medical Centi"" l.£xingtori
Reprint requests: Dr. Burki, University of Kentucky Medical Center,
800 Rose Street, Urington 40536-0084
To the Editor: I have reviewed Dr. Burkfs comments regarding the utility of detenninations of relative dead space to tidal volume (VdlVt) in the cltnical diagnosis of pulmonary embolism. While I agree it is possible that "clinically significant" pulmonary emboli are unlikely to be present in the absence of altered ~s exchan~e, I do not believe that either examination of a single arterial blood gas sample or the detennination of VdlVt and spirography as reported by Dr. Burki (Am Rev Respir Dis 1986; 133:679-85) has been demonstrated to be sufficiently sensitive to obviate further dia~ostic evaluation in most cases where the diagnosis is entertained. Dr. Burki reports that VdNt detenninations combined with spirography is as sensitive and specific as ventilation/perfusion lung scans based on a prospec-
tive measurement of these pararneters in 110 l'onsel'ulive patients with possible pulmonary emholism. In revie\\'inJ,t thfA data prt;-sented, I feel similar limitations are present as detailed in rny earlier eomments on the article hy Cvitanil- and Marino (Ch,',d 1989; 95:48-51). In 29 patients. the dia~nosis of enlholisnl was excluded by either perfusion lung scan or anJ.ti()~raphy, and 16 had normal VdlVt by the authors' definition (VdNt>40 percent), yielding an estimated specificity of 55 percent. Twelve of these 13 patitants had abnornlal spiro~rams sugJ,testinJ,t intrinsil- lun~ diseast;- as a possible cause of the observed elevated Vd!Vt. If these patients are excluded (normal spirogram, normal VdlVt) the specificity is 94 l>ereent (16 of 17). The author states the sensitivity of an elevated VdNt to predict the presence of pulmonilry emholism is 100 pereent, yet in the group of 16 patients \\'ith an~io~raphkally proven pulmonary embolism from \\,hich this nurnher was derivt:'d, only three patients are reported to have had normal spiro~ranls (five did not under~() spirometry and ei~ht had ahnornlal spiro~rams). Comparin~ the mean VdlVt hern.'een the patients with ahnormal spiroW'ams \\rith PE (ei~ht of 16) and \\,ithout PE (20 of 29) yields no statistical difference (Students t test, p = 0.73). sUAAesting that the abnormalities of dead space ventilation ohservt;-d in these patients may he explained solely hy intrinsk lung disease. Thus an ahnormal VdlVt would only he su~estive of ernholistn in the presenee of a normal spiro~ram, which occurred in three of 11 (27 percent) patients \\rith anwo~raphit' evidence of PE. It is quite possihle to overestirnate the sensitivity of VdlVt ahnornlalities in pure pulnlonary emholism if the ~roup from \\rhich the nunlhfAr is derived is lar~ely composed of patients \\,ho have haseline ahl1ortnalities in VdNt; the sensitivity of abnormal VclIVt \\rith a uonnal spirogram cannot be meanin~ully reported hased on three patients. A normal Vd!Vt in the patient \\;th an abnormal spiro~ranl ol'l'urred in ei~ht of 20 (40 percent) of patients without PE and in none of tllt.l ei~ht patients \\rith PE. and may he useful to excludt.l thta diaJ,tllosis althou~h the false negative (norrnal VdNt \\rith PE) is unknown given the small patient number. Overall, 71 perl'ent (66 of 93) of p<\tients had ahllormal spiroJ.traphk- results and would t>xpt-ct to benefit from VdlVt detennination only if it \\'as nornlill \\'ith the realization that only 40 percent of patients without PE will he normal and the uncertainty over the false negative ratt;- ~i\'tan the small numher of patients evaluated. With the above in mind, if one analyzes the J,troup purported to he "hi~h probability" (an~o~ram not done) the author reports that 17 of 18 patients had abnormal veWt; however, ten patients had ahnormal spiro~rams and five were not tested. For ten of 13 patients, the elevated VdlVt cannot he used as su~estive t.avidenl'e of ernholisrn ~iven the spiro~raphic abnormality and should not add to the decision to treat or proceed with an~i()graphy. The (.'()nverse is illustrated in the ~roup felt to he "lower prohahility of pulmonilry embolism"; of the 13 patients with normal spiro~rams, ei~ht (62 percent had elevated VdNt. Either these patients are incorrectly stratified as low probability or the specificity of this findil1~ is not 94 percent. An~io~raphic <..'()nfirmation would he flfACeSsary and useful in a lar~er number of this subgroup. Finally, 59 percent (65 of 110) of patients underg()in~ evaluation for pulmonary embolism did not have a definitive study (normal perfusion scan or angiogram or conversely a study diagnostic of embolism); given that the total number of true positives ilnd true ne~atives have not been identified, definitive data on sensitivity or specificity of any subgroup is potentially frau~ht \\rith em)r. Simply stated, those patients who clinically required pulmonary an~iogra phy may represent a group with larger emboli and subsequently lar~er gas exchange abnormalities <..'ompared to the clinically less dramatic group that may have gone undiagnosed and who may have displayed much less alteration in dead space. Without investi~atin~ all patients suspected of PE until a definitive study (either positive or negative) is obtained, it is impossible to state \\rith certainty that significant emboli are unlikely in the absen('"e of signifieant ~as CHEST I 98 I 3 I SEPTEMBER, 1990
777
Gene L. Colice, AI.D., F.C.C.~, U:terans Administration Medical Center, White River Junction, VT
Diagnosing Pulmonary Embolism To the Editor: I was interested in the letter by Dr. Kufs (Chest 1989; 96:953) referring to the article on arterial blood ~s analysis and anwographically-proven pulmonary embolism (PE) by Cvitanic and Marino (Chest 1989; 95:48-51). While Dr. Kufs is quite correct in cautioning us regarding correctly assessing the sensitivity of various tests for pulmonary embolism, it is certainly worth emphasing that clinically significant pulmonary embolism is highly unlikely to be present in the absence of evidence of abnormal ~s exchange, ie, either hypoxemia or normoxemia in the presence of hypocapnia. In this regard, it is useful to consider a relatively simple test requiring only minimal extra effort over and above the measurement of arterial blood gases, (ie, the VdNt ratio). This has been demonstrated in a prospective study (Am Rev Respir Dis 1986; 133:679-85) to be as useful as the radioisotope scan and may obviate the need for pulmonary anwography in some patients. In that study, the diagnostic sensitivity of a VdlVt value >0.4 in the presen<..'e of a normal spirogram as a positive test of PE, and of a VdNt <0.4 excluding the diagnosis of PE, was 100 percent: specificity was 94 percent. Bayesian analysis of those data indicated that the probability of a correct diagnosis of PE using these criteria in a similar population would be 90.5 percent; for exluding PE, 96.7 percent. These data clearly indicated that, as a diagnostic test in PE, VdlVt measurement is at least <.'Omparable in terms of sensitivity and specificity to radiosiotope lung scannin~ and would certainly, if routinely applied, reduce the requirement fc)r angiography in a proportion of patients suspected to have PE. It should be noted that only relatively simple measurements are involved: simultaneous arterial blood ~as analysis, expired gas collection and analysis, and spirometry. N. K. Burki, M.D., Ph.D., f:C.C.~, University of Kentucky Chadler Medical Centi"" l.£xingtori
Reprint requests: Dr. Burki, University of Kentucky Medical Center,
800 Rose Street, Urington 40536-0084
To the Editor: I have reviewed Dr. Burkfs comments regarding the utility of detenninations of relative dead space to tidal volume (VdlVt) in the cltnical diagnosis of pulmonary embolism. While I agree it is possible that "clinically significant" pulmonary emboli are unlikely to be present in the absence of altered ~s exchan~e, I do not believe that either examination of a single arterial blood gas sample or the detennination of VdlVt and spirography as reported by Dr. Burki (Am Rev Respir Dis 1986; 133:679-85) has been demonstrated to be sufficiently sensitive to obviate further dia~ostic evaluation in most cases where the diagnosis is entertained. Dr. Burki reports that VdNt detenninations combined with spirography is as sensitive and specific as ventilation/perfusion lung scans based on a prospec-
tive measurement of these pararneters in 110 l'onsel'ulive patients with possible pulmonary emholism. In revie\\'inJ,t thfA data prt;-sented, I feel similar limitations are present as detailed in rny earlier eomments on the article hy Cvitanil- and Marino (Ch,',d 1989; 95:48-51). In 29 patients. the dia~nosis of enlholisnl was excluded by either perfusion lung scan or anJ.ti()~raphy, and 16 had normal VdlVt by the authors' definition (VdNt>40 percent), yielding an estimated specificity of 55 percent. Twelve of these 13 patitants had abnornlal spiro~rams sugJ,testinJ,t intrinsil- lun~ diseast;- as a possible cause of the observed elevated Vd!Vt. If these patients are excluded (normal spirogram, normal VdlVt) the specificity is 94 l>ereent (16 of 17). The author states the sensitivity of an elevated VdNt to predict the presence of pulmonilry emholism is 100 pereent, yet in the group of 16 patients \\'ith an~io~raphkally proven pulmonary embolism from \\,hich this nurnher was derivt:'d, only three patients are reported to have had normal spiro~ranls (five did not under~() spirometry and ei~ht had ahnornlal spiro~rams). Comparin~ the mean VdlVt hern.'een the patients with ahnormal spiroW'ams \\rith PE (ei~ht of 16) and \\,ithout PE (20 of 29) yields no statistical difference (Students t test, p = 0.73). sUAAesting that the abnormalities of dead space ventilation ohservt;-d in these patients may he explained solely hy intrinsk lung disease. Thus an ahnormal VdlVt would only he su~estive of ernholistn in the presenee of a normal spiro~ram, which occurred in three of 11 (27 percent) patients \\rith anwo~raphit' evidence of PE. It is quite possihle to overestirnate the sensitivity of VdlVt ahnornlalities in pure pulnlonary emholism if the ~roup from \\rhich the nunlhfAr is derived is lar~ely composed of patients \\,ho have haseline ahl1ortnalities in VdNt; the sensitivity of abnormal VclIVt \\rith a uonnal spirogram cannot be meanin~ully reported hased on three patients. A normal Vd!Vt in the patient \\;th an abnormal spiro~ranl ol'l'urred in ei~ht of 20 (40 percent) of patients without PE and in none of tllt.l ei~ht patients \\rith PE. and may he useful to excludt.l thta diaJ,tllosis althou~h the false negative (norrnal VdNt \\rith PE) is unknown given the small patient number. Overall, 71 perl'ent (66 of 93) of p<\tients had ahllormal spiroJ.traphk- results and would t>xpt-ct to benefit from VdlVt detennination only if it \\'as nornlill \\'ith the realization that only 40 percent of patients without PE will he normal and the uncertainty over the false negative ratt;- ~i\'tan the small numher of patients evaluated. With the above in mind, if one analyzes the J,troup purported to he "hi~h probability" (an~o~ram not done) the author reports that 17 of 18 patients had abnormal veWt; however, ten patients had ahnormal spiro~rams and five were not tested. For ten of 13 patients, the elevated VdlVt cannot he used as su~estive t.avidenl'e of ernholisrn ~iven the spiro~raphic abnormality and should not add to the decision to treat or proceed with an~i()graphy. The (.'()nverse is illustrated in the ~roup felt to he "lower prohahility of pulmonilry embolism"; of the 13 patients with normal spiro~rams, ei~ht (62 percent had elevated VdNt. Either these patients are incorrectly stratified as low probability or the specificity of this findil1~ is not 94 percent. An~io~raphic <..'()nfirmation would he flfACeSsary and useful in a lar~er number of this subgroup. Finally, 59 percent (65 of 110) of patients underg()in~ evaluation for pulmonary embolism did not have a definitive study (normal perfusion scan or angiogram or conversely a study diagnostic of embolism); given that the total number of true positives ilnd true ne~atives have not been identified, definitive data on sensitivity or specificity of any subgroup is potentially frau~ht \\rith em)r. Simply stated, those patients who clinically required pulmonary an~iogra phy may represent a group with larger emboli and subsequently lar~er gas exchange abnormalities <..'ompared to the clinically less dramatic group that may have gone undiagnosed and who may have displayed much less alteration in dead space. Without investi~atin~ all patients suspected of PE until a definitive study (either positive or negative) is obtained, it is impossible to state \\rith certainty that significant emboli are unlikely in the absen('"e of signifieant ~as CHEST I 98 I 3 I SEPTEMBER, 1990
777